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I'm rebuilding my deck , yard space tight so i want to utilize the underdeck space , but the deck height is only about 5' so im wondering if i could flat sheet on the joist w/pt plywood and put 5/4 deck boards on top of the plywood? Would it hold too much moisture? I live in central alberta so not to rainy. Appreciated any advice
 

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That does not sound like something I would recommend. There would be times when the deck boards are sitting in water. If the deck is sloped for drainage as is normal, that would be better, but I still do not like the idea.

It might be better if you put the plywood on the underside of the joists, and shim one end to insure water runs off.
 

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FlyingHammer
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I'd be worried about it holding rain water against the deck boards and possibly channeling it back toward the house. You might also have a problem with leaves and other debris getting stuck in there. Personally I'd give up a bit of head room in the shed and just put up tin roof panels under the joists.
 

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if you have any kind of floor/deck that is exposed, and you have a solid surface, such as plywood, you need to slope it so water won't pool. General rule is quarter inch per foot, or 2% If you use deckboards directly on joists, you don't need to worry about this since the water will drain through.

Best thing if you use plywood and want it to last is to put a membrane over the plywood, then the deckboards. Just make sure there is sufficient slope. All sundecks are sloped like this.
 

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If the deck is sloped properly away from the house, and you don't need a wood look deck floor, you could use a product like Duradek (basically a vinyl covering that is professionally glued/heat moulded on top of your deck floor (which would be plywood) that prevents any moisture from getting underneath).

I haven't used it personally, but know people who have and love it. Comes in all kinds of patterns/looks (floor tiles, wood, rock, etc). But, only if you don't want the 5/4 board look.

This enables you to not have to have two surfaces (5/4 board over plywood below), worry about shimming the flywood under the deck joists for runoff, etc.

http://www.duradek.com/
 
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