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Wood Floor, Concrete slab, and Moisture

Hi all, I had a question about proper installation of a wood floor on a concrete slab.

Assuming the slab has been moisture tested (and passes with flying colors), would it be alright to just put a plywood subfloor and finished wood floor on top? Or would I still be at risk of a mold issue in the future?

Reading about this online has pointed me in a thousand different directions. Some places say just to put tar paper under the subfloor, some places say I need a vapor barrier, some places say I need to float the wood floor, and some places say I need TWO layers of plywood subfloor in different directions. I'm really confused.

Then there's the issue of nailing through the vapor barrier. Am I correct in assuming that by nailing through it into the concrete, I am creating a weak point in the barrier for moisture to come up?

Honestly, I thought installing wood floors would be easy, but this whole moisture factor is opening up a can of worms....

Thanks in advance for any replies.

Asked by J S
Posted Dec 29, 2011 12:10 PM ET


6 Answers

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Hi Jay,
Providing the slab is flat (1/8" within 8' radius: self leveler can correct some deficiencies), I'd suggest installing a high quality floating floor, such as Kahrs. These are made to go over slabs: no additional subfloor required. The underlayment material that accompanies a float-in installation provides all of the waterproofing you'll need, as long as the slab stays as dry as it is now, which it should.
While floating floors are a bit more expensive on the front end than a traditional nail-down floor, in my experience the cost difference is more than made up by eliminating the need for all that additional plywood. They're also a lot quicker to install.
Good luck,
Chris Koehn
TimberGuides Design & Build
Vancouver Island

Answered by Chris Koehn
Posted Dec 29, 2011 12:29 PM ET


There are a lot of variables. Is your slab below grade or above grade?

Answered by Martin Holladay, GBA Advisor
Posted Dec 29, 2011 12:35 PM ET


I would install a poly vapor barrier on top of the slab - then install a high quality subfloor like advantech - we typically install it diagonally in the room - then felt or a product like aquabar and then your floor. A few nails in your vapor barrier will be ok.

Even if the slab test ok now - there is no telling what may happen down the road so I would not leave the vapor barrier out. we just had to tear our an enitre house of hardwoods that was installed over a slab that had been there for 20 years with no problems - out of nowhere, the hardwoods buckled and there was mold on the subfloor. No vapor barrier was previously installed.

Answered by Danny Kelly
Posted Dec 29, 2011 9:32 PM ET


Martin, the area is below grade (basement)

Thank you all for the replies. I'll start moving on with the project and see how the tests go.

Answered by J S
Posted Dec 31, 2011 1:36 PM ET


I don't think the testing is much use. First of all, how are you going to test? Tape plastic to the slab? Buy a test kit? Buy a Tramex meter? Buy a meter for ASTM 2170? Even if the slab seems fairly dry now, I would still stick with an engineered floating floor over a vapor barrier. A slab below grade is very unlikely to stay dry forever, even if it is dry now, so the more precautions you take the better. If it were my job I'd probably try to talk you into a porcelain tile floor.

Answered by David Meiland
Posted Dec 31, 2011 6:59 PM ET


Installing a wood floor over a basement slab is always somewhat risky, because many basement slabs are subject to occasional moisture-entry events.

You didn't mention whether there is any rigid foam under the slab. Ideally, your slab should be insulated.

Here is more information on how other people have install a plywood subfloor over a basement slab:

Fine Homebuilding: The Stay-Dry, No-Mold Finished Basement

Fine Homebuilding Q&A: Finishing a basement floor

GBA Q&A: Basement floor insulation retrofit

GBA Q&A: Floating plywood floor

Answered by Martin Holladay, GBA Advisor
Posted Jan 1, 2012 7:24 AM ET
Edited Jan 1, 2012 7:26 AM ET.

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