Engineered wood flooring is made to be glued directly to a concrete subfloor. Why add plywood? Are you doing it for height? The best method would probably be to install a moisture barrier like Bostik MVP4 or Traxxshield99 and glue the floor over it. If you want to use plywood, the NWFA recommends a poly barrier laid in a bed of cutback mastic underneath.

I HATE the "plywood laying on concrete" concept. It fails so often it is HEART BREAKING!!!! If this is a PROPERLY BUILT sleeper subfloor, it will look like this:

1. Vapour barrier over concrete

2. 2"x4" of 4"x6" laid in a joist system over the plastic sheeting

3. Layer of subfloor grade plywood

4. SECOND layer of subfloor grade plywood to make a total of 3/4" thick

5. Installed Engineered hardwood

This system is the PROPER way to do this. And YES....you will RAISE THE FLOOR as much as 4"...and that's why many people don't do it....and the cost is a lovely reminder as to why they don't do it.

The plywood on concrete is a cheap practice done by people who do not want to glue or float...or don't know how to glue/float an engineered hardwood below grade.

I suggest you find another way to do this. And perhaps another professional to come in and offer solutions.

Good luck. The "plywood over concrete" has tears and frustration waiting for you in just a few years time.

What is the nature of the failure on the plywood you're referring to?

I've probably laid hundreds of thousands of square feet of plywood over concrete and can't think of one failure I could attribute to the installation method (i.e. the failures have all been caused by other factors, like a flood).

Granted we go a little overboard in that we flat trowel cold bond adhesive on the concrete at 50'/gallon, lay in heavy poly sheeting, trowel another layer of mastic over that at 32'/gallon, lay in the plywood, shoot it down with 1-1/2" hardened coil nails and fasten any loose areas with tapcons. Before sheet vinyl fell out of favor we used it under plywood all the time in conjunction with adhesive. I've had a few jobs recently where the client wanted something better that visqueen so we used a membrane like Traxxshield99 with seam tape.

The plywood subfloors I have seen fail due to installation method, are almost always laid dry over a single layer of black felt paper (which is not a suitable moisture barrier), then shot or screwed down.

I don't think there's any need to build up a subfloor like that and affect the relationship to doors and steps, but if the advantages were deemed to be beneficial, I'd recommend a moisture barrier, 1x4s fastened to the concrete 12" on center and a layer of 3/4" plywood over that, with the joints laid on the 1x4s. This can be a little complicated because it would be nice to have p.t. lumber for the 1x4s but they're generally soaking wet upon purchase and there may be an issue using steel fasteners.

jfcwood - the failures you have seen (all having to do with improper installation methods) are what I'm talking about.

You have done installs like this with ALL the precautions (and the price tag to go with it)...you can say you are 1 in 100. The vast majority of plywood-on-concrete failures are BECAUSE of the install...which means very few people will do it properly. Hence the statement: you are 1 in 100.

I caution against the "vast majority". Not the 1 in 100. And homeowners are hard pressed to know the difference between the installer who does know vs the installer who doesn't know....that's why I caution against it. And many home renovation choices are made with an eye on the bottom line. Which makes homeowners balk at the proper install because it is more expensive than the "wrong" install.

I caution against it not be cause it can't be done...but because it is almost always done improperly. Better to avoid the issue all together.