subfloor over concrete


Old 12-10-08, 09:43 PM
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subfloor over concrete

I'm going to float some engineered hardwood over my concrete floor. There is some slight high/low spots on the concrete. Instead of trying to fill in/level the floor could I put down a vapor barrier and then cover that with 3/8 or 1/2 inch plywood nailed down with cement nails? I would then float the hardwood over the plywood. I would prefer the 3/8 inch since I don't want the transition to the tile in my bathroom to be to much.
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Old 12-12-08, 07:06 AM
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i don't see that "flattening" the unflat concrete. but i am no pro.
Old 12-12-08, 03:47 PM
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What good is the vapor barrier going to be, when you poke 32 holes per sheet of plywood, into it??

That idea has failed many, many times.

To do any good, your going to need at least 5/8 thick, or your going to need to flatten the concrete anyway. 3/4 is better.

The better thing to do, is to float the subfloor, too. Cut your plywood into 2 feet or 16" by 8 feet planks, and loose lay them in a staggered fashion, the opposite way you want the finished flooring to run. That way your moisture barrier is intact.

You could set Hardie backer into thinset, and let that cure flat, then cover it with your moisture barrier cushion combo.
Old 12-12-08, 03:55 PM
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Depending on height of high spots, you may get away with using self-leveling compound on the concrete to fill the low spots and smooth out surface to achieve flatness. Otherwise, high spots can be ground and low spots filled with the self-leveling compound.
Old 12-12-08, 09:37 PM
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Thank's for the info. I guess i'm going to spend some time,do it right and try to level the floor the best I can.

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