Plywood subfloor got wet and warped

21 Dec 2010
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United Kingdom

I'm building a garden office, and built the subfloor using this plywood. It rained a bit (not a lot) before I managed to get a tarp over it, and then rained some more with some water coming through the tarp.

Now it's dried out, I can see some areas where the wood has raised and split, as you can see in the photo here:

The wood conforms to BSEN636-2, which means it should be "capable of resisting weather exposure for short periods of time". Though I'd argue it's failed at that.

Have I used the wrong grade or wood, and need to rip it up and start again, or can it be fixed?
My initial reaction was to give it some light 'persuasion' with a hammer to flatten it out, but I'm worried that this will happen more and get worse as the build progresses and I don't want to be replacing the floor when the walls and roof are up!

Any thoughts?


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can't see your photo.

having it laid flat means that even a brief shower will soak in until it has all dried up.

the tarp might have delayed drying.

I fear the layers (especially the top one) will have delaminated.

It's scrap.

You might consider turning it over, depending how deep the damage is.

note that it says "please note that all plywood being used externally must be edge sealed and painted both sides with a sealer"

I used similar ply for the wall of a shed, treated all sides and edges with a water-repellent decking stain, and it's fine, but has never had water lying on it.
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It got wet then you covered it with a tarp so you've inadvertently trapped the moisture in the timber and exacerbated the problem as John says. A hammer won't flatten it out, so the only thing you can try is to lift it and turn it over, and make sure that it's protected in the future

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