Fixing plywood to concrete floor

[Mat and Suzy]

I need to lay a plywood sub-floor (ready for engineered hardwood to go on top) over a concrete floor in my first floor family room. What is the best way to fix it to concrete? I have never done anything like this before, but I understand there are nails that will drive themselves down into concrete....is that right? Are they easy to use?

Would this method be better than some kind of thinset/mortar etc?

TIA

Mat

[Chris Lewis]

According to Mat and Suzy :

There's many options:

1) Concrete nails (what you asked about). Drive with a sledge hammer (full size 10lb or a hand sledge). This gets old very fast if you're trying to drive a lot of nails, which you do. USE PROTECTIVE GEAR! 2) Tapcons (screws you drive into predrilled holes). Relatively easy on the system, but the drilling is a pain. 3) Remmington power drivers. These are .22 shell powered nailers. Easy. Requires care you don't shoot yourself. USE PROTECTIVE GEAR! 4) Construction adhesive (caulking gun application). Trick is getting the ply to stay flat while it's drying. 5) Use sleepers instead of ply, using (1) and/or (4) to fasten them. 6) Combination of above.

I think what I'd specifically suggest is to use (4) and (1) to hold the ply down for set. You won't have to drive so many nails - they're just to keep the ply flat while the adhesive dries.

The adhesive is best overall because it'll do the best job of preventing the plywood from "slapping".

[Joseph Meehan]

Find out what the manufacturer of the hardwood requires. They should have some very specific requirements for concrete floors. Hardwood and concrete (moisture) don't generally get along well.

If it were me and the manufacturer did not have any thing to say and suggested that installation over concrete was ok, I would want to make sure I had a solid vapor barrier and then sleepers then the new subfloor of materials that can handle potential moisture.

[Mat and Suzy]

how to fix it onto concrete.

The best plan is to use a membrane between concrete and plywood, and again between plywood and engineered hardwood. Because of that, I 'm not sure adhesive will work for the concrete/ply joint.

Any ideas anyone?

[Joseph Meehan]

I am not sure I would want to fix it to the concrete. I would rather put it on sleepers. I don't think you want it in direct contact with the concrete that can and will bring in moisture. Concrete is NOT waterproof.

I would not want a trapped space between two membranes. I suggest one barrier on the concrete and then use sleepers treated for moisture. Frankly I would not worry about "attaching" the sleepers to the concrete, I would tend to let the floor float.

However before I made any decisions, I would get back to the manufacturer and find out exactly what they recommend for your situation. Somehow I suspect they are going to tell you not to use their product on concrete, without special prep that they spell out.

BTW is this above or below grade?

[Mat and Suzy]

It is at grade, which is exactly what engineered hardwood is designed for - even OK for basements.