Don't use OSB.pbw said:
There are 2 major grades of ply, that can be found at lowes/home depot. First is sheathing. It often has a mediocre ply on one side, and knotty ole things on the other. It is the cheapest, and is mostly used for sheathing, and subfloors on houses. Occasionally, you can find a pallet of this stuff that has a great ply on one side, and a fairly good one on the other. Sometimes the trees they cut it from don't have many knots, but since it hasn't been gone over with a machine to fix it, they still sell it as sheating.
The next is furniture grade (not sure of the actual name) It has footballs in all the voids on the outside plys. It is probably what you want. It is a little pricier, but worth it.
Now, HD/lowes also carry plys with a veneer of a hard wood on them. They are for making furniture with, where you want the wood finish, but don't want to use the real wood, and can't stand that vinyl wood on the MDF, and LDF that much of the cheap furniture is made from. You don't need this, as it is just extra money for something that will be covered by carpet. Now, there is also Birch ply, which is better for certain things, especially when spar varnish will be used, but you don't want that either, as it is rather pricey.
From what I remember, the furniture grade is only in 1/2 and 3/4. (it is in more, but we are looking at a range from 1/2 to 3/4). The sheathing is available in 5/8 as well. You could do just fine with 1/2, especially seeing how close your structure is. Unless you find a great batch of sheathing, you should stick with the furniture grade 1/2 and a good coat of water sealer.
. Are you going to seal all of the wood bracing before you put the deck down, especially any plywood edges that would soak-up water fairly quick?
