OK, I have recently started two threads about floors over concrete. I sense I am in danger of posting too often. Hope this is the last one. I had/have consider a subfloor product called "subflor advance", which was discussed in an earlier thread. Then I read more about floating subfloors and I think I can save some $$ and 1/8" floor height (the subflor advance is 7/8", versus 3/4" plywood).

So I intend to use 3/4" diagonally on the floor, over a vapor barrier. I have not decided whether to cut the sheets to 2x8 or perhaps even 16" x 8'. If I can get the 4x8 to lay flat, I will just lay them flat, if not, I may start cutting to strips. Kerfing back side is a given. I will level the concrete in advance to the best of my ability as well.

I know a double layer of 1/2" would be better, but I am just not liking the extra 1/4" of height.

The flooring will be a very stable 3/4" quartersawn oak, so that should give the floor a good deal of stability as well.

Question that remains: What sort of plywood? My local big boxes have BC that looks too unstable to float. CDX sure is out of the question. They have some birch plywood listed as hardwood ply that looks and feels very stable. Costs more too. I have not priced baltic birch, but I suspect its not cheap (I have a good deal of cabnetry experience and baltic birch is a real joy to work with!)

Someone must have some experience here.

Thanks in advance.....

Oh -- ordered the Bostitch MIIIFS last night. $399 delivered off of ebay, new with warranty. Hope that was a decent deal.

I'm not sure why you say the BC grade looks unstable. And why do you say the CDX grade is out of the question? Most often, the lower grades are used because they cost less and are going to be covered but they should be no less stable. By cutting the plywood into 16" x 8' strips (panels) you will allow it to conform more easily to the subfloor. Kerfing the backside is recommended as well. A full 4x8 sheet will not usually lay down as flat. The 2' x 8' panel is also a good option. I do not think baltic birch is called for for a subfloor/underlayment. Just my opinion.

When I see a pile of plywood and the top sheets are starting to twist and no longer are laying "like a stack of cards" then I obviously wonder what sorts of twists and bends it will make when I am trying to get it to lay down on my floor with nothing holding it except the weight of the flooring.

So BC or CDX has been done? My concern about using the hardwood plywood, such as birch, is that it is not really rated for any sort of moisture. Of course it will be over the moisture barrier, so under normal conditions it will be dry. But if the floor were ever to get wet (not from below, but say from a leaking pipe) then bad things might happen.

I assume that exterior rated plywood would be the correct way to go, right?

The X in exterior rated means the glue that holds the plywood veneers together is moisture/waterproof adhesive. That's about it.