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Old 06-14-2012, 06:56 PM   #1
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Hardwood over concrete slab


Hi

(Re-posting as a new topic.)

I want to install hardwood (American walnut, 3/4" by 4") in my condo and I have a concrete slab. I have done some research on various sites, so I have read about the old recommended methods that used asphalt, as well as some more recent methods that use two layers of plywood. I have also read a lot about hardwood not being recommended on a concrete slab by some people.

Here is the procedure I have come up with after doing my research. I would like to hear some opinions before I start.

First, I will put a layer of polyethylene sheet, 6 mil, on the concrete slab, as a moisture barrier.

Second, I will put two layers of 3/8" plywood, with the individual sheets staggered. The bottom layer is CDX. The top layer is similar but the upper face is sanded (it costs slightly more but will probably make the work a little easier). I will staple or screw the two layers of plywood together, taking care not to puncture the underlying polyethylene sheet. The two layers of plywood will cover the entire room as if they were a single large, heavy block, floating over the slab. The plywood will not be nailed to the concrete.

Finally, I will nail the hardwood to the plywood using 1-1/2" L-cleats.

Comments?

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Old 06-14-2012, 09:55 PM   #2
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Hardwood over concrete slab


Sounds like a terrible idea to me. What does the manufacturer say about this type of installation? It almost always is a bad idea to attempt 3/4" hardwood when concrete is involved.

Jaz

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Old 06-14-2012, 10:08 PM   #3
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Hardwood over concrete slab


Moisture will accumulate under the plastic and grow mold.
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Old 06-14-2012, 10:13 PM   #4
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Hardwood over concrete slab


For going over concrete this is the method for solid hardwood. It sounds like you have it pretty well figured out.
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Old 06-15-2012, 05:05 AM   #5
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Hardwood over concrete slab


Defiantly get the product recomendations 1st.
Is this a raised slab? Get a moister test done 1st.
I wouldn't use cdx, maybe advantech T&G but not 2 sheets you might as well just raise the floor with pressure treated 1x4s then your ply.
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Old 06-15-2012, 05:35 AM   #6
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Hardwood over concrete slab


Use engineered flooring instead!
Look into it, it's even below grade rated, far more stable then soild wood, same look because it's real wood on the surface, can be refinished.
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Old 06-15-2012, 07:12 AM   #7
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Hardwood over concrete slab


JetSwet:

Thanks for the advice.

To answer your question, the top surface of the slab is one foot above grade (not less).

What's wrong with using two sheets of plywood? Using 1x4s would raise the floor even more and would require more work than using a second layer of 3/8" plywood. Also, if I used 1x4s, where do you suggest I should put the plastic sheet?

Last edited by alessandrot; 06-15-2012 at 07:22 AM.
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Old 06-15-2012, 07:32 AM   #8
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Hardwood over concrete slab


Quote:
Originally Posted by alessandrot
JetSwet:

Thanks for the advice.

To answer your question, the top surface of the slab is about 1 to 2 feet above grade.

What's wrong with using two sheets of plywood? Using 1x4s would raise the floor even more and would require more work than using a second layer of 3/8" plywood. Also, if I used 1x4s, where do you suggest I should put the plastic sheet?
Forget about what I said I was giving idea if u were below the grade.

If your that high off the grade you will be fine with your plan.
Just put one layer of ply down your angled nails go in on a angle not straight down.



Hardwood over concrete slab-image-2694662786.jpg

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Old 06-15-2012, 07:51 AM   #9
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Hardwood over concrete slab


JetSwet:

Thanks. If I understand your picture correctly, the plastic sheet goes between the concrete and the plywood and the roofing felt goes between the plywood and the hardwood, right? I had not considered the roofing felt, but I will certainly add it if it's useful.
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Old 06-15-2012, 08:45 AM   #10
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Hardwood over concrete slab


Quote:
Originally Posted by alessandrot
JetSwet:

Thanks. If I understand your picture correctly, the plastic sheet goes between the concrete and the plywood and the roofing felt goes between the plywood and the hardwood, right? I had not considered the roofing felt, but I will certainly add it if it's useful.
Yes that exactly how it goes. You got it!
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Old 06-15-2012, 01:40 PM   #11
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Hardwood over concrete slab


Quote:
Originally Posted by JetSwet View Post
Defiantly get the product recomendations 1st.
Is this a raised slab? Get a moister test done 1st.
I wouldn't use cdx, maybe advantech T&G but not 2 sheets you might as well just raise the floor with pressure treated 1x4s then your ply.
Pressure treated lumber is never to be used indoors.
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Old 06-15-2012, 02:15 PM   #12
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Hardwood over concrete slab


I would follow the instructions from the mfg before I did anything to see what they recommend so the warranty will be supported. Otherwise, you are on your own if something does south.
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Old 06-15-2012, 02:41 PM   #13
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Hardwood over concrete slab


Quote:
Originally Posted by rusty baker
Pressure treated lumber is never to be used indoors.
Oh please. It's fine if your worried about termites can go with the borate lumber but this option is ruled out anyway.
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Old 06-15-2012, 07:56 PM   #14
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Hardwood over concrete slab


By doing it your way your going to throw off every baseboard, door opening, and where it meets any other flooring.

Why in the work would use you 2, Layers of 3/8's. If you insist on doing it your way use 3/4" Advantec sub flooring.
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Old 06-15-2012, 08:13 PM   #15
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Hardwood over concrete slab


Quote:
Originally Posted by joecaption
By doing it your way your going to throw off every baseboard, door opening, and where it meets any other flooring.

Why in the work would use you 2, Layers of 3/8's. If you insist on doing it your way use 3/4" Advantec sub flooring.
Advantech yes but I dont even think 3/4" is necessary. 1/2"?.... Although the nails might not hold so your right Joe. Nevermind lol

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