Your tiny house subfloor is more than just an underlayer for your flooring. This subfloor protects your home from debris, moisture, and pests when you’re on the road. It’s a barrier separating your home from the outside world and keeps your tiny house safe and secure.
How do you build a tiny house subfloor on top of a trailer? There are two ways to do this: 1) building the subfloor directly onto the trailer; 2) setting a frame and building on top of the subfloor. Subfloor construction on a roving tiny home involves the other steps necessary to build a subfloor on a foundation.
Not sure how to move forward? Look no further! In this article, we discuss everything you need to know about building a tiny house on a subfloor trailer, including the different layers of the subfloor, as well as instructions on how to install on yourself.
Purpose of Subfloor: Is It Necessary?
One of the best things about tiny house construction is flexibility. For instance, if you want to save on your total weight, you can skip traditional framing techniques and use advanced framing. You’ll end up using less wood without compromising the integrity of your tiny house.
But subfloor framing isn’t something you can really skip. Sure, there are ways to reduce the time and cost of building your subfloor, but you’re going to end up with one at the end of the day.
A subfloor does more than stabilize your flooring. A well-constructed subfloor offers the following benefits:
- Maximum floor insulation: More metal means more thermal bridging. With this much metal in contact with your tiny home, you want to make sure you have carefully laid down the insulation in order to protect your floors from heat loss in the winter.
- Effective moisture management: The subfloor can accumulate moisture with mere exposure to the outside world. The inside of the subfloor can rust, degrade, and break down when left unprotected. Installing a vapor barrier and a flashing are two precautions tiny house owners make in order to protect their trailer and their tiny house.
- Barrier between pests and insects: Bugs and insects can easily invade your tiny house through the trailer. Without a reliable flashing material, bugs and even small animals might chew its way up your tiny house while you’re out and about.
All in all, the subfloor is a layer of protection between your home and the outside world. Think of it as wall framing for your tiny house; you wouldn’t build a house without framing so why build a tiny house without a subfloor?
Two Ways to Build a Subfloor
| Trailer as Subfloor | On Top Of Trailer | |
| Plumbing | Plumbing is closer to the ground, which could damage your system during travel | Building on top means more space between your plumbing and the road |
| Time and cost | Faster to do especially if you order a trailer with the flashing built-in. Saves money on subfloor framing by using the trailer | Additional time and cost needed to lay out the wooden frame and sheathing layer |
| Limitations | Tiny house width is limited to the trailer width | The additional framing makes the house heavier, which could make it harder to tow |
| Thermal bridging | More thermal bridging because you’re using the metal as the frame instead of wood | Less thermal bridging and better insulative capabilities |
| Can be DIY? | Yes | Yes |
There are two ways to build a tiny house subfloor on a trailer. One way is to build directly onto the trailer, and the other way is to build a separate framing on top of the trailer.
Trailer as Subfloor
Not all trailer designs allow you to build the subfloor directly on the trailer. Only trailers with evenly spaced crossed members are worth the time and effort of building into directly.
By going this route, you can save about 3 to 6 inches of vertical space depending on materials used. This is the main reason why people will choose to build directly onto the trailer. More head room can really make a difference in a tiny house.
Building On Top of Trailer
Building a subfloor on top of the trailer is the more common route most builders take. It’s a great way to maximize your trailer’s width, since you can build a couple of inches outside the trailer framing to extend the width of your tiny house.
With this design, you don’t have to order a specialized trailer since traditional trailers come with this kind of design.
Parts of a Subfloor: What to Install
Want to try your hand at constructing your tiny house subfloor from scratch? Listed below are the main parts of subfloor framing, from bottom to top:
Flashing
Common materials used: Aluminum or galvanized steel
The flashing is the subfloor’s first layer of defense against moisture, pests, and debris. It’s typically made out of galvanized steel, or aluminum if you don’t mind spending a bit more.
For those who want an even cheaper alternative, a specialized plywood option called marine plywood can also be used. Even though the material is stronger and more water-resistant than normal plywood options, we still recommend using either aluminum or galvanized steel for heavy-duty protection.
You can order a tiny house trailer with the flashing already attached. Attaching the flashing yourself takes 4 days to weeks depending on how big your trailer is.
Can you use more than one type of flashing material for your tiny house?
Yes you can! If your space allows it, then you can definitely use two layers of flashing like what Chris at Oneida Trail did for his tiny house on wheels. He laid down some exterior plywood on top of metal flashing to get double protection.
Most tiny house owners use just one layer of flashing. It’s completely optional to install two layers of subfloor flashing.
Framing
Common materials used: Wood
Framing your tiny house isn’t that different from installing wall framing. Those who are building on top of their trailer like Tiny Zoo Travels can extend the width of their tiny house by 5 to 10 inches or a little bit more depending on your design.
This involves measuring the desired width for your trailer and cutting individual pieces to set up the frame. The frame is then bolted down on the sides of the trailer to ensure the tiny house stays glued on top of the trailer. The cross beams you create with the wood, attached with heavy duty wood glue, is where your chosen insulation material will come in.
Insulation
Common materials used: Sill plate gasket or plywood for the trailer sides (optional) and spray foam or rigid foam board for the frame cavities
Insulating your tiny house subfloor is one of the most important things about this build. The metal framing of the trailer is incredibly conductive. You can expect a lot of thermal bridging between the metal and the outside world, which could mitigate your efforts in properly insulating your tiny house walls if you don’t bother to install your subfloor properly.
Applying spray foam is one of the quickest ways to insulate the cavities of your trailer. Getting a professional to have this done can cost around $200 to $400 depending on the size of the trailer and the company you’re working with.
A great DIY option is using rigid foam board to fill in the cavities. You’ll have to measure the width, depth, and height of the foam cavity for accuracy. Fill in gaps with a canister spray foam to seal off any gaps. XPS, EPS, and Polyiso are the top choices for rigid foam board insulation.
Insulating the trailer frame
This step is completely optional. Most tiny home owners end at this part and just apply the flooring material on top to finish the build. Others take additional precautions to fully insulate the trailer.
The longer route is by installing thin plywood on top of the metal frame to prevent thermal bridging. As a result, thermal bridging is removed because the subfloor will be in contact with the wood layer, not the metal.
An easier way of doing this is by covering exposed metal frames with sill plate gasket. A sill plate gasket can be joined with duct tape, and will provide the extra security you need to prevent thermal bridging, without having to do put in hours of additional work.
Vapor Barrier
Common materials: Polyethylene
The vapor barrier is the last thing you’re installing before the actual subfloor goes on top. While most rigid foam boards have their own vapor barrier, we recommend putting in one anyway for an added layer of protection against moisture.
It’s a thin sheet of plastic, typically made out of polyethylene, that you staple down. Unlike an air barrier, installing a vapor barrier doesn’t have to be 100% sealed. Simply lay it on top of your other layers and install.
If you’re using a glue to install your vapor barrier, make sure it’s compatible with the material. Polyethylene melts when in contact with most sealants. Most builders use an alternative called acoustical sealant to install the vapor barrier.
Subfloor Sheathing
Common materials: Plywood or OSB (oriented strand board)
The subfloor sheathing is the final part of the subfloor. It’s usually made out of plywood or OSB. Choosing your subfloor sheathing material depends on the floor layer you’re putting on top.
Plywood is stiffer and less prone to swelling than OSB, making it the ideal flooring layer for tiles to prevent cracking. OSB is smooth on both sides and is best used as a layer for vinyl and linoleum flooring, as well as other types of flooring that can reflect the subfloor’s imperfections when applied.
We don’t recommend using other traditional subfloor materials such as plank and concrete. Plank can loosen up over time and can deteriorate at a faster rate in a moving tiny house. Concrete, on the other hand, is extremely heavy and won’t be feasible for a roving tiny house.
Subfloor DIY Guide
The techniques involved in building directly onto the trailer and laying a framing above it aren’t so different. The only added step is to add wooden frames on top of the trailer, but other than that the construction of built-in and above-trailer subfloors are similar.
Why Build Directly Onto the Trailer
Using the trailer as a subfloor requires using a specialized trailer that will allow you to build directly into the trailer. Traditional trailers are difficult to work with but you can order specialized trailers built for tiny houses. These trailers have evenly spaced cross beams that can be insulated and sealed off.
Why build directly into the trailer? Building into the trailer means skipping wood framing for your subfloor. This can save you costs and weight if those are two of your concerns. It’s also a great way to save vertical space, allowing you to expand your tiny house upwards without making it any less road-friendly.
How to Build Tiny House Subfloor On a Trailer
1. Choose the Right Trailer
In order to build directly onto the subfloor, you must choose a trailer that can accommodate the subfloor. You can order one from a specialized contractor, but you can also recycle an older one to make it more friendly for tiny homes.
Mic and Lyndi from Tiny It Yourself talk about saving $5,000 on a tiny house trailer by salvaging an old RV trailer and turning that into their tiny house foundation. Their tiny house on wheels was built directly into the trailer, saving them 6 inches of vertical space. The DIY couple was able to pull this off on an RV trailer by adding wooden cross beams onto the platform to try and equalize the spaces for an easier build.
2. Add / Order the Flashing
The first part of your tiny house subfloor is the flashing. If you’re repurposing an old trailer, chances are it won’t have a flashing. You can order one with a flashing already installed for an additional cost, but it’s also possible to do it yourself.
To install the flashing, you will need duct tape and self tapping screws to secure the flashing onto the trailer. Use self tapping screw to attach the flashing by the trailer frame, and duct tape to secure any loose gaps on the sides of the cross beams. We recommend having a creeper while you work to make it easier to move from one area to another.
3. Add the Frames (For Above-Trailer Only)
If you’re not using your trailer as your subflooring frame, you’ll have to lay down some foundation before installing the rest of your subfloor. Use 2 x 4 wood to frame the subfloor of your tiny house. You can build within the trailer size or extend a couple of inches for more width.
Your framing depends on the size and shape of your trailer. We suggest portioning it into three sections: one in front, one around the wheel well, and one at the back.
Bryan from Going Tiny Life explains his plans for his dovetail trailer. Bryan used 2 x 4 and 2 x 6 wood for the framing of his trailer and he intends to install different frames based on the weight distribution of his trailer.
The different widths for the framing account for the different depths in the trailer. The key is to create a stable frame to accommodate the sheathing material.
4. Install the Insulation and Vapor Barrier
Installing the insulation on a subfloor is similar to installing it on a wall. If you’re using rigid foam boards as insulation, prepare to have a spray to seal off any gaps. Fibrous insulation such as rockwool also works well in subfloor. Measure the depth of each cross member cavity to get the best fitting for your insulation.
Further reduce thermal bridging by applying sill plate gasket on the edge of the trailer. You can also cover where the cross beams would touch the wood to prevent any thermal bridging. Attach the sill plate gasket with glue.
After installing the insulation, add an additional vapor barrier to keep moisture from getting to your subfloor. Simply attach the vapor barrier with a nail gun to keep it pinned to your floor. If your vapor barrier starts to condensate from being left outside, we recommend poking a small hole around the condensation build up to let the moisture out.
5. Lay Down the Subfloor
Finally, finish attaching the subfloor to your tiny house trailer by screwing the subfloor onto the trailer. Depending on the size and orientation of your crossbeams, you want to take a look at the “kind” of plywood you’re using to ensure you’re getting the right amount and size of sheathing material for your tiny house.
Check whether you need the scant or full face variety of your plywood and ask your supplier for the exact dimensions.
Lay down the subfloor lengthwise against the cross beams for added strength. To avoid wasting any material, measure every space as you go and fit each sheathing piece before permanently attaching it to your trailer.
DIY With Confidence
Laying down the first subfloor is the first step towards your dream tiny home. Do it correctly and you’ve got a firm foundation that can support the rest of your house. If you’re a first-time builder, we recommend following a blueprint online to prevent any mistakes during the build. If you’re confident with your DIY skills, you can try experimenting with other techniques not mentioned here to improve your subfloor.
It might seem like a hard project at first, but if you buckle down and get to it, you can finish your own subfloor in four days to a week. Ticking this off your tiny house to do list will surely make the rest of the build a seem like a piece of cake. Happy building!
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