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Old 05-29-2007, 05:52 PM   #1
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vapor barrier over concrete floor


Hi. My garage floor is concrete, and I want to put first sheets of plywood (screwed into the concrete) and then T&G wood flooring on top (nailed into the plywood). What should I use as a vapor barrier between the concrete and the plywood? I live in California, and at this point I don't know if we will ever get any rain again, so: do I even need it?
Grateful for any suggestions.

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Old 05-29-2007, 06:51 PM   #2
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vapor barrier over concrete floor


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Originally Posted by cricket View Post
Hi. My garage floor is concrete, and I want to put first sheets of plywood (screwed into the concrete) and then T&G wood flooring on top (nailed into the plywood). What should I use as a vapor barrier between the concrete and the plywood? I live in California, and at this point I don't know if we will ever get any rain again, so: do I even need it?
Grateful for any suggestions.
6 Mil Poly.

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Old 05-30-2007, 12:07 PM   #3
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vapor barrier over concrete floor


O.k., thanks, that sounds easy. But now the question is: Is it o.k. to screw down the plywood through the polyethylene, making wholes into it? Or would it be o.k. to just float the 3/4" ply and then nail the T&G into the plywood?
I would greatly appreciate more info on this, and thanks in advance.
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Old 05-30-2007, 01:35 PM   #4
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vapor barrier over concrete floor


FYI, even with the poly layer, a sheet of tar paper is generally recommended between the flooring and the wood planks also, and that tar paper does serve as a moisture retardent.
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Old 05-30-2007, 02:22 PM   #5
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vapor barrier over concrete floor


Tar paper helps with moisture a bit, but not a great deal. Anything you put nails through will loose a lot of it's ability to stop moisture. Tar paper is generally more to eliminate squeaks between the wood and the subfloor. Personally on concrete I'd use a floating system like Kahrs. It is a hardwood surface on a pine backing the clicks together and floats. It comes with it's own pad that eliminates noise and moisture concerns. Easier and faster to install that real hardwood as well. It also looks better IMO since it doesn't have the micro beveled edges prefinished hardwood has. Price may be an issue with a product like this though. Best part is, no nails going through the vapor barrier.
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Old 05-30-2007, 02:53 PM   #6
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vapor barrier over concrete floor


Thanks to both for your thoughts. I think the Kahrs system will be too costly. This is going to be a studio and I don't want to spend too much on it. This is also the reason why I am going to put down some sort of pine or spruce or fir instead of hardwood. I haven't researched that yet, but I have had very nice success with that kind of wood in my kitchen and bathroom, painted and varnished, and it looks very nice and kind of European. So, to Handy man88, should I put both the poly and the tarpaper under the plywood or the tarpaper between the plywood and the T&G? What about the nails going through the tarpaper into the plywood making holes through it?
I'd love to hear your answer or any other suggestions. Thanks.
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Old 05-30-2007, 04:08 PM   #7
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vapor barrier over concrete floor


You obviously need to put something between the wood and the concrete since you should not have untreated wood in contact with concrete that is in contact with soil.


Both tar paper and poly are vapor retarders and are cheap. Why not use both? There is certainly some merit in the thickness and make-up of the tar paper.
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Old 05-30-2007, 04:17 PM   #8
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I think there are multiple ways you can do this, and it probably depends on how much you want to spend and how much effort you are willing to put into it. Here are some options:

a) lay down poly sheet, screw down plywood over that, caulk over the screw holes, lay down your tar paper, and then install your T&G boards

b) lay down some 1x PT wood strips to be used as furring strips and screw (or glue) these in. Lay down the poly over the furring strips and then screw the plywood onto the PT wood (use stainless steel screws) and caulk over screws. Lay down tar paper, and then install your hardwoods.

You may have to go with option B if all you can find are 2 inch flooring nails or staples. Otherwise, if you're installing these long nails into the tongue of the hardwood, you may hit the concrete slab before the nail/staple can properly seat into the tongue.

Also, you will have a heckuva time predrilling into the slab and then driving in screws. Many, many of them. Gluing PT furring strips, if possible, would ease on that problem.

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Old 05-30-2007, 05:57 PM   #9
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If I put down poly AND tarpaper, couldn't I just lay the plywood on top (staggered of course) WITHOUT screwing it into the concrete and then nail the T&G on top of it at 90 degrees, with nails of a suitable length of course? It would be so much easier! And why wouldn't the plywood stay down? It is held down, after all, by the T&G crosswise nailed on top of it? And the double vapor barrier would protect it from the bottom! Please say yes. Thanks.
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Old 05-30-2007, 07:02 PM   #10
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If you're going to have the poly and tar paper back to back, that would be sort of redundant, and not necessary.

In any regard, I seriously doubt if your pneumatic or manual nailer will be able to drive those hardwood floor nails into the concrete, assuming you are using a floor nailer. If you are not, your life will be very hard if you want to drive a nail through the tongue with a hammer.

First of all, there is no grip on the nails to grip into concrete and second of all, the nails are probably too small in diameter to go through the tongue and then seat into the concrete.

You'll be banging on your nails so hard that you will probably slip and damage the hardwood planks. Those tongues on the planks are very fragile, and whenever I drive nails through those tongues on planks close to the wall, I always predrill.

Sorry, but in conclusion, I don't think it's possible.
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Old 05-30-2007, 07:06 PM   #11
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No, no, Handy man88, I meant the T&G just nailed into the plywood, not all the way down into the concrete. And the whole thing would be sort of lying on top of the vapor barrier. Don't they mean that when they say "floating"? What do you think?
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Old 05-30-2007, 07:23 PM   #12
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I believe there are roll on vapor barrier products that you can use on concrete and then use glue down flooring on top of that. At least I hope there are because I have a customer who had that done (not through me) not long ago.....hope it lasts. I was also told that there is glue for glueing down flooring that has vapor barrier in it.
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Old 05-30-2007, 07:28 PM   #13
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vapor barrier over concrete floor


Thank you, send-it-all, for your reply. Do you by any chance know the name of such a product? I have a feeling it would be very expensive, but I'd like to find out more about it. I'll try and google it too.
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Old 05-30-2007, 09:17 PM   #14
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Tomorrow, Ill try to call the flooring guy that did the job for my customer and ask what he uses.
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Old 05-30-2007, 09:38 PM   #15
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vapor barrier over concrete floor


That is great. Thank you very much.

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