Hey all,
I'm getting ready to put in prefinished wood flooring over a concrete subfloor (only about 600 odd sq. ft. worth). Now I plan on installing a plywood subfloor first, using 5/8'' or 3/4'' plywood but my lack of knowledge on the subject leads me to the following questions:
I know that you can "float" the subfloor, e.g., put down two layers of 1/2'' ply and screw them together and then attach the prefinished floor to that or you an glue and nail down a layer of 3/4'' ply.

With the various searches that I've done I've not really seen/read a comparison of these two methods, e.g., which is better? - I assume that the single layer of 3/4'' glued and nailed down is best but I'm just curious.

I also have a question about the moisture barrier. If I go the route of putting in a single layer of 3/4'' I assume it is necessary to put a moisture barrier down below the ply, e.g., 6 mil plastic or something similar. Further I understand that it is necessary to "glue" down the plastic and then glue the ply on top of that.

What is the process/best adhesives to use for gluing down the moisture barrier and what is the best way to go about doing that?

Thanks much

oh, also, does anyone know anything about westhollow prefinished flooring? I've found it on one website only and it's price is very low so I'm curious if the quality is just as low as the price.

why aren't you gluing the prefinished flooring to the concrete? thats what everyone else does.

Not everyone will glue it directly to the concrete, if it is a ¾" or a thin solid hardwood. Only a few have deep enough pockets to risk it.


You can also float a ¾" plywood. Cut the plywood into 2x8 pieces, and stagger the pieces. Don't fasten them to the concrete.


If you fasten down the plywood with anchors, Use Henrys asphalt mastic, on the concrete, roll out your moisture barrier and broom it over the mastic to get it flat and no air pockets. Fasten down. The asphalt mastic seals where the fastener penetrates the moisture barrier.

thanks much for the tips gentlemen. I'm just not comfortable with gluing solid prefinished (not engineered) right to the concrete. Using the Henrys asphalt mastic and laying down a moisture barrier, can I then glue (liquid nails or some such construction adhesive) the plywood to the moisture barrier or is that a total waste? Another followup - what size anchors should be used, I'll probably be going the hilti route instead of cutnails or tapcon screws (had a horrible experience with tapcons and a very very solid concrete slab).

thanks very much for the very helpful info!

So you can float the 3/4" plywood subfloor? This is good, good news. No glue and no mess! Are there any disadvantages to this approach? Thank you in advance.

cmose, It would be a wasteful mess to try and glue the plywood to the moisture retarder. A ramset works, but the tapcons you hate, are usually used. Did I mention the plywood shold be installed at a 45º to the finished flooring, so a finished flooring seam doesn't fall on a plywood seam. It is rarely done here, but when it does cause a problem, it is noticable and called panelization.




dfigu, With this application you can use a thick poly moisture retarder, and no asphalt mastic.
Don't tear or wrinkle the moisture retarder, and run the 2x8 strips in the opposite direction of the finished flooring. Leave an expansion gap around the perimeter just as you would with the finished flooring. Make sure your finished flooring fasteners don't penetrate the plywood and puncture the moisture retarder, or you will be sorry.

thanks for the insight - is there any negative to using hilti/ramset fasteners instead of tapcon screws? and back to both, what lenghts..?

Thanks very much!