I'm about to install some 3/4" hardwood in my house. The first floor is a slab on grade (poured in 1966 and seemingly very dry) and I'm going to put down 3/4 plywood and fasten the hardwood to that.

My question is, how do i install the plywood. A family friend/flooring contractor told me both glue and nail the plywood to the slab (because the nails don't guarantee to hold in the concrete), which would require me NOT to put down a moisture barrier. He told me to put tar paper between the plywood and the hardwood though. I'm skeptical because everything I've read has stressed a moisture barrier. I live in Austin TX where it's pretty dry most of the year but I'm nervous still about the plywood rotting out from under me in a matter of years.

So, does that sound like a general practice (gluing and nailing the plywood with no moisture barrier)? What other ways are there of installing the plywood to slab?

you can seal the concrete and do a full bed of adhesive or use Vapor lok adhesive by bostik for one product then nail

you can lay your 6 mil plastic down and cut the plywood into 16" strips and float..lay it loose down..stagger end joints and then nail directly to this using 11/2" nails.

you can lay 6 mil then shoot the plywood ion with 11/2" concrete pins.

they will hold..tha'ts what they are designed to do..if they don't hold its because its lightweight concrete.

you can do a abouble3/8 or 1/2 plywood subfloor..first parallel to floor direction,then the top on 45 staples and glued directly to first layer.
this is another floating method..6 mil underneath.


all will work and be within a guideline installation.

6 mil plastic over Roofing tar (the tar seals all the holes that the nails make), then rent the beefiest powder actuated nailer (hilti) you can find and put 20- 1 1/2" nails in each piece of 3/4" plywood.
Then 15# felt (it is more of a moisture retarder not a barrier)
Then your hard wood with 1 1/2" nails.

Look up the manufacturers installation guidelines and make sure the installer adheres to them. We have utilized 2 layers 15# felt adhered with black mastic with 3/4" plywood exposure 1. Then using powder gun nail with a minimum 32 fasteners per sheet. Recently we have used two layers of 3/8" plywood 1st layer with 1/8" spacing 2nd layer over 1st offset from 1st then screw top layer to bottom layer. This will prevent any penetration of the concrete vapor retarder. See NWFA or mfg guidelines. Carefully check moisture content of concrete slabs 4.5% mc,or plywood mc which must be 12% or less.