Building The Subfloor

Building the Subfloor

A tiny bit of this post will overlap with THIS ONE posted last week, but below is about two weeks of our work condensed into a single digestible blog entry for anyone curious about the creation of our tiny house subfloor. 

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After the purchase of our customized 8’x20′ flatbed, we bought these steel sheets. The flatbed was designed with a 3.5″ thick chassis, with 2″ steel crossbars welded across and under the chassis every 16 inches (with a few exceptions in separation where the axles are located). The flatbed is a straight, deck-between trailer with two tandem 5200 lb. axles, weighing about 2,300 lbs. dry but with a gross weight rating of 10,000 lbs. We are aiming and hoping to weigh about 8,000 lbs. dry once the house is complete. Virtually all of the furniture will be built-in to the tiny house, so there won’t be a whole lot of extra weight once our stuff is inside. Anyways, the steel sheets are specifically 20 gauge mild steel, which was cheaper and stronger than aluminum, but because it is steel it should be protected from the elements to ensure a longer life. The purpose of this steel is to lay on the crossbars of the flatbed before laying our subfloor down, hence protecting the wooden subfloor itself from those same elements that will be endured instead by the 20GA steel. We used a rubberizing paint which was like tar and was awful to apply. I’m not sure I would use it again, and certainly there are many ways to go about protecting your subfloor. 

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Before laying down the subfloor, we drilled pilot holes vertically downward through the crossbars. We put almost 50 pilot holes evenly throughout the trailer, intending to later crawl beneath the trailer to drive 1/4″ lag screws up through the steel, sinking them into the wooden subfloor. This is one part of how we will secure the wood to the steel. We also drilled horizontal holes through the chassis itself, and then later through the 2×4’s, in order to place a 5/8″ lag bolt through, securing the two lengthwise sides of the subfloor to the lengthwise sides of the chassis. We used five lag bolts per 20′ side, so 10 lag screws in total. 

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Then we began construction on the subfloor itself, which again is constructed of 2×4’s. Although we will not be using pressure treated wood anywhere else, we did use pressure treated wood for the subfloor, in our attempt to, in any and every way conceivable, preserve the life of our subfloor for as long as possible. In theory the subfloor will be perfectly sealed so that no water ever gets in, but if it does, the pressure treated wood will hold up much longer. And the reason that our chassis is 3.5″ thick is because the 2×4’s are also that same thickness, the idea being, once we drop the subfloor into the chassis, each 2×4 will rest on the steel crossbars and sit flush  with the top of the steel frame of the trailer. Essentially what we’ve done here is gain an extra 3.5″ in living space height (because we’ve built the subfloor between the chassis frame rather than on top of it), as the height of a tiny house must be no more than 13.5′ from ground level in order travel on highways. 

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Now we’ve placed the steel sheets in to rest on the crossbars, and then rested the finished subfloor on top of the steel sheets. We used cans of spray foam insulation and also Tuck Tape to seal around the perimeter or any places where moisture could possibly get in. You can see from the photo above that we have extended the floor past the wheel wells, which will be built over eventually, to give us a full 8 feet of width. 

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Here’s where I crawled underneath the trailer to drive the lag screws up through the crossbars and into the wood framing. I had pre-drilled pilot holes through the steel crossbars, however, now there is also 20GA steel to drill through before getting to the wood. I had to drill a second pilot hole from beneath through the pre-existing pilot hole to get through the steel sheets and then the wood. Only then could I change bits and drive the lag screws through. 

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Now we can start installing the insulation. You can see a bit to the right of Kat’s hand one of the lag bolts that is horizontally bolting the wood and steel together. This is R-12 pink insulation which we obtained brand new and for free. Because we used 2×4’s instead of anything wider, R-12 is about as good as we can do, outside of spending about A Thousand Dollars on spray foaming it all, which possibly could have given us R-18. Between the R-12 pink batts, a radiant barrier (I will mention more about this later), the rubberizing paint beneath, proper heating from inside, and doing everything we can to seal the floors, this home is intended to withstand southern Ontario winters. Our roof will eventually be constructed using 2×6’s, so we can definitely get a better R-value in the roof, which should be more important since heat rises. 

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Here’s the subfloor with most of the pink batts in place. 

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Here is the radiant barrier, which you can buy in rolls. It looks like a silver version of a swimming pool cover, and you might have seen it covering pipes or a water heater in basements. We have cut out each piece to match the batts of insulation, and dropped them into place on top of the batts. 

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Here’s Kat spray foaming around the edges of the radiant barrier, to seal off the crack between the barrier and the 2×4. 

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And this is a 6 millimeter polyethelene sheet, a vapour barrier, which we’ve rolled over top of everything. The only thing left to do is lay down the subfloor sheathing. 

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Most of what we’ve found out there suggests using a 3/4″ tongue and groove plywood. We have used a 1/2″ aspenite board, partly in effort to reduce weight wherever possible, and partly because we had so many sheets of it donated to us. We intend to finish the floors with hardwood, which will hopefully add to the floor’s total strength, and we don’t expect much if any floor sagging even though we’ve used 1/2″ board instead of 3/4″.  

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There it is. Tons of space. Couldn’t ask for more. Now to sit down, and for a brief moment, rest. 

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3 thoughts on “Building The Subfloor

  1. Congratulations on starting your tiny home….will start to follow your blog regularly. I’m rooting for you. Where do you plan on making your final home? Peter from Mississauga

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