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Is there a readily available alternative to plywood that I could use for the walls and floor of a treehouse that would be waterproof (from a will-not-be-damaged-by-water standpoint rather than a perfectly-blocks-water-through-all-cracks-with-no-extra-steps standpoint)? Perhaps some plastic or fiber-cement sheet board?

It would need to have strength similar to plywood so if someone jumped on the floor or fell into the wall, it would not break. Ideally it would be relatively easily workable with regards to cutting and screwing into it (though buying different blades or screws if needed would be fine).

g491
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You might consider High-Density Polyethylene (HDPE) sheets, which in Canada is popularly known as "Puck Board" but might be called something else in other parts of the world. There's a marine version sold under the brand name 'StarBoard'.

It's waterproof and has a strength comparable to an equal thickness of plywood, as far as I know.

Roger
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Pressure treated plywood exists. It's treated with the same chemicals as the PT dimensional lumber that you're probably using to build other parts of the tree house. The glues in it may or may not be especially water resistant, but it's used for exterior applications all over the place and there hasn't been a general uproar about it failing prematurely, so one might assume that it's generally suited to purpose.

You could use the PT plywood for the floor. You could also slope the floor for runoff (the standard 1/4" per foot of run would be fine), just like you'd slope a deck surface. Then, either at every plywood joint or just on the lowest edge, leave a small gap (I'd think 1/4" should do just fine) for the water to run out. This will ensure that any water that does get in has a place to get out and encouragement to do so.

I built a shed a couple of summers ago and I covered the bare (non-PT) studs with T1-11 siding. This is also a PT/weather-proofed material. Once the siding was up, we painted it. Between the built in weather resistance of the T1-11 siding and an occasional refresh of the paint, I'm expecting it to last at least 10 years (and I'm hoping for more like 15-20) before it needs any significant level of repair. Likewise, you could use T1-11 for your siding if you'd like some detail on the sides instead of just flat slabs of plywood.

In any case, some paint, epoxy coating, polyurethane or other protectant will go a long way toward preserving the wood, no matter which product it's applied over.

FreeMan
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Weathering steel will work a trick.

They use it to create shipping containers. No painting necessary. Steel is stronger than plywood !

Cutting and screwing are also quite standard. Tools to cut metal are often the same as the ones used to cut wood just use different blades.

Fresh Codemonger
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