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I am building a shed. The floor consists of 4x4 PT runners, 2x8 PT girders, and 3/4" PT Plywood flooring. Anytime in the past, when I have laid PT lumber next to non-PT lumber, there is mold/mildew on the non-PT lumber after a very short time. Also, anything metal seems to rust quicker when on or next to PT lumber. I wanted a heavier duty shed, so I was thinking of adding a second, non-pressure treated layer of plywood on the first. This would strengthen the floor and also separate the PT wood from the contents in my shed. Is this plan acceptable? Should I put a moisture barrier between a single ply of pressure treated plywood and a second non-PT ply?

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Dont put moisture barrier between, it will just trap moisture and rot quicker. Moisture for shed should be under structure. But if it collects rain water on top you are just making humidity worse. Pt plywood is generally dried out when you buy it so there shouldn't be an issue.

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put the moisture barrier between the runners and the girders.

PT wood doesn't cause mold, moisture does.

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