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Tiny house undercarriage

airie | Posted in Energy Efficiency and Durability on

I have read several threads about tiny house subfloor assemblies. We like the idea of a “DIY SIP” with 4″  high density EPS sandwiched between plywood. This will sit over the 16″s oc steel cross members of the trailer, with only the bottom layer of plywood extending under stud walls. What we have questions about is how to weather/road proof the underside of this assembly (our trailer has no pan).

We have seen/read many tiny home builders use 1′ aluminum coil lashing. Our first thought was to use this idea but instead of sealing with tape, overlap to allow drainage with a layer of house wrap between the flashing and the plywood. The only way of fastening I can see in this case is to bolt the bottom layer of plywood, house wrap (reinforced with tape at puncture points) and flashing to the trailer cross members, with bolt heads protruding slightly into the foam.

Some advice we received since then indicated that glueing the subfloor to the cross members would allow less flex/creaking floors. Our previous idea doesn’t really allow this but it could be achieved by ditching the aluminum and house wrap and either using pressure treated plywood for the bottom layer and/or some kind of boat epoxy to protect the underside of the plywood. Are there concerns around contact between pressure treated plywood and the trailer or insulation foam? Can you foresee any condensation/drying problems with a marine epoxy? Do you think the aluminum is the better idea? Any comments/critiques/ideas welcome.

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  1. Expert Member
    BILL WICHERS | | #1

    I’ve never worked with a tiny house, but I have a LOT of experience working with many of the materials you’re considering in my commercial work.

    Do NOT allow pressure treated lumber to directly contact the frame. The chemicals used in the wood treatment will corrode the steel. This is why you can use regular fasteners in pressure treated wood when building decks, for example.

    I would advise against using aluminum flashing if you plan to travel in areas that salt the roads in the winter. Salt eats aluminum.

    Regarding squeaks and the like, there are adhesives that can bond steel (auto manufacturers use adhesives in place of welds in many car doors, among other places), but I’m not so sure how well adhesives would hold up bonding wood to steel in a bouncing trailer. What I would do instead would be to use some 2x4 or similar lumber bonded and SCREWED (not nailed) to the floor, with the 2x4 paralleling the steel frame members and attached as needed horizontally to the steel frame.

    I would use a stainless steel sheet instead of the aluminum flashing. There are three grades commonly used, 303, 304, and 316. There is also a 400 series but I’ve never used that one. 304 and 316 are common, 316 is most corrosion resistant and highest cost. 304 is a good balance. A sheet metal fabricator should be able to form the sheet for you and you’ll be surprised how inexpensive it is compared with what many think (it’ll be more than aluminum flashing though!). 303 is a cheaper alloy but less common.

    If you want to save money, use stainless steel as an interface material between the steel frame and the treated lumber. This will require less stainless, but will protect the steel frame. You need to use stainless fasteners as well — I wouldn’t use any of the coated ones for treated wood in this application since the road vibration will probably rub the coating off after a while resulting in eventual corrosion of the fasteners and ultimately structural failure.


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