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targeted4u 07-09-2011 03:53 PM

Plywood sub floor over concrete slab
Need a little guidance here....have a concrete slab that was carpeted, mrs. decided she wants wood floor. So carpet is gone, concrete floor is sound, and moisture free. Due to transitions between two other rooms (one is 3/4" hardwood over 1/2" plywood over concrete slab, the other is kitchen ceramic tile).

My "plan" is to lay down 6mil plastic moisture barrier, then 1/2" plywood, and then 3/4" hardwood. This decision was made because I would like the transition from the existing adjoining wood floor to the new one to be "level".

Question is, what is the best way to attach the new 1/2" ply to the concrete? Power Nailer? Adhesive Glue? or?

My first thought was a "engineered" floor and glue down operation....but this is a dining room with some pretty heavy furniture, and not sure how it will hold up with 10 dining room chairs sliding around on it.

Comments and suggestions welcomed! Thanks in advance....

rusty baker 07-09-2011 07:43 PM

The plastic against concrete will cause moisture condensaton and eventually mold.

targeted4u 07-09-2011 07:50 PM


Originally Posted by rusty baker (Post 682853)
The plastic against concrete will cause moisture condensaton and eventually mold.

So what would be the better method of sealing the concrete against moisture forming? I was considering/thinking about sealing/ "painting" the concrete surface first.....then the moisture barrier, then the plywood, then the hardwood. The floor will no doubt outlive me anyway! :no:

JazMan 07-09-2011 09:06 PM

Plywood on a slab is NEVER a good idea. Installing plastic under it is even worse. If there's moisture, where do you think it's gonna go? The moisture has to evaporate into the room. If there's too much moisture, you need to find the fix for that first.


rusty baker 07-09-2011 09:10 PM

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I agree Jaz, plywood on slab is a failure waiting to happen, but some people are convinced it will work.

targeted4u 07-10-2011 01:52 PM

Thanks for your input.....but we have been in this house for 6 years, two of the existing hardwood floor rooms with the same set up that I am trying to duplicate, have shown no evidence of having mold problems (yet).

Bud Cline 07-10-2011 02:17 PM


Thanks for your input.....but...
You guys should have seen that commin'.:)

rusty baker 07-10-2011 03:55 PM

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Originally Posted by Bud Cline (Post 683226)
You guys should have seen that commin'.:)

Yep, that's why I posted.

targeted4u 07-10-2011 06:45 PM

So again I thank ya'll for your informed input, although no one has mentioned my original request for "guidance" on attaching plywood to the dry cement slab.

This wasn't a brain storm I just came up with on my own idle time....did some online research first on vapor barrier/plywood sub/and hardwood install...

One of the references I used I took some reference from:, and their install guidelines.

But as I often tell my clients, opinions are like "noses" everyone has one, and one of my personal favorites......"it must be true, I read it on the Internet".

Again...thanks to all who chimed in.......

PS: Talked to former owner of our house tonight, the "existing wood floor" was installed in if this install lasts another 11 years it will probably outlive me.......... :wink:

rusty baker 07-10-2011 10:00 PM

Good luck. Sometimes things work, even tho they are not done right.

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