Installing plywood subfloor over concrete

politicauxAugust 19, 2009

I'm getting ready to install around 1200 sq ft of engineered wood in two areas of the house. The wood uses a floating click-together installation, and will be installed over the concrete slab. Being engineered wood, I understand it can go straight onto the concrete with a moisture barrier underneath. However...

Here's my issue: we've already done marble tile in the rest of the house, which divides the two areas of wood roughly in half. There are three transition areas where the marble is roughly 1.5" - 2" higher than the concrete the wood will go over. In order to minimize the height difference, I'd like to put plywood underneath the wood to bring the finished wood floor up closer to the level of the marble.

I'm thinking I can just line the concrete with 1/2" or 3/4" pressure treated plywood, then lay the moisture barrier over that and the hardwood on top. Any flaw in my plan? Should the plywood be glued to the slab (easier) or nailed (more difficult and time consuming) or both?

Any other tips for installation would be much appreciated as well!

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You really should not use pressure treated lumber indoors, but a moisture barrier over it would retard migration of any hazardous chemicals into the house air.

I once lifted a customer's floor with portland cement ceramic tile backer board. I had easily 3" difference between the two rooms. I layed down two layers of cement board and reduced that by slightly over 1". I staggered the seams of each layer. It was an inexpensive way to accomplish a lift with a product that was not going to support mold and mildew growth.

This is an 'unorthodox' solution, so 'user beware'...

    Bookmark   August 19, 2009 at 10:22PM
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Hmm... I would've thought backer board would wind up being more expensive, but that might be worth considering. How did you secure the boards to the slab?

As far as using plywood, I'd love to not use pressure treated but wouldn't it slowly rot if it was in direct contact with the cement slab?

    Bookmark   August 20, 2009 at 2:45PM
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Pressure treated plywood will rot, but alot slower than untreated.

I think I used Wonderboard for that one. I didn't fasten it down at all. It is somewhat 'flexible' and heavy, so it just laid flat on its own. The customer installed a floating floor over it. I did patch the seams with a cementious floor patch to even out any slight height difference between panels.

You can get a 4' X 5' panel of Wonderboard for about $10.00...that comes to 50 cents a square foot. In my customer's case, because we did two layers, it came to $1.00 a square foot, much less than a self-leveling underlayment would have cost for the same lift.

    Bookmark   August 20, 2009 at 9:09PM
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I think I would run this by the experts over at : and see what they have to say about it. They are there to help people just like you that know nothing and want to do things the correct way. good luck

    Bookmark   August 20, 2009 at 11:17PM
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Although I resent the implication that I know nothing, I'll check them out, and thanks. You're right though... all I know is what the guy who sold the flooring to me suggested, and I didn't trust a word coming out of his mouth. I'm fairly handy and have some much more handy people helping out, but any expert advice is much appreciated.

    Bookmark   August 24, 2009 at 1:52PM
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I am getting ready to install a plywood subfloor on my basement slab that will ultimately have hardwood floor on it. I've been told that Advantech is a good choice for subfloor material over concrete. Advantech technology mixes wax/parafin or something to that end in the board to prevent moisture from compromising the wood. It will also act as another moisture barrier below your floor and it should not break down.

    Bookmark   August 24, 2009 at 3:58PM
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The wonderboard sheets were 3' X 5', not 4' X 5' as I previously posted.

    Bookmark   August 25, 2009 at 9:56AM
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DLO, on your suggestion I got in touch with the local Advantech distributor last night. He wants $24/sheet for 3 x 5 sheets in 23/32, which works out to about $1.60/sq ft. That's a little out of the budget I've got for this project. Were you able to find it cheaper?

Also, waiting on a call back from the flooring manufacturer's technical department about their recommendations. Hopefully they'll have good news.

    Bookmark   August 25, 2009 at 11:44AM
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Dri Core...more expensive but designed for the purpose...

    Bookmark   August 25, 2009 at 12:10PM
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