Alt Dweller

Building a Tiny House Subfloor

Ever wondered just how you would go about building a tiny house subfloor? My biggest concern when we began our own tiny house on wheels in 2014 was, how do we make sure that the house will be sufficiently attached to the trailer bed, for when we go about towing it down a road or even a highway one day?!

Moving a tiny house

Chances are, if you’re building a tiny house on a trailer bed, it’s because you want it to be portable. Perhaps though, there are other considerations also. For instance we once met somebody who was not allowed to build an addition on to the front of his “regular” house, so he built the addition onto a trailer bed, parked the trailer next to his house where he wanted the addition to be, and then attached the two together. This was acceptable for his local bylaws!

(On a side note, the most frequent questions we are asked surround the conversation of bylaws and “how are we getting away with it!?” I will say in very brief for now, bylaws are set by your most local municipality, whether a city, or township, or otherwise, and bylaws will differ from area to area. So start by checking with your city or town hall!)

Flat Bed Trailer
8′ x 20′ deck-between flatbed trailer

Above you can see our flat bed trailer, and in this photo the only work that we’ve done so far is to lay 20GA sheet metal over top of the two-inch crossbars that run across the trailer bed every two feet. We ordered a deck-between flatbed trailer from Longhaul Trailer Sales in Atwood, Ontario, Canada.

It’s called a deck-between because the deck of the trailer sits between the wheel wells on either side. If we had ordered a deck-over style trailer, the deck would run over the wheel well, which would start our floor an extra several inches higher from ground level. This is not desired since there is a maximum towing height of 13 feet, 6 inches. So the lower you can have your floor sitting to the ground level, the more interior height you will be able to have in your tiny house on wheels, without making your house taller than the maximum allowable towing height.

Tiny House Trailer Bed
Kat applying the tar to the sheet metal

We ordered the sheet metal from the Kitchener location of Metal Supermarkets. They cut the sheets to the specific sizes that would fit onto our flatbed trailer, and even delivered the metal to us in Guelph, at least a 30 minute drive away. The purpose of this sheet metal is to be a protective underbelly for our tiny house on wheels. It protects it while in transport from things like little rocks kicking up at the floor as we drive, and also from things like moisture or mice when we are parked places for extensive periods!

Building a tiny house subfloor
Friends helping to build our tiny house subfloor

So to protect the metal that would protect our tiny house underbelly, we first coated it in this tar-like material we found at Canadian Tire. I can’t remember the name of this product now… I’m going to find out and then I’ll update this post!

Stick-frame subfloor
L to R: Handel, Marco, and myself

This black tar-like material protects the metal against rusting and moisture, and even adds a slight layer of extra insulation. Our tiny house is situated in southern Ontario, Canada, so while our winters are not usually extreme, they do get quite cold for a few months on end every winter, comparable to neighbouring U.S. states like Michigan or New York. I’ll be writing posts soon about cold weather considerations when building a tiny house on wheels, so stay tuned!

Tiny House Subfloor

In my mind, the subfloor is a critical part of the tiny house on wheel’s overall integrity. This is why we opted for a brand new trailer, and why we used pressure-treated wood to build the subfloor. The reasoning here is, if we had tried to restore an old trailer bed, chances are it would begin to rust out sooner than a new trailer, and I am not an expert metal worker of any kind. I’d rather build our tiny house on a foundation that will be reliable for decades, hopefully! And as for the pressure-treated wood, even though we would use reclaimed heat-treated lumber for the rest of the stick-frame build, if moisture or water were going to somehow permeate our subfloor, at least the pressure-treated lumber would stand up to the moisture and be less susceptible to rotting away.

Pilot holes for lag screws

Once the subfloor was placed on top of the sheet metal that was sitting on top of our trailer bed, I crawled underneath and drilled several pilot holes through each of the metal crossbars. After drilling the pilot holes, I ran lag screws up from the bottom, so that they would suck the wood joists down and secure them tightly to the metal crossbars. I did at least 20 of these, evenly spread throughout the trailer bed.

Lag Bolts

In addition to the lag screws coming up from the bottom of the trailer, I also drilled about 10 horizontal holes through the side walls of the trailer bed, and secured the subfloor to the trailer with lag bolts as well. So there’s about 20 vertical screws holding the wood floor down to the metal crossbars, and about 10 bolts spread around the perimeter of the trailer holding the subfloor to the sides of the trailer also.

Next we will get into insulating the floor, and covering it with sheets of plywood, giving a strong foundation on which to build the rest of your tiny house on wheels!

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