I am preparing to install 9/16" cypress plank flooring (5" & 7" widths) over a 30-year-old concrete slab that is in good shape. My plan is to start with a layer of plastic as a vapor barrier, then 3/4" of plywood, then a layer of roofing felt, then my cypress face-nailed into the plywood.
My question is regarding the plywood subfloor. My original plan was to use concrete screws to screw a single layer of 3/4" subfloor down over the plastic and leave 1/2" expansion gaps around each piece. I ran across a suggestion to do it in two 3/8" layers to total 3/4". Neither layer would be fastened to the slab, they would only be fastened to each other. This would prevent having to puncture the vapor barrier and would allow the floor to "float" on the slab. The two layers would be run at either 45 or 90 degrees to each other to prevent gaps and increase the stability of the sub-floor.
The floor is above grade in a 30-year-old home in South Mississippi.
Does anyone know of anything to recommend one approach over the other? One layer or two? Thanks. -Tom