Natural stone

The natural stones used in building may be classified by reference to their origin as: (1) igneous, such as granite and basalt; (2) sedimentary, such as limestones and sandstones; and (3) metamorphic, such as slates and marble. Natural stone has been used in the construction of buildings because it was thought that any hard, natural stone would resist the action of wind and rain for centuries. Many natural stones have proved to be extremely durable for 100 years or more; however, there have been some failures due to the poor selection of the material, poor workmanship and/or poor detailing. Variation within the stone may cause localised weathering problems on exposure to rain and chemical pollutants in the air.

Some natural stones are comparatively soft and moist when first quarried but will gradually harden. Building stones should be seasoned (allowed to harden) for periods of up to a few years, depending on their size and type. Natural stones that are stratified, such as limestone and sandstone, must be used so that they lie on their natural bed to support compressive stress.

The strength of building stone lies in its very considerable compressive strength. The ultimate or failing stress of stone used for walling is about 300–100 N/mm³ for granite, 195–27 N/mm³ for sandstone and 42–16 N/mm³ for limestone. The use of stone as a facing material makes little use of the inherent compressive strength of the material.

Reconstituted (reconstructed) stone

Reconstructed stone is made from an aggregate of crushed stone, cement and water. The stone is crushed so that the maximum size of the particles is 6 mm, and it is mixed with cement in the proportions of one part cement to three or four parts of stone. Portland cement, white cement or coloured cement may be used to simulate the colour of a natural stone as closely as possible. A comparatively dry mix of cement, crushed stone and water is prepared and cast in moulds. The mix is thoroughly consolidated inside the moulds by vibrating and is left to harden in the moulds for at least 24 hours. The stones are then taken out of the moulds and allowed to harden gradually for 28 days.

Reconstructed stone has much the same texture and colour as the natural stone from which it is made, and can be cut, carved and dressed just like natural stone. It is not stratified, is free from flaws and is sometimes a better material than the natural stone from which it is made. Moulded cast stones made by repetitive casting can often be produced more cheaply than similar natural stones, which have to be cut and shaped.

A cheaper form of cast stone is made with a core of ordinary concrete, faced with an aggregate of crushed natural stone and cement. This material should more properly be called ‘cast concrete’. The core is made from clean gravel, sand and cement, and the facing is made from crushed stone and cement to resemble the texture and colour of a natural stone. The crushed stone, cement and water is first spread in the base of the mould to a thickness of about 25 mm, the core concrete is added and the mix consolidated. If the stone is to be exposed on two or more faces, the natural stone mix is spread up the sides and the bottom of the mould. This type of cast stone cannot be carved as it has only a thin surface of natural‐looking stone.

Clay bricks

Clay differs quite widely in composition from place to place, and the clay dug from one part of a field may differ from that dug from another part of the same field. Clay is ground in mills, mixed with water to make it plastic and then moulded, either by hand or machine, to the shape and size of a brick. Bricks that are shaped and pressed by hand in a sanded wood mould and then dried and fired have a sandy texture, are irregular in shape and colour, and are used as facing bricks due to the variety of their shapes, colour and texture. Machine‐made bricks are either hydraulically pressed in steel moulds or extruded as a continuous band of clay. The continuous band of clay, a section of which is the length and width of a brick, is cut into bricks using a wire frame. Bricks made this way are called ‘wire cuts’. The moulded brick is baked to dry out the water and is burnt at a high temperature so that part of the clay fuses the whole mass of the brick into a hard, durable unit. Because there is wide variation in the composition of the clays suitable for brickmaking, and because it is possible to burn bricks over quite a wide range of temperatures sufficient to fuse the material into a durable mass, a large variety of bricks is available.

Calcium silicate (sand–lime) bricks

Calcium silicate bricks are generally known as ‘sand–lime bricks’. The bricks are made from a carefully controlled mixture of clean sand and hydrated lime, which is mixed together with water, moulded to brick shape and then hardened in a steam oven. The resulting bricks are very uniform in shape and colour and are normally a dull white. Coloured sand–lime bricks are made by adding a colouring matter during manufacture. The material from which they are made can be carefully selected and accurately proportioned to ensure a uniform hardness, shape and durability that are quite impossible with the clay used for most bricks.

Concrete bricks

Concrete bricks are manufactured in the same size as clay bricks. They tend to be more consistent in shape, size and colour than clay bricks, and come in a variety of colours and finishes. Appearance and properties vary between manufacturers, although the concrete brick does have a different appearance from clay bricks, which extends the choice available.

Brick classifications

Bricks may be classified in accordance with their uses as noted in the following text, although they should be specified according to the required performance requirements – that is, strength, thermal performance, water absorption, frost resistance and so on.

• Commons. These are bricks that are sufficiently hard to safely carry the loads normally supported by brickwork, but are used for situations where they will not be exposed to view, owing to their dull texture or poor colour.
• Facings. This includes any brick that is sufficiently hard‐burnt to carry normal loads. Manufacturers offer an extensive range of colours and textures from which to choose.
• Engineering bricks. These are bricks that have been made from selected clay and carefully burnt to make a very solid and hard brick that is capable of safely carrying much heavier loads than other types of bricks. These bricks are mainly used for walls carrying exceptionally heavy loads, brick piers, general engineering works and especially for works below ground.
• Special bricks. Specials are made for specific uses in fairface brickwork. (The word ‘fairface’ describes a brick wall finished with a reasonably flat and level face for the sake of appearance.) These bricks are made from fine clays to control and reduce shrinkage deformation during firing. Figure 5.8 is an illustration of some typical (standard) specials. Some specials may be available from stock, others will need to be made to order.
• Special specials. Manufacturers also make purpose‐made specials to order, known as ‘special specials’, to suit particular design requirements. Additional time will be needed to design and manufacture the special specials, and this will need to be considered in the planning of the construction works.

Physical properties of bricks

When specifying bricks, a performance specification will need to consider the following parameters: size and type, compressive strength, frost resistance, water absorption and suction, soluble salt content and visual appearance. This information is provided by brick manufacturers. It is common practice to specify facing bricks by name (manufacturer and brick name), which will automatically confirm the brick’s characteristics.

• Compressive strength. The compressive strength of bricks is found by crushing 12 of them individually until they fail or crumble. The average compressive strength of the brick is stated as the newton per millimetre of surface area (N/mm²) required to ultimately crush the brick. The crushing resistance varies from about 3.5 N/mm² for soft facing bricks up to 140 N/mm² for engineering bricks.
• Water absorption and suction. The amount of water a brick will absorb is a guide to its density and therefore its strength in resisting crushing. The level of water absorption is most critical for bricks to be used for DPCs or below DPC level. Absorption rates vary between 1% and 35%. Bricks with high suction rates absorb water rapidly from the mortar, and hence they are more difficult to reposition as work proceeds than bricks with medium to low suction rates.
• Thermal and moisture movement. All building materials move as a result of the expansion and contraction caused by temperature or moisture changes. The amount of movement depends on the materials and conditions. Allowance must be made for movement in masonry walls through careful positioning of control joints (Table 5.2). Brick manufacturers offer guidance based on current standards. Photograph 5.1 shows a movement joint in a brick and flint clad wall. Expansion in long walls without adequate control joints may result in the bricks at the end of the wall oversailing the DPC (Figure 5.9), or the corner of walls cracking (Figure 5.10). Where buildings or different structures meet, it is always advisable to have control or movement joints. Photograph 5.2 shows a movement joint at the junction between two properties, which forms a party wall between the dwellings.

For clay bricks, it is recommended that the joint be capable of accommodating 10 mm movement. Because materials will be used to construct and fill the joint, this may mean that the joint will be 12–20 mm wide. Calcium silicate bricks suffer from contraction rather than expansion. Where joints are used to accommodate shrinkage, the wall should be structurally sound when the joint is at its widest (when the wall has shrunk to its full extent). Control joints should be located where there is lateral support (Figure 5.11).

Extra ties at 300‐mm centres should be used on each side of the joint. Wire and plastic wall ties can accommodate differential movement between the skins of cavity walls by bending and flexing. In some situations, it may be necessary to place a wall tie directly across the movement joint. Specially designed sleeves can be used with straight wall ties: these allow for movement along the length of the ties but resist movement in other directions (Figure 5.12).

• Movement due to differential settlement. Where it is anticipated that differential settlement may occur due to different ground conditions, foundation designs or building loads, control joints should be used to allow separate parts of the building to move without causing cracking to the facing or structural elements. In long buildings or terraced housing, it may be necessary to separate the buildings and incorporate control joints that allow for differential settlement while retaining the appearance of the building.
• Appearance. There is a large range of bricks from which to choose, and choice is often made on personal preference for the colour and texture of the brick, assuming it meets the set performance requirements. Choice is also influenced by the bricks used on neighbouring buildings, local town planning authority requirements and the existing bricks when working on repair and alteration works.
• Frost resistance. If bricks are saturated and the water within them freezes (expanding as it freezes up to −4 °C), it is likely that small cracks will develop, causing the face of the brickwork to break away (Photograph 5.3). Care is required in exposed areas – for example, parapet walls, chimney stacks and around DPCs – in relation to the selection of bricks and the detailing of the respective junctions to ensure durability. With increased emphasis on higher thermal standards and thermally upgrading existing buildings, the additional insulation results in less heat passing through the wall. During the winter months, this means that the masonry, which is directly exposed to the external environment, receives less heat energy from the interior of the building, and as such is more susceptible to damage from frost. Hence, the durability of the masonry needs careful consideration, especially in exposed locations.
• Efflorescence. Clay bricks contain soluble salts that migrate, in solution in water, to the surface of brickwork as water evaporates to outside air. These salts will collect on the face of brickwork as an efflorescence of white crystals that appear in irregular, unsightly patches. This efflorescence of white salts is most pronounced in parapet walls, chimneys and below DPCs, where brickwork is most liable to saturation. The concentration of salts depends on the soluble salt content of the bricks and the degree and persistence of saturation of brickwork. The efflorescence of white salts on the surface is unsightly and usually causes no damage. In time, the salts may be washed away from surfaces by rain. Heavy concentration of salts can cause spalling and powdering of the surface of bricks, particularly those with smooth faces. This effect is sometimes described as ‘crypto‐efflorescence’. The salts trapped behind the smooth face of bricks expand when wetted by rain and cause the face of the bricks to crumble and disintegrate. Efflorescence may also be caused by the absorption of soluble salts from a cement‐rich mortar or from the ground. There is no way of preventing the absorption of soluble salts from the ground by brickwork below the horizontal DPC level, although the effect can be reduced considerably by the use of dense bricks below the DPC. Soluble salts can migrate through mortar and natural stone in the same way as a brick wall.

When brickwork is persistently wet – such as in foundations, retaining walls, parapets and chimneys – sulphates in bricks and mortar may in time crystallise and expand, causing mortar and renderings to disintegrate. To minimise this effect, bricks with low sulphate content should be used.

Concrete blocks

These are used extensively for both loadbearing and non‐loadbearing walls. A concrete block wall can be laid in less time than a similar brick wall. Lightweight aggregate concrete blocks have good insulating properties against the transfer of heat and are used for the inner leaf of cavity walls. Concrete blocks may be used as a fairface external wall finish. Blocks are accurately moulded to uniform sizes and are made from aggregates to provide a variety of colours and textures.

Concrete blocks are manufactured from cement and either dense or lightweight aggregates as solid, cellular or hollow blocks, as illustrated in Figure 5.13. A cellular block has one or more holes or cavities that do not pass wholly through the block, and a hollow block is one in which the holes pass through the block. The thicker blocks are made with cavities or holes to reduce weight and drying shrinkage.

The most commonly used size of both dense and lightweight concrete blocks is 440 mm long × 215 mm high. The height of the block is chosen to coincide with three courses of bricks for the convenience of building in wall ties and bonding to brickwork. The length of the block is chosen for laying in stretcher bond. For the leaves of cavity walls and internal loadbearing walls, 100‐mm‐thick blocks are used. For non‐loadbearing partition walls, 75‐mm‐thick lightweight aggregate blocks are used. Either 440 × 215 mm or 390 × 190 mm blocks may be used. Concrete blocks may be specified by their minimum average compressive strength for:

• All blocks not less than 75 mm thick
• All blocks less than 75 mm thick used for non‐loadbearing walls, which should be specified by transverse strength

The usual compressive strengths for blocks are 2.8, 3.5, 5, 7, 10, 15, 20 and 35 N/mm². The compressive strength of blocks used for the walls of small buildings of up to three storeys, recommended in Approved Document A, is between 2.8 and 7 N/mm², depending on the loads carried. Concrete blocks may also be classified in accordance with the aggregate used in making the block and some common uses.

Dense aggregate blocks for general use

The blocks are made of cement, natural aggregate or blast‐furnace slag. The usual mix is one part of cement to six or eight parts of aggregate by volume. These blocks are as heavy per cubic metre as bricks, they are not good thermal insulators and their strength in resisting crushing is less than that of most well‐burnt bricks. The colour and texture of these blocks is far from attractive, and they are usually covered with plaster or a coat of rendering. These blocks are used for internal and external loadbearing walls, including walls below ground.

Lightweight aggregate concrete blocks for general use in building

The blocks are made of cement, and one of the following lightweight aggregates: granulated blast‐furnace slag, foamed blast‐furnace slag, expanded clay or shale, or well‐burnt furnace clinker. The usual mix is one part cement to six or eight of aggregate by volume.

Lightweight aggregate concrete blocks are used primarily for internal non‐loadbearing walls. These blocks, usually 75 or 100 mm thick, are made with the same lightweight aggregate as those in Class 2. These blocks are manufactured as solid, hollow or cellular, depending largely on the thickness of the block. The thin blocks are solid and either square‐edged or with a tongue and groove in the short edges, so that there is a mechanical bond between blocks to improve the stability of internal partitions. The poor structural stability may be improved by the use of storey‐height door linings, which are secured at floor and ceiling levels. Thin block internal partitions afford negligible acoustic insulation and poor support for fittings, such as bookshelves secured to them. The thicker blocks are either hollow or cellular to reduce weight and drying shrinkage.

As water dries out from precast concrete blocks, the shrinkage that occurs, particularly with lightweight blocks, may cause serious cracking of the plaster and rendering applied to the surface of a wall built with them. Obviously, the wetter the blocks, the more they will shrink. It is essential that these blocks be protected on building sites from saturation by rain, both when they are stacked on site before use and while walls are being built.

Clay blocks

Hollow clay building blocks are made from selected brick clays that are press moulded and burnt. These hard, dense blocks are hollow to reduce shrinkage during firing, and also to reduce their weight. They are grooved to provide a key for plaster, as illustrated in Figure 5.14. The standard block is 290 mm long × 215 mm high, and 75, 100 or 150 mm thick. Clay blocks are comparatively lightweight, do not suffer moisture movement and have good resistance to damage by fire and poor thermal insulating properties. In the UK, these blocks are mainly used for non‐loadbearing partitions. They are extensively used in southern Europe as infill panel walls to framed buildings, where the tradition is to render the external face of buildings on which the blocks provide a substantial mechanical key for rendering.

Interlocking and recycled blocks

A number of manufacturers have developed interlocking block systems that provide improved thermal resistance as compared to traditional construction. Products tend to be based on polystyrene – or similar materials, such as recycled wood – that forms a lightweight, hollow block. The blocks are interlinked and tied together, usually with uPVC ties, and then filed with a material such as concrete to provide the necessary strength and stability to the wall. These systems tend to appeal to self‐builders and small developers because of their simplicity and ease of construction. The hollow block acts as a type of permanent or lost formwork, with the advantage of providing good thermal insulation. Reinforcement can be inserted between the blocks used to provide additional tensile strength when required. These systems rely on a render to the external face, or the application of some form of facing to provide additional weather protection and an aesthetically pleasing finish.

Cement mortar

When fine cement powder is mixed with water, a chemical action takes place between the water and cement. At the completion of this reaction, the nature of the cement has so changed that it binds itself very firmly to most materials. As the reaction takes place, the excess water evaporates, leaving the cement and sand to harden gradually into a solid mass. The hardening of the mortar becomes noticeable some few hours after mixing, and is complete in a few days. The usual mix of cement and sand for mortar is from one part cement to three or four parts sand, to one part cement to eight parts of sand by volume, mixed with just sufficient water to render the mixture plastic. A mortar of cement and sand is very durable and is often used for brickwork below ground level and brickwork exposed to weather above roof level, such as parapet walls and chimney stacks. When used with some types of bricks, it can cause efflorescence. Because cement mortar has greater compressive strength than required for most ordinary brickwork, and because it is not very plastic by itself, it is sometimes mixed with lime and sand.

Lime mortar

Lime is manufactured by burning limestone or chalk, and the result of this burning is an off‐white, lumpy material known as ‘quicklime’. When quicklime is mixed with water, a chemical change occurs, during which heat is generated, and the lime expands to about three times its former bulk. This change is gradual, taking some days to complete, and the quicklime afterwards is said to be ‘slaked’ – that is, it has no more thirst for water; or, more precisely, the lime is said to be ‘hydrated’. Lime for building is delivered to site ready‐slaked, and is termed ‘hydrated lime’. When mixed with water, lime combines chemically with carbon dioxide in the air, and it gradually hardens into a solid mass in undergoing this change, which firmly binds the sand. The particular advantage of lime is that it is a cheap, reusable, readily available material that produces a plastic material ideal for bedding bricks and stone. Its disadvantages are that it is a messy, laborious material to mix, and it gains strength slower than cement. Because it is, to some extent, soluble in water, it will lose its adhesive property in persistently damp situations, crumble and eventually fall out of joints. Protected from dampness, lime mortar will serve as an effective mortar for the life of most buildings. Lime mortar is usually mixed with one part of lime to three parts of sand by volume. The mortar is plastic, easy to spread and hardens into a dense mass of good compressive strength. Although lime mortar has been replaced with cement mortar in most new building projects, it is essential for maintenance work in historic buildings.

Hydraulic lime

Hydraulic lime is made by burning a mixture of chalk or limestone that contains clay. Hydraulic lime is stronger than ordinary lime and will harden in wet conditions – hence the name. Ordinary Portland cement, made from similar materials and burnt at a higher temperature, has largely replaced hydraulic lime.

Ready‐mixed mortar

Ready‐mixed mortars have come into use particularly on sites where extensive areas of brickwork are laid. A wide range of lime and sand, lime cement and sand, and cement and sand mixes is available. The sand may be selected to provide a colour and texture, or the mix may be pigmented for the same reason. Lime mortar is delivered to site ready to use within the day of delivery. Cement mix and cement lime mortar are delivered to site ready‐mixed with a retarding admixture. The retarding admixture is added to cement mix mortars to delay the initial set of cement. The initial set of ordinary Portland cement occurs some 30 minutes after the cement is mixed with water, so that an initial hardening occurs to assist in stiffening the material for use as rendering on vertical surfaces, for example. The advantages of ready‐mixed mortar are consistency of the mix, the wide range of mixes available, saving in site labour costs and reduction in wastage of material, which is common with site mixing.

Proportions by volume

Mortar for general brickwork may be made from a mixture of cement, lime and sand, in the proportions set out in Table 5.3. These mixtures combine the strength of cement with the plasticity of lime, have much the same porosity as most bricks and do not cause efflorescence on the face of the brickwork. The mixes set out in Table 5.3 are tabulated from rich mixes (1) to weak mixes (5). A rich mix of mortar is one in which there is a high proportion of matrix – that is, lime or cement or both – to sand, as in the 1:0:3 mix, and a weak mix is one in which there is a low proportion of lime or cement to sand, as in the 1:3:12 mix. The richer the mix of mortar, the greater its compressive strength; and the weaker the mix, the greater the ability of the mortar to accommodate moisture or temperature movements. The general uses of the mortar mixes given in Table 5.3 are as mortar for brickwork or blockwork, as follows:

• Mix 1: for cills, copings and retaining walls
• Mix 2: parapets and chimneys
• Mix 3: walls below DPC
• Mix 4: walls above DPC
• Mix 5: internal walls and lightweight block inner leaf of cavity

Mortar plasticisers

Liquids known as ‘mortar plasticisers’, when added to water, effervesce. If very small quantities are added to mortar, when it is mixed, the millions of minute bubbles that form surround the hard, sharp particles of sand, and so make the mortar plastic and easy to spread. The specific application of these mortar plasticisers is that, if used with cement mortar, they increase its plasticity, and there is no need to use lime. It seems that the plasticisers do not adversely affect the hardness and durability of the mortar, and hence they are commonly and successfully used in mortars.

Jointing

‘Jointing’ is the word used to describe the finish of the mortar joints between bricks or blocks, in brickwork or blockwork that is finished fairface (not subsequently covered with plaster, rendering or other finish). Flush joints are generally made as a ‘bagged’ or a ‘bagged in’ joint. The joint is made by rubbing coarse sacking or a brush across the face of the brickwork to rub away all protruding mortar, and hence leave a flush joint. This type of joint, illustrated in Figure 5.15, can most effectively be used on brickwork in which the bricks are uniform in shape and comparatively smooth faced, where the mortar will not spread over the face of the brickwork.

A bucket handle joint is a concave, slightly recessed joint, as illustrated in Figure 5.15. A bucket handle joint may be formed by a jointing tool with or without a wheel attachment to facilitate running the tool along uniformly deep joints. The advantage of the bucket handle joint is that the operation compacts the mortar into the joint and improves weather resistance to some extent. Flush and bucket handle joints are mainly used for jointing as the brickwork is raised, the joint being made after the mortar has hardened sufficiently, or ‘gone off’.

The struck and recessed joints shown in Figure 5.15 are more laborious to make, and therefore more expensive. The struck joint is made with a pointing trowel that is run along the joint either along the edges of uniformly shaped bricks or along a wood straight edge, where the bricks are irregular in shape or coarse textured, to form the splayed back joint. The recessed joint is similarly formed with a tool shaped for the purpose. Of the joints described, the struck joint is mainly used for pointing the joints in old brickwork, and the recessed joint to emphasise the profile, colour and textures of bricks for appearance’s sake to both new and old brickwork.

Pointing

‘Pointing’ is the operation of filling mortar joints with a mortar selected for colour and texture to either brickwork or blockwork. Mortar for pointing is a special mix of lime, cement and sand (or stone dust), chosen to produce a particular effect of colour and texture. The overall appearance of a fairface brick wall can be altered by the selection of mortar colour and type of pointing. The joints in new brickwork are raked out about 20 mm deep when the mortar has gone off sufficiently and before it has set hard, and the joints are pointed as scaffolding is removed.

The mortar joints in old brickwork that was laid in lime mortar may, in time, crumble and be worn away by the action of wind and rain. The joints are raked out to a depth of about 20 mm and pointed with a mortar mix of cement, lime and sand that has roughly the same density as the brickwork. The operation of raking out joints is laborious and messy, and the job of filling the joints with mortar for pointing is time consuming; hence, the pointing of old work is expensive. Pointing or re‐pointing old brickwork is carried out both as protection for the old lime mortar to improve weather resistance and to improve the visual appearance of a wall.

Stretcher bond

The four faces of a brick, which may be exposed in fairface brickwork, are the two long stretcher faces and the two header faces, as illustrated in Figure 5.17. The face on which the brick is laid is the ‘bed’. Some bricks have an indent or ‘frog’ formed in one of the bed faces. The purpose of the frog or indent is to assist in compressing the wet clay during moulding. The frog also serves as a reservoir of mortar onto which bricks in the course above may more easily be bedded.

Brick length can vary appreciably, especially those that are hand moulded and those made from plastic clays that will shrink differentially during firing. It was common practice to describe the thickness of a wall by the length of a brick – that is, ½ B, 1 B, 1¹/₂ B or 2 B walls – rather than by using a precise dimension (the walls in Figures 5.18 and 5.19 may be described as ‘¹/₂ B thick walls’, and the walls in Figure 5.20 as ‘1 B thick walls’). This convention is used in this book too; however, dimensions should always be specified to avoid any possible confusion.

The external leaf of a cavity wall is often built of brick. The most straightforward way of laying bricks in the outer leaf of a cavity wall is with the stretcher face of each brick showing externally. To ensure that bricks are bonded along the length of the wall, they are laid with the vertical joints between bricks lying directly under and over the centre of the bricks in the courses under and over. This is described as ‘stretcher bond’, as illustrated in Figure 5.18.

At the intersection of two half‐brick walls at corners or angles and at the jambs, the bricks are laid so that a header face shows in every other course to complete the bond, as illustrated in Figure 5.18. The appearance of a wall laid in a stretcher bond may be somewhat monotonous because of the mass of stretcher faces showing. To provide some variety, the wall may be built with snap headers, so that a stretcher face and a header face show alternately in each course, with the centre of the header face lying directly under and over the centre of the stretcher faces in courses below and above, as illustrated in Figure 5.19. This form of fake Flemish bond is achieved by using half‐bricks, and hence the name ‘snap header’. The combination and variety in colour and shape can add appreciably to the appearance of a wall. The additional labour and likely wastage of bricks add somewhat to the cost.

English and Flemish bonds

A solid wall, 1 B and thicker, is bonded along its length and through its thickness. The two basic ways in which a solid brick wall may be bonded are with every brick showing a header face and each header face lying directly over two header faces below, or with header faces centrally over a stretcher face in the course below. A bond in which only header faces show is termed a ‘heading’ or ‘header’ bond. This bond is little used, as the great number of vertical joints and header faces is generally considered unattractive. A bond in which header faces lie directly above and below a stretcher face is termed the ‘Flemish bond’ (Figure 5.20b). This bond is generally considered the most attractive bond for facing brickwork, because of the variety of colour shades between header and stretcher faces dispersed over the whole face of the walling.

The English bond, illustrated in Figure 5.20c, avoids the repetition of header faces in each course by using alternate courses of header and stretcher faces, with a header face lying directly over the centre of a stretcher face below. The colour of header faces may differ from the colour of stretcher faces. In the English bond, this difference is shown in successive horizontal courses.

Bonding at angles and jambs

At the end of a wall at a stop end, at an angle or quoin and at jambs of openings, the bonding of bricks has to be finished up to a vertical angle. A brick ¼ B wide has to be used to close or complete the bond of the ¼ B overlap of face brickwork. A brick, cut in half along its length, is used to close the bond at an angle. This cut brick is termed a ‘queen closer’, and is illustrated in Figure 5.21. If the narrow‐width queen closer is laid at the angle, it might be displaced during bricklaying. To avoid this possibility, the closer is laid next to a header, as illustrated in Figure 5.22. The rule is that a closer should be laid next to a quoin (corner) header.

There is often an appreciable difference in the length of facing bricks, so that a solid wall 1 B thick may be difficult to finish as a fairface wall on both sides. Where a 1 B wall is built with bricks of uneven length, it may be necessary to select bricks of the same length as headers and use longer bricks as stretchers. Walls 1¹/₂ B thick may be used for substantial walling for larger buildings where the walling is finished fairface on both sides.

To complete the bond of a solid wall 1¹/₂ B thick in double Flemish bond, it is necessary to use cut half‐bricks in the thickness of the wall, as illustrated in Figure 5.22a. At angles and stop ends of a wall, queen closers are laid next to quoin headers, and a three‐quarter‐length cut brick is used. Cutting half‐length (¹/₂ bats) and three‐quarter‐length bricks and closers is time consuming and wasteful. A 1¹/₂ B–thick wall, finished fairface on both sides and showing English bond on both sides, requires considerably less cutting of bricks to complete the bond, as illustrated in Figure 5.22b. It is only necessary to cut closers and three‐quarter‐length bricks to complete the bond at angles and stop ends.

Garden wall bond

Garden wall bonds are designed specifically to reduce the number of through headers to minimise the labour in selecting bricks of roughly the same length for use as headers. Usual garden wall bonds are three courses of stretchers to every course of headers in an English garden wall bond and one header to every three stretchers in Flemish garden wall bond, as illustrated in Figure 5.23. The reduction in the number of through headers does, to an extent, weaken the through bond of the brickwork. Other combinations such as two or four stretchers to one header may be used. Tops of garden walls are finished with special coping bricks, or a brick or stone capping or coping, to provide weather protection to the top of the wall.

Lead

Lead is an effective barrier to moisture and water. It is liable to corrosion when in contact with freshly laid lime or cement mortar, and should be protected by a coating of bitumen or bitumen paint applied to the mortar surface and both surfaces of the lead. Lead is durable and flexible, and can suffer distortion due to moderate settlement in walls without damage. It is an expensive material and is little used today other than for ashlar stonework or as shaped DPCs in chimneys. Cheaper ‘lead substitute’ materials, which have very similar properties to lead, are widely available. A lead DPC should weigh not less than 19.5 kg/m² (Code No. 4, 1.8 mm thick). Lead should be laid in rolls across the full thickness of a solid wall and each leaf of a cavity wall. The lead should be lapped at joints along the length of the wall and at all intersections, with the lap being at least 100 mm or the width of the DPC.

Copper

Copper is an effective barrier to moisture and water, is flexible, has high tensile strength and can suffer distortion due to moderate settlement in a wall without damage. It is an expensive material, and is rarely used today. Copper as a DPC should be annealed, at least 0.25 mm thick and have a nominal weight of 2.28 kg/m².

Bitumen DPC

Bitumen DPCs are reasonably flexible and can withstand distortion due to moderate settlement in walls without damage. Bitumen DPCs, which are made in rolls to suit the thickness of walls, are bedded on a level bed of mortar and lapped at least 100 mm or the width of the DPC at running joints and intersections. Bitumen is economical, flexible, reasonably durable and convenient to lay. Bitumen is used in conjunction with other materials such as hessian, fibre, rubber or polymers to make a durable and workable DPC. The combination of a mortar bed, bitumen DPC and the mortar bed over the DPC for brickwork makes a comparatively deep mortar joint.

Polyethylene sheet and thermoplastic polymeric products

Polyethylene DPCs have largely been replaced by thermoplastic polymeric products. The flexible polymer DPCs are the most cost‐effective and are most commonly used for both horizontal and vertical applications in domestic construction. Most DPCs are manufactured from reprocessed materials and supplied in 30‐m‐long rolls and widths to suit standard brick and block walls. The polymer roll is a clean and easy‐to‐handle material that remains flexible and workable at low temperatures. It can withstand distortion due to moderate settlement in a wall without damage, and is an effective barrier against moisture. The sheets should have an embossed surface to increase mechanical slip resistance, reducing the possibility of the brickwork moving on the DPC. It is laid on an even bed of mortar and lapped at least at the width of the DPC at running joints and intersections. Being a thin sheet material, it makes a thinner mortar joint than bitumen DPC, and is sometimes preferred for that reason. Its disadvantage as a DPC is that it is fairly readily damaged by sharp particles in mortar or the coarse edges of brick. All laps and joints should be fully lapped by at least 100 mm and sealed using a jointing tape in accordance with the manufacturer’s instructions.

Around door openings and windows, where the internal skin of blockwork needs to be returned and the cavity closed, a vertical DPC is installed to prevent the passage of water (Photograph 5.4). Cavity trays are often formed out of polythene and are often used as DPCs. Where the cavity is bridged, such as above window and door openings, a cavity tray may be used to ensure that water is guided away from the internal skin (Photograph 5.5).

Polymer‐based sheets

Polymer‐based sheets, such as pitch polymer and copolymer thermoplastic, are thinner than bitumen sheets and are used where the thicker bitumen DPC mortar joint would be unsightly. This DPC material, which has its laps sealed with adhesive, may be punctured by sharp particles and edges. Copolymer thermoplastic DPCs have better tensile strength and higher tear and puncture resistance as compared to pitch polymer DPCs.

Ethylene propylene diene monomer (EPDM) rubber sheets

EPDM rubber sheet materials are also available.

Mastic asphalt

Mastic asphalt, spread hot in one coat to a thickness of 13 mm, forms a semi‐rigid DPC, impervious to moisture and water. Moderate settlement in a wall may well cause a crack in the asphalt, through which moisture or water may penetrate. It is an expensive form of DPC, which shows on the face of walls as a thick joint, and is rarely used.

Slate

Thin slates are sufficiently impermeable to water to serve as an effective DPC in solid walls of brick or stone. Slates are laid on a bed of mortar in two courses, with staggered joints, as illustrated in Figure 5.27. Because of the small units of slate, and the joints being staggered, this DPC can remain reasonably effective where moderate settlement occurs. To be effective, the edges of the slates should be exposed on a wall face and not be covered, which makes a deep joint and a strong architectural statement. This form of construction is rarely used today, but is seen in existing buildings, and knowledge of the technology is of use when renovating such buildings.

Smooth or wood‐float finish

Smooth (wood‐float finish) rendering is usually applied in two coats. The first coat is spread by trowel and struck off level to a thickness of about 11 mm. The surface of the first coat is scratched before it dries to provide key for the next coat. The first coat should be allowed to dry out. The next coat is spread by trowel and finished smooth and level to a thickness of about 8 mm. The surface of smooth renderings should be finished with a wood float rather than a steel trowel. A steel trowel brings water and the finer particles of cement and lime to the surface, which, on drying out, shrink and cause surface cracks, whereas a wood float (trowel) leaves the surface coarse‐textured and less liable to surface cracks. Three‐coat rendering is used mostly in exposed positions to provide a thick protective coating to walls. The two undercoats are spread, scratched for key and allowed to dry out to a thickness of about 10 mm for each coat; the third or finishing coat is spread and finished smooth to a thickness of 6–10 mm.

Spatterdash

Smooth, dense wall surfaces, such as dense brick and cast in‐situ concrete, afford poor key and little suction for renderings. Such surfaces can be prepared for rendering by the application of a spatterdash of wet cement and sand. A wet mix of cement and clean sand (mix 1:2, by volume) is thrown on to the surface and left to harden without being trowelled smooth. When dry, it provides a surface suitable for the rendering, which is applied in the normal way.

Scraped‐finish rendering

An undercoat and finish coat are spread as for a smooth finish, and the finished‐level surface, when it has set, is scraped with a steel straight edge or saw blade to remove some 2 mm from the surface to produce a coarse‐textured finish.

Textured finish

Textured rendering is usually applied in two coats. The first coat is spread and allowed to dry, as previously described. The second coat is then spread by trowel and finished level. When this second coat is sufficiently hard, but still wet, its surface is textured with wood combs, brushes, sacking, wire mesh or old saw blades. A variety of effects can be obtained by varying the way in which the surface is textured. An advantage of textured rendering is that the surface scraping removes any scum of water, cement and lime that may have been brought to the surface by trowelling and which might otherwise have caused surface cracking.

Pebbledash (drydash) finish

Pebbledash finish is produced by throwing dry pebbles, shingle or crushed stone on to, and lightly pressed into, the freshly applied finish coat of rendering, so that the pebbles adhere to the rendering but are mostly left exposed as a surface of pebbles. Pebbles of 6–13 gauge are used. The undercoat and finish coat are of a mix suited to the background, and are trowelled and finished level. The advantage of this finish is that the pebbledash masks any hair cracks that may open due to the drying shrinkage of the rendering.

Roughcast (wetdash) finish

A wet mix of rendering is thrown on to the matured undercoat by hand to a thickness of 6–13 mm to produce a rough, irregular‐textured finish. The gauge of the aggregate used in the wet mix determines the finish.

Machine‐applied finish

A wet mix of rendering is thrown on to a matured undercoat by machine to produce a regular, coarse‐textured finish. The surface texture is determined by the gauge of the aggregate used. This may have the natural colour of the materials or be coloured to produce what are called ‘Tyrolean finishes’.

Maintenance of render

Render is subject to staining over time as rain and moisture combine with pollutants in the air to discolour the render. This is often linked to run off from projections, such as window cills and wall vents. When exposed to solar radiation in the summer months, the staining may disappear in some instances, but not on shaded and north‐facing rendered walls. Regular cleaning will be required, as determined by the render finish and the micro‐climate of the building.

Internal insulation

Internal insulation is used where solid walls have sufficient resistance to the penetration of rain, and/or alteration to the external appearance is not permitted or desirable. A disadvantage of internal insulation is that, as the insulation is at, or close to, the internal surface, it will prevent the wall behind from acting as a heat store where constant low‐temperature heating is used. There is also potential for interstitial condensation to form on the face of the wall; therefore, a vapour barrier should be used on the surface of the insulation to ensure that moisture is restricted from meeting the cold face of the wall. Photograph 5.6 shows 150 mm of polyisocyanurate (PIR) insulation fixed to the internal side of a solid 250‐mm‐thick clay brick wall. The insulation on the plane elements provides an approximate U‐value of 0.15 W/m²K.

It is usual to cover the insulating layer with a lining of plasterboard or plaster, so that the combined thickness of the inner lining and the wall have the required U‐value. Internal linings for thermal insulation are either preformed laminated panels that combine a wall lining of plasterboard glued to an insulation board, or separate insulation material fixed to the wall and then covered with plasterboard or plaster. The method of fixing the lining to the inside wall surface depends on the surface to which it is applied.

Adhesive fixing

Adhesive fixing directly to the inside wall face is used for preformed laminate panels and rigid insulation boards. Where the inside face of the wall is clean, dry, level and reasonably smooth, the panels or boards are secured with an organic‐based gap‐filling adhesive, applied in dabs and strips to the back of the boards or panels, or to both the boards and the wall. The panels or boards are then applied and pressed into position against the wall face, and their position adjusted with a foot lifter.

When the surface of the wall is uneven, the panels or boards are fixed with dabs or bonding, applied to both the wall surface and the back of the lining. Dabs are small areas of wet plaster bonding applied at intervals on the surface with a trowel, as a bedding and adhesive. The lining is applied and pressed into position against the wall. The wet dabs of bonding allow for irregularities in the wall surface, and also serve as an adhesive. Some lining systems use secondary fixing in addition to the adhesive – for example, non‐ferrous or plastic nails, or screws, driven or screwed through the insulation boards into the wall. In order to prevent air leakage around the edges of plasterboards applied using dot and dab, a ribbon (continuous strip) of plaster or fixing adhesive should be positioned around the perimeter of the plasterboard. Figure 5.33 is an illustration of laminated insulation panels fixed to the inside face of a solid wall.

Mechanical fixing

As an alternative to adhesive fixing, the insulating lining and the wall finish can be fixed to wood battens (that are nailed to the wall) with packing pieces, as necessary, to form a level surface. The battens should be impregnated against rot and fixed with non‐ferrous fixings. The insulating lining is fixed either between or across the battens, and an internal lining of plasterboard is then nailed to the battens, through the insulation. The thermal resistance of wood is less than that of most insulating materials. When the insulating material is fixed between the battens, there will be thermal bridges through the battens, which may cause staining on wall faces.

Internal wall finish

An inner lining of plasterboard can be finished by taping and filling the joints, or with a thin‐skim coat of plaster. A finish of lightweight plaster and finishing coat is applied to the ready‐keyed surface of some insulating boards, or to expanded metal lathing fixed to battens. Laminated panels of insulation, lined on one side with a plasterboard finish, are made specifically for the insulation of internal walls. The panels are fixed with adhesive or mechanical fixings to the inside face of the wall.

Vapour barriers and checks

Moisture vapour pressure from warm moist air inside insulated buildings may find its way through internal linings and condense on cold outer faces. Where the condensate is absorbed by the insulation, it will reduce the efficiency of the insulation, and where condensation saturates battens, they may rot. With insulation that is permeable to moisture vapour, a vapour check should be fixed on the room side of the insulation. A vapour barrier is one that completely stops the movement of vapour through it, and a vapour check is one that substantially stops vapour. As it is difficult to make a complete seal across the whole surface of a wall, including all overlaps of the barrier and at angles, it is in effect impossible to form a barrier, and the term ‘vapour check’ should more properly be used. Sheets of polythene with edges overlapped and taped together are commonly used as a vapour check.

External wall insulation

Insulating materials by themselves do not provide a satisfactory external finish to walls against rain penetration, nor do they provide an attractive finish; thus, they are covered with a finish of cement rendering, paint or a cladding material such as tile, slate, profiled sheeting or weatherboarding. For rendered finishes, one of the inorganic insulants, rockwool or cellular glass in the form of rigid boards, is most suited. For cladding, one of the organic insulants is used, because it is thinner for a comparable U‐value.

Because the rendering is applied over a layer of insulation, it will be subject to greater temperature fluctuations than if it were applied directly to a wall, and so it is more liable to crack. To minimise cracking due to temperature change and moisture movements, the rendering should be reinforced with a mesh securely fixed to the wall, and movement joints should be formed at intervals of not more than 6 m. The use of a light‐coloured finish and rendering incorporating a polymer emulsion can also help to reduce cracking. As the overall thickness of the external insulation and rendering is too great to be returned into the reveals of existing openings, it is usual to return the rendering by itself, or to fix some non‐ferrous or plastic trim to mask the edge of the insulation and rendering. This will result in the reveals of openings acting as thermal bridges, which is not desirable. Figure 5.34 is an illustration of insulated rendering applied externally.

Slabs of compressed mineral wool are secured to the external face of the wall with stainless steel brackets, fixed to the wall to support and restrain the blocks that are arranged with either horizontal, bonded joints or vertical and horizontal continuous joints.

The insulation can be fixed by purpose‐made plugs and mechanical fixings. These fixings reduce the thermal bridges that can occur when frames are used to attach the insulation.

Jambs of openings

The jambs of openings for windows and doors in solid walls may be plain (square) or rebated. Plain or square jambs are used for small‐section window or door frames of steel, and also for larger‐section frames where the whole of the external face of frames is to be exposed externally. The bonding of brickwork at square jambs is the same as for stop ends and angles with a closer next to a header in alternate courses to complete the bond. Rebated jambs are used to provide some protection to window/door frames (Figure 5.35).

Rebated jambs

Historically, rebated jambs have been used as a feature to protect window and door frames. Figure 5.35 is a diagram of one rebated jamb, illustrating some commonly used terms. The thickness of brickwork that shows at the jamb of openings is described as the ‘reveal’. With rebated jambs, there is an ‘inner reveal’ and an ‘outer reveal’, separated by the rebate. Figure 5.36 shows the brick arrangements for forming a rebated jamb in a Flemish bond. The amount of labour involved in creating such features makes them expensive, and they are hence very rarely used nowadays.

Head of openings in solid walls

Solid brickwork over the head of openings has to be supported by either a lintel or an arch. The brickwork that the lintel or arch has to support is an isosceles triangle with 60° angles, formed by the bonding of bricks, as illustrated in Figure 5.37. The vertical joints between bricks, which overlap ¼ B, form the triangle. In a bonded wall, if the solid brickwork inside the triangle were taken out, the load of the wall above the triangle would be transferred to the bricks of each side of the opening, in what is termed ‘the arching effect’.

Lintel (alternatively spelt ‘lintol’) is the name given to any single solid length of timber, stone, steel or concrete built in over an opening to support the wall above it, as shown in Figure 5.37. The ends of the lintel must be built into the brick or blockwork over the jambs to convey the weight carried by the lintel to the jambs. The area of wall on which the end of a lintel bears is termed its ‘bearing at ends’. The wider the opening, the more weight the lintel has to support, and the greater its bearing at ends must be to transmit the load it carries to an area capable of supporting it. For convenience, its depth is usually made a multiple of brick course height (75 mm), and the lintels are not usually less than 150 mm deep. Hardwood timber lintels have largely been replaced by concrete and steel lintels.

Casting lintels

The word ‘precast’ indicates that a concrete lintel has been cast inside a mould, and has been allowed time to set and harden before it is built into the wall. The words ‘in‐situ cast’ indicate that a lintel is cast in position inside a timber mould fixed over the opening in walls. Precast lintels may be used for standard door and window openings, the advantage being that, immediately after the lintel is placed in position over the opening, brickwork can be raised on it, whereas the concrete in an in‐situ cast lintel requires a timber mould or formwork and must be allowed to harden before brickwork can be raised on it. Lintels are cast in situ if a precast lintel would have been too heavy or cumbersome to have been hoisted and bedded in position. Precast lintels must be clearly marked to make certain that they are bedded with the steel reinforcement in its correct place, at the bottom of the lintel. Usually, the letter ‘T’ or the word ‘top’ is cut into the top of the concrete lintel while it is still wet.

Prestressed concrete lintels

A prestressed lintel is made by casting concrete around high‐tensile, stretched wires, which are anchored to the concrete, so that the concrete is compressed by the stress in the wires (see also Barry’s Advanced Construction of Buildings). The load applied by the stressed wires, which compress the concrete, has to be overcome before the lintel will bend. Two types of prestressed concrete lintels are available: composite lintels and non‐composite lintels.

Composite lintels are stressed by wires at the centre of their depth, and are used with brickwork, which acts as a composite part of the lintel in supporting loads. These comparatively thin precast lintels are built in over openings, and brickwork is then built over them. Prestressed lintels over openings more than 1200 mm wide should be supported to avoid deflection until the mortar in the brickwork has set. When used to support blockwork, the composite strength of these lintels is considerably less than when used with brickwork.

Non‐composite prestressed lintels are made for use where there is insufficient brickwork over to act compositely with the lintel, and also where there are heavy loads. These lintels are made to suit brick and block wall thicknesses, as illustrated in Figure 5.39.

Prestressed lintels may be used over openings in both internal and external solid walls. In external walls, prestressed lintels are used where the wall is to be covered with rendering externally, and for the inner leaf of cavity walls where the lintel will be covered with plaster. Precast reinforced concrete lintels may be exposed on the external face of both solid and cavity walling where the appearance of a concrete surface is desired.

Concrete boot lintels

The lintel is boot shaped in section, with the toe part showing externally. The toe is usually made 65 mm deep. The main body of the lintel is hidden inside the wall, and it is this part of the lintel that does most of the work of supporting the brickwork (Figure 5.40). The detail is sometimes built such that the toe of the lintel finishes 25 or 40 mm back from the external face of the wall, as shown in Figure 5.40, to improve the visual appearance. The brickwork built on the toe of the lintel is usually ½ B thick for openings up to 1.8 m wide. The 65‐mm deep toe, if reinforced as shown, is capable of safely carrying the two or three courses of ¹/₂ B–thick brickwork over it. Because the bricks are bonded, the brickwork above the top of the main part of the lintel bears mainly on it. If the opening is wider than 1.8 m, the main part of the lintel is sometimes made sufficiently thick to support most of the thickness of the wall over, as shown in Figure 5.40. The bearing at ends, where the boot lintel is bedded on the brick jambs, should be of the same area as for ordinary lintels.

Although this section does not deal with cavity wall construction, Figure 5.41 illustrates how the concrete lintel used in solid wall construction evolved for use in cavity walls. Boot lintels (Figure 5.41c) should not be used in cavity wall construction, because the solid concrete provides a thermal bridge across the cavity wall. Concrete lintels can be used in a cavity wall where two separate lintels are used to support each skin (Figure 5.41). The gap between the two skins of brickwork and blockwork can be filled with an insulated cavity closer. Further information on cavity wall construction can be found later in this section.

Semi‐circular arch

The most efficient method of supporting brickwork over an opening is by the use of a semi‐circular arch, which transfers the load of the wall it supports most directly to the sides of the opening through the arch (see Figure 5.45). A segmented arch, which takes the form of a segment (part) of a circle, is less efficient, in that it transmits loads to the jambs by both vertical and outward thrust.

Rough and axed arches

The two ways of constructing a curved brick arch are with bricks laid with wedge‐shaped mortar joints or with wedge‐shaped bricks with mortar joints of uniform thickness, as illustrated in Figure 5.46. An arch formed with uncut bricks and wedge‐shaped mortar joints is termed a ‘rough brick arch’, because the mortar joints are irregular and the finished effect is rough. In time, the joints tend to crack and emphasise the rough appearance; thus, rough arch‐work is rarely used for fairface work.

Arches in fairface brickwork are usually built with bricks cut to a wedge shape, and mortar joints of uniform width. Bricks are cut (either on‐ or off‐site) to the required wedge shape by gradually chopping them to shape – hence the name ‘axed bricks’. A template, or pattern, is cut from a sheet of zinc to the exact wedge shape to which the bricks are to be cut. The template is laid on the stretcher or header face of the brick, as illustrated in Figure 5.47. Shallow cuts are made in the face of the brick on each side of the template. These cuts are made with a hacksaw blade or file, and are intended to guide the bricklayer in cutting the brick. Then, holding the brick in one hand, the bricklayer gradually chops the brick to the required wedge shape. When the brick has been cut to a wedge shape, the rough, cut surfaces are roughly levelled with a coarse rasp. From the description, this appears to be a laborious operation, but a skilled bricklayer can in fact axe a brick to a wedge shape in a few minutes. The axed wedged‐shaped bricks are built to form the arch, with uniform 10‐mm mortar joints between the bricks.

Gauged brick arch

Gauged bricks are those that have been accurately prepared to a wedge shape so that they can be put together to form an arch with very thin joints between them. This does not improve the strength of the brick arch, and is done for appearance. Hard‐burnt clay facing bricks cannot be cut to the accurate wedge shape required for this work, because the bricks are too coarse‐grained. One type of brick used for gauged brickwork is called a ‘rubber brick’ (or ‘brick rubbers’), because its composition is such that it can be rubbed down to an accurate shape on a flat stone. Rubbers are made from fine‐grained sandy clays. They are moulded and then hardened by baking at a lower temperature than that at which clay bricks are burnt, the aim being to avoid fusion of the material of the bricks, so that they can easily be cut and accurately rubbed to shape.

Sheet zinc templates are cut to the exact size of the wedge‐shaped brick voussoirs. These templates are placed on the stretcher or header face of the brick to be cut, and the brick is sawn to a wedge shape with a brick saw. Then they are carefully rubbed down by hand on a large flat stone until they are the exact wedge shape required as indicated by the template. The gauged rubber bricks are built to form the arch, with joints between the bricks as thin as 1.5 mm. Mortar used between the gauged bricks is composed of either fine sand, cement and lime, or lime and water, depending on the thickness of joint. The finished effect of accurately gauged red bricks with thin white joints between them is considered very attractive. Gauged bricks are used for flat camber arches.

Two‐ring arch

Rough and axed bricks are used for both semi‐circular and segmental arches, and gauged brick for segmental and flat camber arches, to avoid the more considerable cutting necessary with semi‐circular arches. Rough, axed or gauged bricks can be laid so that either their stretcher or their header face is exposed. Semi‐circular arches are often formed with bricks showing header faces to avoid the excessively wedge‐shaped bricks or joints that occur with stretcher faces showing. This is illustrated in Figure 5.48 by the comparison of two arches of similar spans, first with stretcher face showing and then with header face showing. If the span of the arch is of any considerable width, say, 1.8 m or more, the practice is often to build it with what is termed ‘two or more rings of bricks’, as illustrated in Figure 5.48. An advantage of two or more rings of bricks showing header faces is that the bricks bond into the thickness of the wall. Where the wall over the arch is more than 1 B thick, it is practical to effect more bonding of arch bricks in walls or viaducts by employing alternate snap headers (half‐bricks) in the face of the arch.

Segmental arch

The curve of this arch is a segment, that is, part of a circle, and designers of a building can choose any segment of a circle that they think suits their design. By trial and error over many years, bricklayers have worked out methods of calculating the segment of a circle related to the span of this arch, which gives a pleasant‐looking shape, and which, at the same time, is capable of supporting the weight of the brickwork over the arch. The recommended segment is such that the rise of the arch is 130 mm for every metre of span of the arch.

Centring

Temporary support for brick arches is necessary, and this is usually in the form of a timber framework to the profile of the underside of the arch, called ‘centring’. It is fixed and supported in the opening while the bricks of the arch are being placed and the coursed brickwork over the arch laid. When the arch and brickwork are finished, the centring is removed. A degree of both skill and labour is involved in arch building – in setting out the arch, cutting bricks for the arch and the abutment of coursed brickwork to the curved profile of the arch. Thus, an arched opening is more expensive than a plain lintel head.

Flat camber arch

A flat camber arch is not a true arch, as it is not curved, and might well be more correctly called a flat brick lintel with voussoirs radiating from the centre, as illustrated in Figure 5.49. The bricks from which the arch is built may be either axed or gauged to the shape required, so that the joints between the bricks radiated from a common centre and the widths of voussoirs measured horizontally along the top of the arch are the same. This width will be 65 mm or slightly less, so that there are an odd number of voussoirs; the centre is the key brick.

The centre from which the joints between the bricks radiate is usually determined either by making the skew or slating surface at the end of the arch 60° to the horizontal, or by calculating the top of this skew line as lying 130 mm from the jamb for every metre of span. If the underside or soffit of this arch were made absolutely level, it would appear to be sagging slightly at its centre. This is an optical illusion, and it is corrected by forming a slight rise or camber on the soffit of the arch. This rise is usually calculated at 6 or 10 mm for every metre of span, and the camber takes the form of a shallow curve. The camber is allowed for when cutting the bricks to shape. In walls built of hard, coarse‐grained facing bricks, this arch is usually built of axed bricks. In walls built of softer, fine‐grained facing bricks, the arch is usually built of gauged rubbers, and is termed a ‘flat‐gauged camber arch’. This flat arch must be of such height on face that it bonds in with the brick course of the main walling. The voussoirs of this arch, particularly those at the extreme ends, are often longer overall than a normal brick, and the voussoirs have to be formed with two bricks cut to shape.

Flat‐gauged camber arch

The bricks in this arch are jointed with lime and water, and the joints are usually 1.5 mm thick. Lime is soluble in water and does not adhere strongly to bricks, and in time the jointing material between the bricks may perish and the bricks might slip out of position. To prevent this, joggles are formed between the bricks. These joggles take the form of semi‐circular grooves cut in both bed faces of each brick, as shown in Figure 5.49, into which mortar is run.

Rubble walling

Rubble walling is extensively used for agricultural buildings in towns and villages in those parts of the country where a local source of stone is readily available. The various forms of rubble walling may be classified as ‘random rubble’ and ‘squared rubble’.

Random rubble

Uncoursed random rubble stones of all shapes and sizes are laid in mortar, as illustrated in Figure 5.50a. No attempt is made to select and lay stones in horizontal courses. There is some degree of selection to avoid excessively wide mortar joints, and also to bond stones by laying some longer stones, both along the face and into the thickness of the wall, so that there is a bond stone in each square metre of walling. At quoins, angles and around openings, selected stones or shaped stones are laid to form roughly square angles.

Random rubble brought to course is similar to random rubble uncoursed, except that the stones are selected and laid such that the walling is roughly levelled in horizontal courses at vertical intervals of 600–900 mm, as illustrated in Figure 5.50b. Transverse and longitudinal bond stones are used.

Squared rubble

Squared rubble uncoursed is laid with stones that come roughly square from the quarry in a variety of sizes. The stones are selected at random, are roughly squared with a walling hammer and laid without courses, as illustrated in Figure 5.51a. As with random rubble, both transverse and longitudinal bond stones are laid at intervals. ‘Snecked rubble’ is a term for squared rubble in which a number of small squared stones, ‘snecks’, are laid to break up long continuous vertical joints. Snecked rubble is often difficult to distinguish from squared random rubble.

Squared rubble brought to course is constructed from roughly square stone rubble, selected and squared so that the work is brought to courses every 300–900 mm intervals, as illustrated in Figure 5.51b. Squared rubble course is built with stones that are roughly squared, so that the stones in each course are roughly the same height and the courses vary in height, as illustrated in Figure 5.52a. The face of the stones may be roughly dressed to give a rock‐faced appearance, or dressed smooth to give a more formal appearance.

Polygonal walling

Stones that are taken from a quarry where the stone is hard have no pronounced laminations and come in irregular shapes that can be laid as polygonal walling. The stones are selected and roughly dressed to fit when laid, to an irregular pattern, with no attempt at regular courses or vertical joints. At corners and as a base, roughly square‐edged stones are used, as illustrated in Figure 5.52b.

Flint walling

Flint walling is traditional to East Anglia and the south and southeast of England. Both field and shore flints are used. The flints used for walling are up to 300 m in length and 75–250 mm in width and thickness. The flints or cobbles may be used whole or split to show the heart of the flint, and also knapped or snapped so that they show a roughly square face. Flint walling is built with a dressing of stone or brick at angles and in horizontal lacing courses that level the wall at intervals. Figure 5.53a is an illustration of whole flints laid without courses in brick dressing to angles and as lacing, and Figure 5.53b is an illustration of knapped flints laid to courses in stone dressing.

Ashlar masonry joints

Ashlar stones may be finished with smooth faces and bedded with thin joints, or the stones may have their exposed edges cut to form a channelled or V joint to emphasise the shape of each stone and to give the wall a heavier, more permanent appearance. The ashlar stones of the lower floor of large buildings are often finished with channelled or V joints and the wall above with plain ashlar masonry to give the base of the wall an appearance of strength. Ashlar masonry finished with channelled or V joints is said to be ‘rusticated’. A channelled joint (rebated joint) is formed by cutting a rebate on the top and one side edge of each stone, so that, when the stones are laid, a channel rebate appears around each stone, as illustrated in Figure 5.54a. The rebate is cut on the top edge of each stone, so that, when the stones are laid, rainwater, which may run into the horizontal joint, will not penetrate the mortar joint. A V joint (chamfered joint) is formed by cutting all four edges of stones with a chamfer, so that, when they are laid, a V groove appears on the face, as illustrated in Figure 5.54b. Often, the edges of stones are cut with both V and channelled joints to give greater emphasis to each stone.

Tooled finish

Plain ashlar stones are usually finished with flat faces. The stones may also be finished with their exposed faces tooled to show the texture of the stone. Some of the tooled finishes used with masonry are illustrated in Figure 5.55. It is the harder stones such as granite and hard sandstone that are more commonly finished with rock face, pitched face, reticulated face or vermiculated face. The softer, fine‐grained stones are usually finished as plain ashlar.

Cornice and parapet walls

It is common practice to raise masonry walls above the levels of the eaves of a roof, as a parapet. The purpose of the parapet is partly to obscure the roof and also to provide a depth of wall over the top of the upper windows for the sake of improving the appearance in the proportion of the building as a whole. In order to provide a decorative termination to the wall, a course of projecting moulded stones is formed. This projecting stone course is termed a ‘cornice’, and it is generally formed of one or more courses of stone below the top of the parapet (Figure 5.56). This projection provides some protection to the wall below.

The parapet wall usually consists of two or three courses of stones capped with coping stones bedded on a DPC of sheet metal. The parapet is usually at least 1 B thick, or of such thickness that its height above the roof is limited by the requirements of the Building Regulations. The parapet may be built of solid stone or stones bonded to a brick backing. The cornice is constructed of stones of about the same depth as the stones in the wall below, cut such that they project, and are moulded for appearance sake. Because the stones project, their top surface is weathered (slopes out) to throw water off.

Saddle joint

The projecting, weathered top surface of coping stones is exposed, and the rain water running off it will, in time, saturate the mortar in the vertical joints between the stones. To prevent rain soaking into these joints, it is usual to cut the stones to form a saddle joint, as illustrated in Figure 5.56. The exposed top surface of the stones has to be cut to slope out (weathering), and a projecting quarter circle of stone is left on the ends of each stone when this cutting is executed. When the stones are laid, the projections on the ends of adjacent stones form a protruding semi‐circular saddle joint, which causes rain to run off away from the joints.

Weathering to cornices

Because cornices are exposed and liable to saturation by rain and possible damage by frost, it is good practice to cover the exposed top surface of cornice stones cut from limestone or sandstone with sheet metal. The sheet metal covering is particularly useful in urban areas, where airborne pollutants may gradually erode stone. Sheet lead is preferred as a non‐ferrous covering because of its ductility, which facilitates shaping, and its impermeability. Sheets of lead, Code No. 5, are cut and shaped for the profile of the top of the cornice, and are laid with welted (folded) joints at 2‐m intervals along the length of the cornice. The purpose of these comparatively closely spaced joints is to accommodate the inevitable thermal expansion and contraction of the lead sheet. The top edge of the lead is dressed up some 75 mm against the parapet as an upstand, and turned into a raglet (groove) cut in the parapet stones and wedged in place with lead wedges. The joint is then pointed with mortar. The bottom edge of the lead sheets is dressed (shaped) around the outer face of the stones and welted (folded). To prevent the lower edge of the lead sheet from lifting up in high winds, 40‐mm‐wide strips of lead are screwed to lead plugs that are set in holes in the stone at 750‐mm intervals, and are folded into the welted edge of the lead, as illustrated in Figure 5.57. Where cornice stones are to be protected with sheet lead weathering, there is no purpose in cutting saddle joints.

Cement joggle

Cornice stones project, and one or more stones might in time settle slightly, such that the decorative line of the mouldings cut on them would be broken, ruining the appearance of the cornice. To prevent this possibility, shallow V‐shaped grooves are cut in the ends of each stone, so that, when the stones are put together, these matching V grooves form a square hole, into which cement grout is run. When the cement hardens, it forms a joggle, which locks the stones in their correct position.

Dowels

To maintain parapet stones in their correct positions in a wall, slate dowels are used. The stones in a parapet are not kept in position by the weight of walling above, and these stones are, therefore, usually fixed with slate dowels. These dowels consist of square pins of slate that are fitted to holes cut in adjacent stones, as illustrated in Figure 5.56.

Cramps

Coping stones are bedded on top of a parapet wall as a protection against water soaking down into the wall below. There is a possibility that the coping (capping) stones may suffer some slight movement, and cracks may develop in the joints between them. Rain may then saturate the parapet wall below, and frost action may contribute to some movement and eventual damage. To keep coping stones in place, a system of cramps is used. Either slate or non‐ferrous metal is used to cramp the stones together. A short length of slate, shaped with dovetail ends, is set in cement grout (cement and water) in dovetail grooves in the ends of adjacent stones, as illustrated in Figure 5.58a. As an alternative, a gunmetal cramp is set in a groove and mortise in the end of each stone and bedded in cement mortar, as illustrated in Figure 5.58b. For coping stones cut from limestone or sandstone, a sheet metal weathering is sometimes dressed over coping stones. The weathering of lead is welted and tacked in position over the stones.

Lintels

A stone lintel for small openings of up to about a metre width can be formed of one whole stone, with its ends built into jambs and its depth corresponding to one or more stone courses. The poor tensile strength of stone limits the span of single‐stone lintels, unless they are to be disproportionately deep. Over openings wider than about a metre, it is usual to form lintels with three or five stones cut in the form of a flat arch. The stones are cut such that the joints between the ends of stones radiate from a common centre, so that the centre, or key stone, is wedge shaped, as illustrated in Figure 5.59. The stones are cut so that the lower face of each stone occupies a third or a fifth of the width of the opening.

To prevent the key stone from sinking due to settlement and so breaking the line of the soffit, it is usual to cut half‐depth joggles in the ends of the key stone to fit to rebates cut in the other stones. The joggles and rebates may be cut to the full thickness of each stone and show on the face of the lintel, or more usually the joggles and rebates are cut on the inner half of the thickness of stones as secret joggles, which do not show on the face, as illustrated in Figure 5.60. The depth of the lintel corresponds to a course height, with the ends of the lintel built in at jambs as end bearing. Stone lintels are used over both ashlar and rubble walling. The use of lintels is limited to comparatively small openings due to the tendency of the stones to sink out of horizontal alignment. For wider openings, some form of arch is used.

Arches

A stone arch consists of stones specially cut, such that the joints between stones radiate from a common centre; the soffit is arched, and the stones bond in with the surrounding walling. The individual stones of the arch are termed ‘voussoirs’, the arched soffit the ‘intrados’ and the upper profile of the arch stones the ‘extrados’ (see Figure 5.59) The voussoirs of the segmental arch are cut with steps that correspond in height with stone courses, to which the stepped extrados is bonded. The stones of an arch are cut so that there are an uneven number of voussoirs with a centre or key stone. The key stone is the last stone to be put in place as a key to the completion and the stability of the arch – hence the term ‘key stone’.

Crossetted arch

The semi‐circular arch, illustrated in Figure 5.61, is formed with stones that are cut to bond into the surrounding walling to form stepped extrados, and also to bond horizontally into the surrounding stones. The stones, voussoirs, are said to be ‘crossetted’, or crossed. The stones radiate from a common centre, with an odd number of stones arranged around the half‐circle, so that there is a central key stone. The extent to which the crossetted top of each stone extends into the surrounding masonry is not necessarily dictated by the stretcher bond of the main walling. Any degree of bond into the main wall may be chosen and the bond of masonry adjusted accordingly. The effect of the crossetted voussoir is to emphasise the arch as structurally separate from the coursed masonry, and it is chosen for that effect. There is some waste involved in cutting stones to achieve this effect.

Wall ties

Early metal ties were prone to corrosion (Figure 5.63), and many (non‐galvanised) steel ties used during 1930s–1950s have had to be replaced. Where ties break or corrode, the external skin of stone or brick will bow and deform as it expands and contracts with thermal and moisture movement. Galvanised steel, stainless steel and plastic cavity ties were introduced to resolve the problem. The ties had clips and deformations at their centre, known as a ‘drip’, designed to encourage any moisture that passed on to the tie to drip off at the deformation. The moisture collected on the drip drops down into the cavity and out through the external leaf of the cavity.

Wall ties must be protected during delivery, storage, handling and use against the inevitable knocks that may perforate the toughest coating to mild steel, and the consequent probability of rust occurring. Wall ties made from stainless steel will not suffer corrosion during the useful life of buildings. Sharp edges of steel wall ties can pose a hazard to bricklayers. It is common practice to build up one leaf of the cavity wall to a reasonable height (one lift) with the wall ties built in. The problem that bricklayers face is that, as they build the second leaf of brickwork, they have to avoid the protruding sharp metal wall ties as they bend down to lay the second leaf, and a number of eye injuries have resulted. Plastic wall ties, which do not suffer from corrosion, are now much more commonly used, because they do not have sharp edges and do not pose the same risk as metal ties.

Standard section wall ties, illustrated in Figure 5.64, are the vertical twist strip, the butterfly and double‐triangle wire ties. As a check to moisture that may pass across the tie, the butterfly type is laid with the twisted wire ends hanging down into the cavity to act as a drip. The double‐triangle tie may have a bend in the middle of its length, and the strip tie has a twist as a barrier to moisture passing across the tie. Of the three standard types, the butterfly is more likely to collect mortar droppings than the others.

The wall tie illustrated in Figure 5.65 is made from corrosion‐resistant stainless steel. The ridge at the centre of the length of the tie is designed for strength and to provide as small a surface as possible for the collection of mortar droppings. The perforations are intended for improving the bond to mortar. The lengths of wall ties vary to accommodate different widths of cavity and the thickness of the leaves of cavity walls. For a 50‐mm cavity with brick leaves, a 191‐ or 200‐mm‐long tie is made. For a 100‐mm cavity with brick leaves, a 220‐mm‐long tie is used. The increased demands for higher levels of insulation have led to the development of cavity walls with 150‐mm cavities; extra‐long wall ties with increased stiffness are now available for such cavities.

Spacing of ties

The spacing of wall ties built across the cavity is usually 900 mm horizontally and 450 mm vertically, or 2.47 ties per square metre, and staggered, as illustrated in Figure 5.66. The spacing is reduced to 300 mm around the sides of windows, door openings and where movement joints are used. In practice, a block with a mortar joint is 225 mm deep; therefore, ties are usually positioned every block course around the openings (Figure 5.67). In Approved Document A to the Building Regulations, the practical guidance for the spacing of ties is given, respectively, as 900 and 450 mm horizontally and vertically for 50–75 mm cavities; 750 and 450 mm horizontally and vertically for cavities 76–100 mm wide; and 300 mm vertically at unbonded jambs of all openings in cavity walls within 150 mm of openings to all widths of cavities.

Lateral support to walls

Walls that provide support for timber floors are given lateral support by 30 × 5 mm galvanised or stainless steel ‘L’ straps fixed to the side of floor joists at not more than 2 m centres for houses up to three storeys and 1.25 m centres for all storeys in all other buildings.

Lateral support from timber floors, where the joists run parallel to the wall, is provided by 30 × 5 mm galvanised iron on stainless steel strap anchors, secured across at least three joists at not more than 2 m centres for houses up to three storeys and 1.25 m for all storeys in all other buildings. The ‘L’ straps are turned down a minimum of 100 mm on the cavity side of the inner leaf of cavity walls and into solid walls. Solid timber strutting is fixed between joists under the straps, as illustrated in Figure 5.68. Solid floors of concrete provide lateral support for walls where the floor bears for a minimum of 90 mm in both solid and cavity walls.

Where joists, floor intersection or rebates are cut into external walls, it is necessary to ensure that that they do not provide a passage for air. To improve the energy efficiency of buildings, the air permeability is controlled and reduced to a minimum. For example, in Passivhaus construction, this is currently restricted to an airtightness of 0.6 air changes per hour at 50 Pa. To achieve airtight buildings, the wall needs to be effectively sealed. It is common to parge masonry walls with a thin coat of mortar or plaster, and to seal all junctions and interfaces.

Cills and thresholds of openings

A cill is the horizontal finish to the wall below the lower edge of a window opening. The function of a cill is to protect the wall below a window. Cills are shaped or formed to slope out and project beyond the external face of the wall, so that water runs off. A cill should project at least 45 mm beyond the face of the wall below, and have a drip on the underside of the projection. The cavity insulation shown in Figure 5.74 is carried up behind the stone cill to avoid a thermal bridge, and a DPC is fixed behind the cill as a barrier to moisture penetration. A variety of materials may be used as a cill, such as natural stone, cast stone, concrete, tile, brick and non‐ferrous metals. The choice of a particular material for a cill depends on cost, on availability and, to a large extent, on appearance. As a barrier to the penetration of rain to the inside face of a cavity wall, it is good practice to continue the cavity up and behind the cills.

The threshold to door openings serves as a finish to protect a wall or concrete floor slab below the door. Thresholds were commonly formed as part of a step up to external doors with the top surface of the threshold sloping out to avoid water penetration to the interior. This is no longer common practice because a level entrance is required to allow ease of ingress and egress.

Steel lintels

Most loadbearing brick or blockwork walls over openings, where the cavity insulation is continued down to the head of the window or to the door frame, are supported by steel section lintels. The advantage of these lintels is that they are comparatively lightweight and easy to handle; they provide adequate support for walling over openings in small buildings; and, once they are bedded in place, the work can proceed. The lintels are formed either from a mild steel strip that is pressed to shape, and galvanised with a zinc coating to inhibit rust, or from stainless steel. The lintels for use in cavity walling are formed with either a splay to act as an integral damp‐proof tray, or as a top hat section over which a damp‐proof tray is dressed. Typical sections are shown in Figures 5.75, 5.76, 5.77 and Photograph 5.8.

For insulation, the splay section and top hat section lintels are filled with EPS. The top hat section steel lintel is built into the jambs of both the inner and outer leaves to provide support for both leaves of the cavity wall, as illustrated in Figures 5.76 and 5.78. The two wings at the bottom of the lintel provide support for the brick outer and block inner leaves over the comparatively narrow openings for windows and doors. Where the cavity is partly filled with insulation, it is usual to dress a flexible DPC from the block inner leaf down to a lower brick course or down to the underside of the brick outer leaf. The purpose of the DPC or tray is to collect any water that might penetrate the outer leaf and direct it to weep holes in the wall.

The splay section lintel is built into the jambs of openings to provide support for the outer and inner leaves of the cavity wall over the openings, as illustrated in Figure 5.77. Where the cavity is filled with insulation, there is no need to build in a DPC or tray. Any water that might penetrate the outer leaf will be directed towards the outside by the splay of the lintel. Unless the window or door frame is built‐in or fixed with its external face close to the outside face of the wall, the edge of the wing of the lintel will be exposed on the soffit of the opening. Fairface brickwork supported by steel lintels may be laid as horizontal course brickwork or as a flat brick on edge or end lintel.

Thermal bridging through lintels

Where there is a continuous mass of material across a cavity, the heat flow through that material is likely to be considerable. Even with pressed steel lintels, which incorporate insulation (Photograph 5.8), there is likely to be a continuous piece of steel that links the internal and external environments. Figure 5.79 illustrates how heat energy finds its way across the cavity. The use of two separate concrete lintels is more effective at reducing the heat flow and thermal (cold) bridging.

Concrete lintels

As an alternative to the use of steel lintels, reinforced concrete lintels may be used to support the separate leaves over openings (Figure 5.80). A range of precast reinforced concrete lintels is available to suit the widths of most standard door and window openings, with adequate allowance for building in the ends of lintels on each side of the openings. For use with fairface brickwork, the lintel depth should match the depth of brick course heights to avoid the cutting of bricks around lintel ends. These comparatively lightweight lintels are bedded on walling as support for outer and inner leaves.

Cavity trays

In positions of severe exposure to wind‐driven rain, the outer leaf of a cavity wall may absorb water to the extent that rainwater penetrates to the cavity side of the outer leaf. It is unlikely, however, that water will enter the cavity unless there are faults in construction. Where the mortar joints in the outer leaf of a cavity wall are inadequately flushed up with mortar, or the bricks in the outer leaf are very porous and where the wall is subject to severe or very severe exposure, there is a possibility that wind‐driven rain may penetrate the outer leaf to the cavity.

It has become common practice to build in some form of DPC or tray of flexible, impermeable material to direct any water out to the external face of walls. A strip of polymer‐based polythene, bitumen felt or sheet lead is used for the purpose. The DPC and tray shown in Figure 5.80 is built in at the top of the inner lintel and dressed down to the underside of the outer lintel over the head of the window. As an alternative, the DPC tray could be built in on top of the second block course and dressed down to the top of the outer lintel, with weep holes in the vertical brick joints.

Partial fill

Partial fill construction requires the use of insulating material in the form of boards that are sufficiently rigid to be secured against the inner leaf of the cavity (Photograph 5.9). A 25‐mm‐wide air space between the outer leaf and the cavity insulation should be adequate to resist the penetration of rain, providing the air space is clear of all mortar droppings and other building debris that might serve as a path for water. In practice, it is difficult to maintain a clear 25‐mm‐wide air gap, because of protrusion of mortar from joints in the outer leaf and the difficulty of keeping so narrow a space clear of mortar droppings. Good practice, therefore, is to use a 50‐mm‐wide air space between the outer leaf and the partial fill insulation.

To meet insulation requirements with a 100‐mm cavity and partial fill, it may be economic to use a lightweight block inner leaf to augment the cavity insulation. The usual practice is to build the inner leaf of the cavity wall first, up to the first horizontal row of wall ties, then to place the insulation boards in position against the inner leaf. As the outer leaf is built, a batten may be suspended in the cavity air space and raised to the level of the first row of wall ties to stop mortar droppings from falling down the cavity. The batten is then withdrawn and cleared of droppings. Insulation‐retaining wall ties are then bedded across the cavity to tie the leaves and keep the insulation in position. This sequence of operations is repeated at each level of wall ties. The suspension of a batten in the air space and its withdrawal and cleaning at each level of ties do considerably slow down the process of brick and block laying, but it is essential that the procedure be followed.

Care should be taken to ensure that mortar does not fall on cavity ties. Where mortar falls on the tie, a thermal bridge and potential path for moisture to enter the building are created (Photograph 5.10).

Insulation‐retaining ties are usually plastic, standard galvanised steel or stainless steel wall ties to which a plastic disc is clipped to retain the edges of the insulation, as illustrated in Figure 5.81. The ties may be set in line one over the other at the edges of boards, so that the retaining clips retain the corners of four insulation boards. The materials used for partial fill insulation should be of boards, slabs or batts that are sufficiently rigid for ease of handling and to be retained in a vertical position against the inner leaf inside the cavity without sagging or losing shape, so that the edges of the boards remain close‐butted throughout the useful life of the building.

Total fill

Insulation that is built in as the walls are raised, to fill the cavity totally, will to an extent be held in position by the wall ties and the two leaves of the cavity wall. To maintain a continuous, vertical layer of insulation inside the cavity, one of the mineral fibre semi‐rigid batts or slabs should be used. Fibreglass and rockwool semi‐rigid batts or slabs in sizes suited to cavity tie spacing are made specifically for this purpose. As the materials are made in widths to suit vertical wall tie spacing, there is no need to push them down into the cavity after the wall is built, as is often the procedure with loose fibre rolls and mats, which can displace freshly laid brick or blockwork.

The most effective way of insulating an existing cavity wall is to fill the cavity with some insulating material that can be blown into the cavity through small holes drilled in the outer leaf of the wall. The injection of the cavity fill is a comparatively simple job. The complication arises in forming sleeves around air vents penetrating the wall and sealing gaps around openings. Glass fibre and granulated rockwool of EPS beads are used for the injection of insulation for existing cavity walls. These materials can also be used for blowing into the cavity of newly built walls.

Insulation materials

The materials used as insulation for the fabric of buildings may be grouped as inorganic and organic insulants.

Inorganic insulants are made from naturally occurring materials that are formed into fibre, powder or cellular structures that have a high void content, such as, for example, glass fibre, mineral fibre (rockwool), cellular glass beads, vermiculite, calcium silicate, magnesia and compressed cork. Inorganic insulants are generally incombustible, do not support the spread of flames, are rot‐ and vermin‐proof and generally have a higher U‐value than organic insulants. The inorganic insulants most used in the fabric of buildings are glass fibre and rockwool – in the form of loose fibres; mats and rolls of felted fibres and semi‐rigid and rigid boards; batts and slabs of compressed fibres; cellular glass beads fused together as rigid boards; compressed cork boards; and vermiculite grains.

Organic insulants are based on hydrocarbon polymers in the form of thermosetting or thermoplastic resins to form structures with a high void content, such as, for example, polystyrene, polyurethane (PUR), isocyanurate and phenolic. Organic insulants generally have a lower U‐value than inorganic insulants, are combustible, support the spread of flame more readily than inorganic insulants and have a comparatively low melting point. Organic insulants most used for the fabric of buildings are EPS in the form of beads or boards, extruded polystyrene (XPS) in the form of boards and PUR, isocyanurate, and phenolic foams in the form of preformed boards or spray coatings.

The materials normally used for cavity insulation are glass fibre, rockwool and EPS, in the form of slabs or boards, in sizes to suit cavity tie spacing. With increased requirements for the insulation of walls, it may well be advantageous to use one of the more expensive organic insulants such as XPS, PIR or PUR because of their lower U‐value, where a 50‐mm clear air space can be maintained without greatly increasing the overall width of the cavity.

With growing attention to building in a greener and more sustainable manner, some new materials are starting to be used. These include products made from natural materials such as hemp and sheep’s wool, and products made from recycled products, such as newspaper.

Sheep’s wool thermal insulation products are made of 100% wool, or a mixture of wool and other fibres. Wool has a thermal conductivity of around 0.04 W/mK, with 100 and 200 mm depths of insulation, giving U‐values of 0.40 and 0.20, respectively, and is better suited to roof insulation (see also Chapter 6) or to the wide cavities afforded by timber‐framed construction. Cellulose thermal insulation products are made from recycled newspapers, typically with a thermal conductivity of around 0.036 W/mK.

For calculating the thickness of insulation required in a wall, the following formula is useful as a reference:

• R = d/λ = thermal resistance (m²K/W)
• λ = thermal conductivity (W/mK)
• d = thickness of material (in metres)
• U = U‐value = thermal transmittance (W/m²K)
• Hence, the thermal transmittance (U‐value) is expressed as U = 1/ΣR.

The thermal resistance of a cavity wall is calculated as follows.

• If a wall requires a U‐value of 0.35 W/m²K, the resistance required can be calculated by transposing the following formula:
• U‐value = 1/ΣR
• Thus, ΣR (required) = 1/U‐value
• Hence, the thermal resistance required = 1/0.35 = 2.86 m²K/W

  Component/element	Thickness (metres), ‘d’	Thermal conductivity (W/mK), ‘λ’	Thermal resistance (m2K/W), ‘R’ = d/λ
  External surface resistance (RSo)	–	–	0.04
  102‐mm brick outer leaf	0.1025	0.84	0.122
  50‐mm cavity	0.050	0.27	0.185
  100‐mm blockwork aircrete	0.100	0.11	0.909
  13‐mm plasterboard	0.013	0.50	0.026
  Internal surface resistance (RSi)	–	–	0.13
  ΣR	–	–	1.421

• Thermal resistance required = 2.86 m²K/W
• Therefore, 2.86 − 1.421 = 1.439 m²K/W
• Transposing the formula, R = d/λ
• Thus, R × λ = d
• 1.439 × 0.03 = 0.043 m, or 0.043 × 1000 = 43 mm

Additional resistance to be provided by insulation = 2.86 − 1.421 = 1.439 m²K/W. Assuming that insulation with a thermal conductivity of 0.03 W/mK is used, the thickness of insulation required will be obtained by rearranging the formula R = ^d/_λ. Thus, R × λ = d, using the simple multiplication 1.439 × 0.03 = 0.043 m, or 0.043 × 1000 = 43 mm. Hence, the required thickness of insulation is 43 mm, or the closest convenient thickness available from a manufacturer, say, 45 or 50 mm of insulation board.

Sealing cavities in party walls

Cavity barriers are used to reduce flanking sound and also to reduce air circulation within the cavity (Figure 5.83). By closing the cavity with barriers, air movement from one part of the cavity to the other is less likely to occur, and because open spaces within the cavity are reduced, the effect of convection currents, which can manifest as a result of differing temperatures, is also minimised. All parts and edges of the cavity must be sealed, including the top and bottom of the cavity.

Cavity barriers are also required to withhold the passage of smoke and fire. Gaps in the cavity barriers will allow fire to pass around them or flash through the voids, as fire follows sources of oxygen. It is important that junctions be designed and detailed to prevent air flow within the cavity, and also that the barriers be fitted carefully and then independently checked.

If there are any imperfections in the building envelope that connect to the cavities, cold air will enter the building and circulate around the structure and its connecting cavities. The ingress of cold air (especially when exposed to high wind pressure) and the escape of the warmer air will lead to thermal bypasses, cause cold draughts and result in cold spots within the building (as the cold air cools the fabric). Where cavities are not required to be ventilated, they should be sealed to prevent thermal and acoustic exchanges and to reduce the potential penetration of smoke and fire.

Effective U‐values of party walls separated by cavities

The heat flow in relatively stable conditions typically ranges from 0.5 to 0.7 W/m²K, if the cavity is not sealed. Unless edge sealing is used in party walls, a default U‐value of 0.5 W/m²K must be used (see Part L of the Building Regulations). With effective edge sealing, the U‐value is assumed to be 0.2 W/m²K. Only a fully filled insulated cavity, effectively sealed at all edges, can be assumed to have a U‐value of 0.0 W/m²K (Figures 5.84, 5.85, 5.86, 5.87, 5.88, 5.89, 5.90, and 5.91).

Advantages of straw bale construction

In addition to the simplicity of design and ease of adaptability, there are a number of performance advantages that a straw bale house has over conventional loadbearing masonry or framed construction.

Straw bale structures are highly fire resistant. The compressed bales contain enough air to provide good insulation values, but, because they are compacted firmly, they do not hold enough air to permit combustion. Combined with render and plaster surface finishes, a high degree of fire resistance is possible. The type of straw and the moisture content of the bales will mainly determine the thermal characteristics of the wall. With infill construction, the frame will also be a determining factor of the overall thermal performance of the wall. Acoustic insulation is considerably better than a conventional wall structure.

Closely packed straw bales, covered with render on the outside face and plaster on the inner face, provide good air leakage control. Coupled with simple geometric design, well‐built and well‐maintained straw bale construction provides a building with very little air leakage. The render and plaster finishes can be left unpainted and are inexpensive to maintain. The construction cost of a straw bale wall should be significantly less than that for a comparative wall with the same thermal and acoustic performance.

Disadvantages of straw bale construction

The main disadvantage of straw bale construction relates to concerns over durability, particularly moisture‐related damage. Straw bale construction is most suited to dry climates (hot and cold). In the UK, the construction of straw bale houses is a relatively recent innovation; thus, it is difficult to state with any certainty what the durability of the structure will be over the longer term. As more straw bale buildings are built in the UK and more research is conducted into their durability, designers and builders should have more information from which to make informed decisions.

Fungal rot represents the greatest threat to the life of a straw bale building; thus, careful detailing is required to prevent the straw bales from becoming wet. Foundations and roof construction are critical in preventing unwanted rain and moisture penetration, and the construction of the wall must be done in such a way as to avoid any possibility of interstitial condensation. Fungi and mites can live in wet straw; therefore, the bales must be bought when they are dry, kept dry until needed and then sealed into the wall construction with plaster and render to eliminate any chance of access for pests. Paint for interior and exterior wall surfaces should be permeable to water vapour, so that moisture does not get trapped inside the wall. Services, especially sealing around outlets, need special attention.

Straw bale walls are considerably thicker than the more traditional construction methods (at least double), and, hence, where land is at a premium, a straw bale house will provide less internal space for the same external dimensions as compared to a traditional construction. This may be of little concern in rural locations. The appearance of straw bale buildings may not always appeal to a more urban environment, although there are a few examples of straw bale construction being used in densely populated urban environments, usually in conjunction with other materials as part of a composite construction.

Selection of straw bales

Automatic straw balers create tight blocks of straw that provide a relatively easy‐to‐manoeuvre building block. Sizes of bales vary depending on the baler; however, typical bale dimensions and weight when dry are:

• Two‐wire bale: 450 × 350 × 900 mm, approximately 25 kg
• Three‐wire bale: 600 × 400 × 1050 mm, approximately 35 kg

Although the smaller‐sized bale is easier to handle, the medium‐sized three‐wire bale provides better structural, thermal and acoustic performance. Larger cubical and round bales are also available, but these require lifting by mechanical means. There have been moves in the US to develop a ‘construction‐grade’ straw bale. Ideally, bales should be twice as long as they are wide to simplify and maintain a running bond in the courses.

Straw bales should be purchased immediately after the harvest, when they are abundant, fresh and dry. Dealing directly with a farmer is the cheapest and perhaps most reliable method of selecting the best‐quality bales, since quality is largely judged on visual appearance and touch. Transportation to the building site will need to be addressed, perhaps independently of the arrangement with the farmer if the bales are to be transported out of the local area. Straw bale merchants provide an alternative source of supply and will have an established transport infrastructure and storage facilities. Once again, the selection of the bales, especially if sourced from more than one farmer, is an important consideration.

Bales should be tied tightly with polypropylene string or bailing wire and should not twist or sag when lifted. All bales should be uniformly compacted and contain thick, long‐stemmed straw; any bales comprising short, thin straw should be rejected. Old and/or damaged bales should be rejected. If practical, the construction of a straw bale building should be undertaken immediately after the main grain harvest, when the bales are fresh and dry, with bales transported to the site, positioned and protected from the weather as quickly as possible. If this is not an option, then the bales should be selected and stored under dry, ventilated, conditions. Bales should be tested for moisture content, which must be 14% or below when purchased and when used for building. All bales must be stored under dry conditions until required for building purposes. Protection of the bales from the weather is also necessary during construction.

Foundation construction and site drainage

The base of a straw bale construction must be kept dry at all times; thus, the manner in which the straw bale interacts with the foundation will be a crucial factor in determining the durability of the wall. The position of the lowest straw bale in relation to the finished ground level and the detailing of the external wall finish at this junction are particularly important. Similarly, site drainage must be designed to get water away from the base of the walls as quickly as possible (Figure 5.92).

Guidance on the minimum distance between the finished ground level and the lowest bale position varies from around 225 mm upwards to 300–450 mm. Knowledge of local rainfall patterns, the extent of the roof overhang in relation to the height of the wall and the ground finish adjacent to the wall will be determining factors. As a general guide, the bales should be positioned at least 450 mm off the finished ground level to avoid splashback of rain, bouncing off the ground and on to the face of the wall. The material under the straw bales should be good‐quality stone, engineering brick or dense blockwork. The DPC should be positioned between the solid foundation and the timber base or sole plate. The manner in which the external render is finished at the junction of foundation and wall is also important. A drip should be formed at the bottom of the render to help throw any water that runs down the wall off and away from the base of the wall.

Care should be taken to not compromise the site drainage through inappropriate surface finishes. The ground around the structure must be well drained, and kept dry with adequate surface and below‐ground drainage. The position of the water table and the history of the proposed site with regard to flooding should be investigated, and appropriate decisions taken to protect the construction from damp and/or wet conditions. Self‐draining foundations are one method of helping to keep the base of the wall free of water. Vermiculite is placed above the cavity tray. Below the cavity tray, the cavity should be filled with drainage gravel, and weep holes provided at regular intervals (Figure 5.93).

Roof construction

The design and construction of the roof is another critical area. The roof should be constructed with a large overhang to provide protection to the wall below from rain and snow. Leaks in the roof and/or inappropriate detailing at the wall‐to‐roof junction will compromise the long‐term durability of the structure. A large roof overhang on single‐storey buildings will also provide good protection to the base of the wall. Roof pitch is another consideration, with steep pitches and a good‐quality roof covering to facilitate the rapid discharge of rainwater from the roof considered important for the durability of the roof. Timber roof construction is the usual method, with the roof loads transferred to the foundations via the loadbearing straw bale wall or via the timber frame construction (described further in more detail). Roof coverings may be of reed thatch, tile or lightweight finishes, with lightweight roof structures and coverings preferred for loadbearing straw bale walls.

Wall construction

The main principle of straw bale design is based on the use of full bales as a building module – simplicity is key. Half‐bales will be required for bonding purposes, the same principle as that for brick and block wall construction. Bales smaller than a half‐size should not be used, because of their lack of compressive strength. Careful planning is required before any drawings are finalised, simply because the actual size of bales varies between suppliers. Once a bale supplier (and, hence, dimension) has been determined, it is possible to design and detail the building plan and also the opening sizes (doors and windows) and their positions within the wall. The main principles relating to the strength and stability of the wall are similar to those for loadbearing or framed construction. Where possible, doors and windows should be selected to suit the bale module size, or be manufactured/built to suit the bale module. When using loadbearing straw bale construction, there will be some settlement of the wall after the roof has been completed. The straw bales must be sealed against the weather and pests. External surfaces are finished in lime render (stucco). Internal surfaces are plastered. The render and plaster provide an attractive surface finish, which also adds to the structural integrity of the wall.

Straw and clay building

A traditional and durable construction method is to use a ‘batter’ of clay and water stirred into loose straw to produce a straw‐reinforced clay mud. The mixture is packed tightly into a lightweight timber ladder framework to create partition walls, or infill panels for framed wall construction.

Pressed straw panels

Pressed straw panels are made by compressing straw under temperature to produce a panel made of 100% straw. The combination of compressed straw, recycled paper lining and adhesive will form a board that generally complies with BS 4046. The result is a low‐cost, versatile product with environmental benefits. The panels can be used as a self‐supporting, non‐loadbearing partition system, and also for roof decking. The technique was first introduced to the UK and developed into a sophisticated straw board product by Stramit, in Suffolk, UK. Pressed straw panels may also be used in roofs and floors. Research and development is currently being conducted by several companies and research organisations into the use of pressed straw panels for use in a structural capacity.

Advantages of earth‐sheltered structures

An earth‐sheltered house provides several benefits over more traditional structures. The biggest advantage is that the building is protected from the weather and hence is not subject to deterioration by, for example, solar radiation or wind damage. In very exposed locations, protection from adverse weather may be a considerable benefit. Protecting the building with earth will reduce or even eliminate the need for regular painting, cleaning of gutters and repair. Thus, maintenance is considerably less onerous for the majority of earth‐sheltered designs.

The earth provides natural insulating properties to the structure, providing a relatively stable indoor temperature that is less affected by external air temperature fluctuations. Thus, less energy is required to maintain a stable internal temperature. The earth also provides considerable sound dampening, in addition to that provided by the structure of the building.

Earth‐sheltered structures will blend into the landscape and may be an ideal solution in areas where the visual appearance of a structure would cause a problem. This does not mean, however, that approval from the local town planning department will be forthcoming, since other factors, such as land use and access, will also need to be considered.

Disadvantages of earth‐sheltered structures

There are a number of potential disadvantages associated with earth‐sheltered structures. Exclusion of moisture is perhaps the most obvious. Waterproofing and tanking is a primary concern, and often carries a relatively high initial cost. Surface water and underground drainage is key. Seasonal or regular surface water flows must be channelled away from the structure. Underground drainage should be designed to remove water (and water pressure on the walls) quickly. Regular maintenance of drainage channels to avoid blockages is also essential if problems are to be avoided.

The initial cost of an earth‐sheltered structure can be as much as 20% higher than a comparable structure built above the ground. Exact figures will depend on the nature of the ground and topography of the site (e.g. gently or steeply sloping). However, there may be considerable cost savings made during the life of the building, which must be considered in a whole life costing approach.

Earth‐sheltered houses tend to be characterised by high levels of humidity; therefore, air exchange is important. Some designs may also result in pockets of stale air, unless some form of air flow is introduced. Ensuring adequate air flow can be achieved through the use of passive or mechanical ventilation systems. Passive ventilation can be provided through vented skylights, which are required to get natural light into the internal spaces. Linking the air exchange system to an earth‐to‐air heat exchanger may help to keep energy costs to a minimum.

Repairs and remedial work to the structure of the building may be expensive, since it is difficult to get to the walls without removing a great deal of earth. This places additional emphasis on the quality of the detailing and the quality of the work undertaken on site. Careful and systematic inspection of the waterproofing work as it proceeds is essential.

Thermal insulation

Insulation is usually placed on the exterior face of the walls, after the waterproofing system has been completed. In this way, the heat generated, captured and absorbed within the earth‐sheltered envelope is retained by the building’s interior. Rigid insulation sheets help to protect the waterproofing against punctures from sharp stones in the soil. Thin protective boards are often used as a barrier between the insulation and the earth.

Adobe

Adobe is compressed earth, rammed into moulds or pressed into blocks while damp, and sun‐dried to form a building block or brick. Adobe is a common construction method in many parts of the world (mainly places with a dry climate). The best adobes are made from soil that is high in clay content. Mechanical presses can be used to form adobe blocks directly from the building site’s soil. These blocks are then used to build loadbearing (or infill) walls, laid without mortar. Blocks are laid in a walling bond, with the connecting surfaces wetted with water to help provide a bond between the individual units when dry. The walls are then rendered (stuccoed). Compaction of the earth is critical, and over‐compaction can lead to fracture of the material. Scientific analysis of the soil is therefore crucial.

Adobe and rammed earth walls absorb solar heat during the day and radiate heat back into the air at night, thus providing a comfortable and relatively constant internal temperature. Hence, adobe tends to be used in dry and hot locations. Both exterior and internal adobe walls provide good thermal mass and can form part of a passive design. The principles of weather protection and durability of earth are similar to that of straw – namely, protection from moisture. In damp climates and areas of heavy rainfall, it might be prudent to use adobe only for internal walls.

Rammed earth

‘Rammed earth’ is a term used to describe the mixing of cement and earth, which is then packed into wall forms with a pneumatic tamper – hence the term ‘rammed’ earth. The result is a material that resembles a sedimentary rock, with compressive strengths about half that of concrete. The strength of the material will vary, depending on the ratio of cement to soil (and soil type); thus, the testing of batches is necessary to determine the loadbearing capacity of the resulting wall. Since it uses soil from the building site, rammed earth can prove to be a relatively cheap material, with reasonable environmental credentials. The use of the material is similar to that described for concrete walls. By increasing the thickness of the walls, the loadbearing capacity may be increased.

Hempcrete

Hemp is a low‐impact, sustainable plant material that is easy to grow, reaching heights of up to 4 m. The fibres are separated from the woody core, shredded and then graded for use in construction. The fibres are then mixed with quicklime (calcium oxide) to provide a homogeneous material that can be used as insulation and non‐structural walling material. When used within a timber frame, hempcrete has good insulation properties, and, with the application of an air barrier, it can achieve the same standard of airtightness as other building materials.

Hempcrete can be sprayed onto a background to provide an insulation layer. Alternatively, it can be prepacked into prefabricated panels that can be delivered to site and erected in the same way as other off‐site‐engineered panels, or the panels can be placed in situ between the timber frames. Formwork is used to provide the temporary mould for the hempcrete, which can be removed after 24 hours. The drying times of hemp vary, and the typical conductivity of hempcrete is 0.07–0.09 W/mK. For a 500‐mm‐deep wall, in a timber frame, it is possible to achieve a U‐value of 0.21 W/m²K.