I have decided to go with a solid wood floor over plywood. There are 2 commonly accepted methods of securing the plywood to the slab, powder actuated nails and glue (mastic, poly, 4×4′ kerfed plywood).
The second method is attractive because I don’t have to bother with powder actuated nails. Are there any disadvantages to the plywood gluedown method?
I’m sure that some of the glue will fail over time but is this really a problem?
Well the reason I didn’t mention your proposed method in the web article on slab installations is the fact that plywood sheets are rarely flat and neither are concrete slabs. The bigger the sheet of material you want to glue down, the more of a problem this becomes. I would like to see slabs perfectly flat so the glue will sick and stay, without being stressed when you walk on it. If these plywood subfloors floors fail (a void starts popping) in any spot they will create a difficult repair job.
But I guess this method is done in the flooring industry, a trade fraught with bad tradesmen and methods. So here’s what I would suggest. Certainly saw kerf the first layer of plywood and use 4 by 4 foot sheets. Lay the plywood kerf down side in that double layer mastic method you speak of. Glue trowel size is really important, spread the correct amount.
I might say at this point that the new urethane adhesives are touted to be totally water proof, so I really feel that the poly layer in this case may be redundant. Just do a really smooth even glue troweling.
Then use full sheets of plywood for the next layer, and lay this at a 45 degree diagonal to the first layer. The top layer of 1/2″ plywood doesn’t need a full troweling of glue, just spots or squiggles of the adhesive around where the screws will go. The 3/4″ wood screws installed every 1 foot square on the top layer will then smush these together quite effectively. This mixes up the more numerous joints that the 4 by 4 plywood has created. You should have no joint in both layers of plywood that go together. Joints should be kept about 1/8″ apart for expansion in both layers.
With the top layer diagonal, and the new floor running straight you shouldn’t ever have any problems, until this glue fails in about 50 years. Oh, and let’s discuss the glue. It should always be a urethane adhesive as they contain no water. Find one from Bostik that will be suited to this specific application. As you are doing the first layers of glue make sure that each one is fully dry before you apply the next layer, or like I said skip the poly film altogether.
Here is the Bostik glue link
If you are doing this in a really dry climate, urethane adhesive won’t cure very fast below an EMC of about 8%. So we always in these cases spray the slab with a light mist of water. A LIGHT mist. But check the glue makers to see the formulation that works the best in your climate.
They sell this stuff in tubes also, and be sure to get the special hand cleaner, or else your hands will be black for days. Good Luck. By the way I have to go into the hospital tomorrow, so I will be unavailable for at least 2 weeks.