I have secured the plywood to the tapcons. I have 1/2" expansion gap around the plywoods perimeter and 1/4"-1/2" between the sheets. I also plan on 1/2" expansion gap for my hardwood instead of 3/4". Is 1/2" expansion gap for plywood and hardwood acceptable? I felt the 3/4" expansion gap is overkill and decided to go with 1/2" instead. I just want to be sure before I lay down my hardwood floor.

Thank you in advance.

I meant to say 3/4" plywood is secured to the concrete slab with tapcon screws.

I don't know, sounds good as long as the concrete moisture vapors are not sky high. After all, you poked a lot of holes in the moisture barrier, compromising its intended purpose.

Thank you, "Floorguy".

Yes, I guess I poked a lot of holes with 32 holes per 4x8 sheet. However, I'm not sure if this is any different than what NOFMA or Bruce people recommend for installing hardwood over the concrete slab---one of their options is to lay down 6 Mil plastic and then secure plywood with the power actuated NAILs (one for each square foot per Bruce or 20 nails per 4x8 sheet per Mike Moore). Are you saying Bruce people and NOFMA got it wrong or taking a big risk?

Before those instructions, it refers to having the concrete tested for high vapor emissions. You did that part though, since your sticking to NOFMA and the old NWFA standards to the "T", right????


The old way was with two layers of asphalt mastic and asphalt saturated felt paper, before the plastic, to seal the fasteners penetration.


Yes, it is a gamble, if you neglected to properly test for vapor emissions!!!

I'm pretty sure NOFMA states:
trowel layer of mastic on concrete
place one layer of plastic
trowel second coat of mastic
one more layer of plastic/roofing felt
nail 3/4" plywood

OR:
layer of plastic
loose lay (after scerfing plywood backing) strips of plywood 2' wide on top of plastic.

that is the short of it.
In other words:
nail/affix through mastic because it seals around the screws.

Or:
Loose lay 3/4" ply over plastic without screwing through it.

www.nofma.org for full installation procedures.