Choosing Your Foundation
In This Chapter
- Understanding the pros and cons of each foundation type
- Knowing the types of trailers appropriate for your Tiny House On Wheels (THOWs)
- Determining when to build a tiny house on a permanent foundation
- Learning the best permanent foundation options for your build
Foundation options are not typically considered a tantalizing topic of conversation. Gather a few tiny housers at a cocktail party, though, and the lively exchange on trailers, skids, and fixed foundation systems can easily continue into the wee hours of the morning. Determining the best foundation option for your build will take some time and research.
In this chapter, we cover the pros and cons of the various tiny house–appropriate foundations. We look at trailer options and how much each can tow. Lastly, we go over fixed foundation options best suited for tiny houses on foundations.
When designing a tiny house, the type of foundation you choose will be one of the most important decisions you’ll make. It will have resounding impacts on nearly every aspect of your build, from design all the way to final construction details. There are advantages and disadvantages to each scenario, so let’s go over each option.
Pros of Building on a Trailer
Even though people have lived in small houses for eons, the tiny house movement itself began with a simple concept: build an affordable house that’s easy to move. It’s this vision that inspired early pioneers to build their homes atop a trailer. Today, THOWs have garnered the imagination of thousands of others and subsequently, trailer options have increased dramatically. Following are the pros of building a tiny house on a trailer.
Location flexibility. Today’s work environment is very different than it was in the past. The opportunities for finding secure, long-term positions in companies are far fewer than they once were. Long-term employment has been replaced by high-turnover jobs forcing many to move more often than they’d like. As such, buying a home no longer makes sense for many people. Tiny houses on wheels are excellent home solutions in these cases; simply hook up and tow them to a new destination. There’s no need for new furniture, high deposits, or the pandemonium of fighting for a decent rental.
Ease of tax burden. Tiny houses on wheels are not considered permanent real estate because they’re not connected to the ground. As such, they’re not currently assessed for property taxes. THOWs are often considered RVs and only assessed sales tax at time of purchase or construction. Some states do charge annual motor vehicle fees or taxes, but these fees are relatively low.
No building codes. Because these tiny houses are relatively new in the construction industry, the government has yet to create residential building codes to oversee the building process. This lack of permits and inspections simplifies a build. A simple trailer registration with your local department of motor vehicles may be all that’s needed to make your Tiny House On Wheels (THOWs) road legal.
Cons of Building on a Trailer
There are also some drawbacks to building a Tiny House On Wheels (THOWs) on a trailer. These drawbacks might not be significant enough to deter you from choosing this option, but only you can determine that. Let’s take a look at the most common challenges associated with tiny houses on trailers.
No building codes. The lack of inspections and codes for tiny houses on wheels is a mixed blessing. Many who build their own tiny houses have limited construction experience, so technical building errors are very common. Even though you’ll save money by avoiding permit fees and save time by skipping inspections, you may very well find yourself ultimately paying more down the road. This is because mistakes that would have been caught by building inspectors may go unnoticed until a problem emerges.
Greater risk in high-wind zones. Tornadoes and other high-wind events often wreak havoc in mobile home and RV parks. Damage is usually higher there because these structures are extremely vulnerable to high winds because they aren’t attached to the ground like permanent housing.
TINY WARNING
Any residence not permanently anchored to the ground is at higher risk when faced with natural disasters such as tornadoes, hurricanes, and the like. However, with enough advanced warning, a Tiny House On Wheels (THOWs) might be able to be evacuated to a safe area until the threat passes.
Greater vulnerability to earthquakes. A house improperly anchored to the ground has very little chance of faring well in an earthquake, which poses a serious threat to its inhabitants. Though there are ways to properly anchor a THOWs, owners that frequently move their homes don’t generally apply those systems because they can be tedious to install.
Increased potential of freezing water lines. Water service and wastewater lines must be protected from freezing temperatures in a THOWs, which can prove to be a challenge. Insulating plumbing within walls is generally not the issue. Rather, the risk comes from the locations in which utilities are exposed to the elements (for example, waste lines and water service lines leading into a THOWs).
Design limitations. Both state and national highway transportation departments impose safety limits, which create significant design limitations for THOWs:
- Vehicular width (typically 8'6")
- Vehicular height (typically 13'6")
- Vehicular weight (varies with road)
Anything beyond those limits requires special permits and extra fees.
Need for a large towing vehicle. If you intend on towing your THOWs more than a few times, it probably makes sense to purchase a truck and tow it yourself. This might mean a financial investment of $30,000 or more for a rig that can adequately do the job. Considering that many owner-built tiny homes cost $20,000-$30,000 total, it could be a stretch to also spend more for a capable towing rig. Furthermore, this truck typically becomes the only household vehicle, and it consumes significantly more fuel during day-to-day driving than a smaller car would.
Pros of Building on a Foundation
As more people join the tiny house movement, many have realized that being mobile is not their priority. Rather, their chief priority is living simply and within their means, and they’re perfectly happy to stay in one place. For these folks, a Tiny House On Foundation (THOFs) is the best approach. There are numerous foundation options available: concrete slabs, stem walls, pad and pier, and more. Following are the pros of staying put.
Construction code compliance. The biggest advantage of building tiny houses on foundations is that they’re considered legal homes in the current International Residential Code (IRC)—the model code for the majority of residential construction in the United States. In fact, you can build a code-approved house smaller than 100 square feet and still receive a certificate of occupancy (COO), assuming that local zoning permits it. This is a huge advantage when you consider that there aren’t any national residential construction standards allowing for a tiny house to be built on a trailer.
Ease of design process. Designing a THOFs is relatively easy compared to a THOWs because there aren’t as many restrictions in terms of maximum width and height. Details such as stairs, head heights, emergency egress, and so on become significantly easier to mitigate without the highway standard restrictions.
Structural integrity. A fixed foundation creates a permanent point of attachment to the ground, enabling a THOFs to resist high winds and seismic activity much better than a THOWs. It also provides much better protection for incoming and exiting water service lines from freezing temperatures.
TINY TIP
You can save a lot of time and money on any construction project when you opt to use conventional materials and techniques. Any time you do something outside the norm, your challenges (and sometimes costs) will increase.
More builder/contractor options. Choosing a Tiny House On Foundation (THOFs) also means more options when it comes to builders. Although any experienced contractor can build a Tiny House On Wheels (THOWs), many are uncomfortable taking on projects with which they don’t have experience. Constructing a THOFs, on the other hand, is a simple process for any qualified builder.
Easier resale. The resale value of a Tiny House On Foundation (THOFs) is typically higher than that of a Tiny House On Wheels (THOWs) because a THOFs is considered real estate, having been issued an official certificate of occupancy, while a THOWs is typically considered a recreational vehicle. Generally speaking, real estate grows in value over time, while RVs depreciate.
Cons of Building on a Foundation
There are a few challenges associated with foundation-built tiny homes that you need to consider as well.
Planning and zoning oversight. Because THOWs aren’t currently considered permanent housing, it’s become fairly common for people to live in them under the radar of local building officials. If a THOWs’ owner is turned in to local authorities, they generally just pick up and move. With a THOFs though, that’s clearly not an option. All planning and zoning oversight must be addressed from the very beginning.
Incurring taxes. Any residence with a COO is subject to taxes, no matter how small it is. How much you’ll pay depends on where you live. Generally, it will be a small fraction of taxes incurred by living in a large house.
Lack of mobility. If flexibility and the option of taking your home with you is top priority, building a Tiny House On Foundation (THOFs) is not your ideal approach. Though it’s possible to move a house on a foundation, this option is not simple or inexpensive.
Pros of Building on Skids
Skids are large wooden beams structurally attached to not only the floor joists in a house, but also a concrete pad-and-pier foundation with steel connectors. Skids can be detached from a foundation, if necessary, enabling a house to be moved to a new location. Using skids for a foundation is somewhat of a hybrid of a trailer and permanent foundation. It’s this best-of-both-worlds approach that makes skids so appealing. The top three advantages are outlined below.
Code compliance. Once you’ve received a COO for your Tiny House On Skids (THOSs), you just apply for a new foundation permit and a new COO each time you move. Because building codes vary regionally due to climactic and natural disaster differences, you’ll need to meet the provisions of the codes in the area to which you’re moving.
Flexibility. Although skids are considered permanently attached to a foundation, they can be removed with relative ease. Once freed from the concrete pad and piers, your home can be moved to a new spot close by or placed on a trailer and moved across the country if necessary. This provides you with great location flexibility coupled with the perk of a COO.
Design options. It’s common for professionals, not individual owners, to move homes built on skids. Those haulers can easily acquire any special permits and have the appropriate rigs to move larger houses, saving you the stress of needing to tow your tiny house.
Cons of Building on Skids
When considering the cons of building on skids, be sure to compare them with foundation-bound and trailer-built tiny houses. Because skids are considered a hybrid between those two, they’ll have some of the cons I’ve mentioned previously.
Design limitations. Even though a THOSs provides more design options than a THOWs, fewer options exist than those found on THOFs. Road restrictions, which place limitations on the width and height of a home, must be considered.
TINY WARNING
A Tiny House On Skids will require hiring a professional hauler because it must be pulled onto a trailer bed with heavy duty equipment. This increases your costs significantly, so if you plan on moving your tiny house more than once or twice, this house type may not be your best option.
Foundation skirting. A Tiny House On Skids (THOSs) is built on a raised foundation system, which requires skirting to enclose the space underneath. Although several options exist, none are as good as a permanent foundation wall. If you’re not careful when choosing skirting, your house may look temporary and low quality.
Limited foundation options. The most common foundation type for a home built on skids is a pad-and-pier foundation. This system type provides relatively easy access to any plumbing lines below your house and meets building code requirements. If, however, your location site’s soils or topography aren’t conducive to a pad-and-pier foundation, an expensive engineered alternative may be your only option available.
What Are My Trailer Options?
Let’s assume you’ve considered all the foundation options and have decided to build your Tiny House On Wheels (THOWs). Next on your agenda is deciding what type of trailer you should build on.
Although there are many options for stock and used trailers, we highly recommend a custom-designed one specifically for your project. Everything you build on top of it will be influenced by your trailer’s strength, stability, and overall quality.
The most common type of trailer hitch set up is a bumper pull, which allows a trailer to be towed from a receiver hitch at bumper level. There are five classes of receiver hitches, and they all have different ratings for what they can pull.
- Class 1 and 2 hitches can only tow up to 3,500 pounds, so they’re too small for towing a tiny house that can easily weigh 10,000 pounds and up.
- Class 3, 4, and 5 hitches can haul a lot more weight and are compatible with weight distribution systems. A weight distribution system is a receiver hitch attachment designed to distribute tongue weight across all axles including the tow vehicle and the trailer.
At no time should the tongue weight exceed 15 percent of the gross trailer weight. It’s better to keep things closer to the 10 percent range. If you overload the tongue, you might encounter trailer sway or worse—the hitch may rip off the tow vehicle.
A Class 3 hitch has a Gross Trailer Weight Rating (GTWR) of 8,000 pounds. However, if you’re using a weight distribution system, that limit can often be bumped up to 12,000 pounds. The receiver tube for the hitch is typically 2" × 2" (larger than what’s found on a smaller vehicle set up). Class 4 hitches also utilize 2" × 2" receiver tubes and can be used in tandem with weight distribution systems. This bumps their towing capacity up from 10,000 pounds to 14,000 pounds.
DEFINITION
Tongue weight is the load placed at the point of connection between the vehicle and the trailer, typically located at the hitch. Also referred to as hitch weight, this measurement is taken when the trailer is loaded to capacity and ready for towing.
Class 5 hitches are a whole different beast. They’re the biggest bumper pull hitches available and have a towing capacity of up to 20,000 pounds GTWR. Because their towing capacity is very high, the use of weight distribution systems does little to increase that limit. We do still recommend these systems, though, because they help level the trailer and hauler, ensuring a safer and smoother ride. A Class 5 hitch uses either a 2" × 2" receiver tube or a 2 ½" × 2 ½" receiver tube. These hitches aren’t commonly used for passenger vehicles, but if your truck can handle the tow capacity, you can certainly install one.
Ever hear of a pintle hitch? Pintle hitches are most commonly used in military applications and not as much in civilian day-to-day life. Pintle hitches extend from the bumper region of your vehicle but not from the bumper itself. They’re secured to the tow vehicle’s frame and are very easy to hook up to. If your tow vehicle can handle it, a pintle hitch can haul anywhere from 10,000 to 60,000 pounds.
A potential downside of using a pintle is that there can be some movement in the coupling between the hitch and the trailer, causing some rattle and noise during towing. That said, if you need to haul a massive amount of weight, they can’t be beat.
To be clear, Class 1–5 and pintle hitches aren’t the same as bumper hitches. A bumper hitch attaches directly to your vehicle’s bumper, limiting your vehicle’s tow capacity to nothing much more than a bicycle rack. It’s important to ensure your hitch matches the tow capacity of your vehicle.
DEFINITION
Gross Trailer Weight Rating (GTWR) is the total weight of a trailer when loaded to capacity. The GTWR accounts for the weight of the trailer plus any loads including fluids and cargo.
To break bumper trailers down further, there are three main styles available:
Car haulers. A car hauler is probably what most people think of when they picture smaller trailers. The wheel wells are exposed above the deck height and the trailer deck is confined between the two wheel wells. The main advantage here is that the deck is relatively low to the ground, translating to more head room in a tiny house. The main disadvantage of car hauler trailers is that their decks are narrow, preventing one from taking advantage of maximum road width limits. A narrow house can feel uncomfortable and make it more challenging to accommodate life’s necessities since there’s less square footage.
Deck overs. A deck over is a trailer where the deck is located over the top of the wheel wells, which provides room for a wider trailer than car hauler styles. However, because the deck is so high off the ground, it limits the available interior height. As such, most deck-over trailers are used for single story tiny homes.
Custom bumper-pull trailers. In our opinion, the best option for a movable tiny house is a custom trailer. With a custom setup, you can incorporate the best qualities of a car hauler and deck-over trailer. Your trailer manufacturer can build the deck as low as possible and extend the edges beyond the wheel wells, maximizing your head height as well as trailer width. The only downside of this setup is that the wheel wells will protrude into your tiny house structure. However, with creative design, you should be able to incorporate the wheel wells so they can’t even be seen.
This illustration shows the difference between car haulers, deck overs, and custom tiny house trailers. The custom option maximizes the deck height and width.
Goosenecks and Fifth-Wheel Trailers
Gooseneck and fifth-wheel trailers are very similar in design. Both connect via a hitch in the tow vehicle’s bed, slightly in front of the tow vehicle’s rear axle. These hitch systems are mounted to the tow vehicle’s frame with an installation kit enabling for very high tow capacities. It’s common for fifth-wheel trailer hitches to accommodate a GTWR of 25,000 pounds. A gooseneck hitch can typically handle a GTWR of 30,000 pounds.
The biggest differences between a gooseneck and fifth-wheel trailer are the hitch and ball configurations as well as how the connection is made. In a gooseneck, the ball is located on the tow vehicle and the hitch-receiving connector is on the trailer. In fifth-wheel trailers, the opposite is true. Because the point of attachment is located at the tow vehicle’s rear axle, the tongue weight of gooseneck and fifth-wheel trailers is well distributed across the tow vehicle. This creates a smoother towing experience and provides a larger turn radius than a bumper pull setup.
We recommend that any THOWs above 28' be upgraded from a bumper pull to a gooseneck or fifth-wheel trailer. The area located over a gooseneck assembly is often used as a bedroom or other habitable space. The head height in a room over the gooseneck is often quite generous and enough for an adult to stand in.
Permanent Foundations for Wheeled Structures
In order for a movable tiny house to be considered a permanent residence by the IRC, it must be permanently anchored to the ground. Exactly how you meet that requirement is up to you and your building department. There are several permanent foundation options available within the current IRC that a THOWs or THOFs could be built atop of.
In some regions where the 2015 IRC’s section R402.1 is acceptable, you can install a permanent wood foundation for your Tiny House On Wheels (THOWs). This system is relatively inexpensive and can be packed up and moved to a new location if need be. All that’s required is plastic, gravel, pressure-treated lumber, and fasteners.
You can pour a concrete slab foundation with embedded anchors and attach your tiny house to it. This system would meet code requirements by resisting seismic and wind loads and is worth pursuing even though it’s not specifically addressed in the IRC.
TINY TIP
The intent of the IRC is to keep the public safe. Keeping this in mind when designing your project is important because if you can show your design meets it, you’ll have a better chance of winning COO approval. We highly recommend you hire a structural engineer to review your architectural plans. Their stamp of approval lends significant clout within building official circles.
Ground-Bound Options
Perhaps you’ve decided not to build on a trailer but rather to build directly on a permanent foundation. There are various factors that help determine what options are available such as soils, slope, site drainage, elevation above sea level, thermal mass potential, climate conditions, and budget.
Slab-on-Grade Foundations
Slab-on-grade foundations are the most common foundation systems in the United States. They’re simple to build and deliver not only a foundation, but also a finished floor. When oriented to maximize solar gain, they also provide excellent thermal mass. They’re easy to maintain and clean and have very long-life expectancies.
On the downside, they’re very hard and uncomfortable under foot. If you’ve ever looked at cashiers working on concrete floors, you might have noticed that they stand on thick rubber matts to minimize discomfort.
Pouring a concrete slab foundation is within the skill range of a well-researched DIYer. It’s also quite easy to find a local concrete foundation contractor in nearly every community. The cost of a concrete slab foundation is relatively low compared to other options.
Perimeter Stem Wall Foundations
Perimeter stem wall foundations, sometimes referred to as perimeter foundations, are quite common. They can be created with poured concrete or a combination of poured concrete and masonry block. Perimeter foundations require either a crawlspace beneath the floor or a basement. In areas in which deep excavation is required to get concrete footings below frost line, perimeter foundation walls make the most sense.
TINY WARNING
While there are perks to building with stem walls, there are also downsides. They are typically more expensive and take longer to assemble. They also require more excavation and backfill as well as more material to assemble.
Pad-and-Pier Foundations
A pad-and-pier foundation is a simple system composed of two connected elements. The first is a concrete pad poured below frost line to support the loads placed upon it. The second is the pier, which is connected directly to the pad. In some cases, the piers consist of tall columns of concrete connected to the pad via rebar. In other cases, the piers are simply steel connectors embedded in the pads.
The adjustable length of the piers makes this system ideal for uneven ground. You’re able to create level floor systems through the pier length adjustments with relative ease. Much is adjusted in the formwork, so you can check and double check your work before any concrete is brought onsite. Keep in mind that once cured, concrete is very difficult to adjust, so it’s very important your forms are accurate.
Skids—The Secret Weapon
Skids take pad-and-pier foundations to the next level. A home built on skids can be placed on a pad-and-pier foundation for permanent attachment and then removed at a later date for relocation. This is possible because of the connection between the two. The assembly between the skids and the foundation is typically made with steel connectors, which are nailed or bolted onto the skid beams. Removing those connectors is all it takes to free the skids from the foundation, enabling you access to load the home on a trailer.
This construction sketch illustrates how a skid can be attached to a post-and-pier foundation system. The “T” strap can easily be disconnected from the framing post, freeing the skid itself if/when you want to move your THOSs.
Engineered Alternatives
There are other options available for foundations other than those we referenced previously. These include foundations of permanent wood, poles, rubble trench, urbanite, stabilized earth, and more. Most of these foundations have been used for thousands of years, yet are not specifically called out in our codes.
If you want one of these systems, you’ll need to hire an engineer to stamp your drawings. An engineer’s stamp will greatly increase the potential of your project being approved. Sometimes building outside of code can be a great approach if the end result is safe and aligns with your overall priorities.
- There are numerous pros and cons associated with the various tiny house foundation options.
- Several hitch classes exist for bumper pull trailers so ensure you get the right one for towing your tiny house.
- Goosenecks and fifth-wheel trailers are ideal for any THOWs over 28' in length.
- Various foundation options exist for THOFs, ranging from concrete slabs to stem walls to pad and piers to skids.