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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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  • Would just painting/lacquering the plywood be an option? – MiG Aug 29 '22 at 16:18
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    There is marine grade plywood that should last for years. There are also a few deck boards made from plastics, that should be strong enough to be jumped on. – crip659 Aug 29 '22 at 16:18
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    Yes, as crip says, "Marine-grade plywood, often simply called marine plywood, is not what it's often claimed to be. That is, it's not waterproof. It is a good-quality, hardwood plywood made with waterproof glue, but since it's not treated with chemicals, it is not rot-resistant. Better grades also tend to be lightweight, strong, and virtually free of defects. These qualities are what make this plywood a popular choice for building boats and boat parts. It's also a smart option for durable outdoor furniture projects in coastal areas, especially after it receives a strong protective finish. " – Armand Aug 29 '22 at 17:21
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    We had an open treehouse built with standard untreated dimensional lumber that lasted fine for 2 decades until we removed it due to tree health issues. No leaf cover during the rainy winters, so you may not need anything too special as long as it doesn't trap water or have direct contact with the ground. – Armand Aug 29 '22 at 17:24
  • More information would help. Quite sure there are some products out there that will last forever, but you might need to sell the house to pay for them. I have a couple of small decks made with PT lumber with no maintenance(painting,staining) from the 90s, I can still jump on. – crip659 Aug 29 '22 at 18:09
  • @Armand when you say "better grades also..." is the "marine grade" plywood what you're referencing by better grades, or is it something different? – g491 Aug 29 '22 at 18:24
  • @g491 Yes, better grades of marine plywood. – Armand Aug 30 '22 at 11:13
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    You're building a deck. Why not use decking? Synthetic options abound. – isherwood Aug 30 '22 at 12:50
  • @isherwood it's because it will also be the indoor floor of the treehouse – g491 Aug 30 '22 at 13:06
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    What difference does it make if it's an indoor treehouse floor or an outdoor deck floor? Synthetic decking will work for either situation. Granted, it's not cheap, and I think some of it requires closer floor joist spacing, but it seems like it will fit your requirements. – FreeMan Aug 30 '22 at 13:07
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    I don't know what that means. Countless covered porches have decking on the floor. – isherwood Aug 30 '22 at 13:07

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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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  • One of the distinct advantages of HDPE is that it is extremely slick. I'm taking advantage of this slipperiness to make runners for my table saw cross-cut sled. Unfortunately, "slick" is _not_ a feature one usually wants in a flooring system, especially one that's outdoors and might be used when wet. – FreeMan Aug 30 '22 at 13:01
  • Also, some reference to the strength of a horizontal sheet of HDPE when used as flooring would be most helpful. And, in small quantities, the stuff isn't too pricey. In large enough quantities to do the floor/walls of a tree house, one might need to mortgage the children to afford it. – FreeMan Aug 30 '22 at 13:17
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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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  • Kids banging knees into steel might hurt more than wood :( – Armand Aug 30 '22 at 11:12
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    It's good, @Armand, for kids to learn to deal with some pain at a young age. Makes them tougher and less whiny when they're older. – FreeMan Aug 30 '22 at 13:02