[BigTy]

For an underdeck shed , advice on sheeting the joist with pt plywood and putting deckboards on the plywood. Trying to make the "shed" as waterproof as possible and its 5' or so , so would prefer to try and keep as much headroom as possible. Any advice is appreciated

 

[chrisBC]

So this underdeck shed will be subject to rain and water I take it?

if you are putting plywood down on the joists, you should create a slope with sleepers, and then put your plywood down. If you put down deckboards with a proper space between each board, then you do not have to worry about water pooling, like you do with the plywood. Hence the need to create slope.

 

[Ron6519]

For an underdeck shed , advice on sheeting the joist with pt plywood and putting deckboards on the plywood. Trying to make the "shed" as waterproof as possible and its 5' or so , so would prefer to try and keep as much headroom as possible. Any advice is appreciated

I wouldn't do this. Water would be trapped on top of the plywood. Pressure treated or not, it will rot.
Ron