How do I float 3/4" plywood sheets over above grade concrete without nailing, screwing or gluing to the slab?

Question

I have solid 3/4" oak hardwood strip that I am looking to nail into the 3/4" plywood that I bought for subflooring. I made this purchase over the pre-made systems because this is the greenest plywood I could find and I am very sensitive to some of the adhesives (VOCs, Urea-Formaldehyde etc...) used in lower grade products. The problem is these are plain old 4x8 sheets that do not have and sort of mechanism to fit together. I cannot nail or glue to the concrete since it is a condo. I was hoping a good underlayment will sort of hold things in place (Wishful thinking I know). I need a scheme to get the plywood down so I can finish my floor.

The concrete is lightweight concrete. I also do not want to pursue a staggered and stacked plywood scheme since I am already adding 1.5" of height to the floor.

Can I just lay the plywood down? Also how big an expansion gap do I need for the plywood subfloor? Is it possible to cut my own groves into the side? Could I maybe staple them together? In the videos I have seen to just tap the boards together.

Answer 1

you can just rabbet each sheet with a halflap type edge (i would make it about 2 inches wide), then glue and screw through both to attach the system together. a little more work than usual, but relatively simple once you get going. you can build a simple jig for your circular saw out of plywood to ensure it all lines up properly. if you drill the upper lip on the edge, the screw will suck the joint up and tight.

FYI, if you are going to float ply over a slab, lay down and tape seal a poly vapour (6mil or heavier) barrier before you put down the ply. this will prevent moisture from being absorbed by the subfloor and oak and avoid rot and warpage.

Answer 2

What about screws?plywood subfloor on concrete