Installing a Fir Floor Over a Dirty/Rough Concrete Slab

Old 10-31-06, 02:22 PM
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Installing a Fir Floor Over a Dirty/Rough Concrete Slab

I'm planning to install a T&G fir floor over a concrete slab. The concrete slab surface is rough (broom-swept) and dirty; it was a garage floor in a past life.

I've read the previous posts describing all of the precise leveling & surface cleaning required prior to installing T&G fir flooring directly over the concrete slab. My concrete slab surface is in such poor condition that, I fear, I might not be able to achieve the required smooth, clean surface.

Why couldn't I just level the concrete slab surface to +- 1/8" in 6 feet radius -- as described in the previous posts -- and then use an adhesive to install 4x8 sheets of 1/2" plywood subflooring directly over the concrete slab? Then I could simply nail the fir flooring to the plywood subflooring?

The advantage of this approach is that I wouldn't be forced to create a perfectly smooth and perfectly clean concrete-slab surface -- an impossible task, I think, given the poor condition of the existing concrete slab.

Will this work?

Old 10-31-06, 03:11 PM
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Concrete needs to be dry and flat. Moisture test should be done. High places can be ground down. Low places can be filled with self-leveling compound.

Solid hardwood can be installed on concrete above grade. A vapor retarder is required. There are two methods recommended for 3/4" hardwood over slabs--plywood over concrete or sleeper system both with vapor retarders. Cut-back asphalt mastic can be used to hold a recommended vapor retarder and/or to glue a plywood sub floor to a slab.

Go to and click publications to download for free the National Oak Flooring Manufacturers Association technical manual for hardwood flooring installation.

When using adhesive over concrete, there will be adhesion problems if there is oil, grease, tar, sealants, moisture, etc. Grinding surface to remove sealants is recommended. There are masonry cleaners available. Pressure washing may also be helpful. Strippers are not recommended because they can cause adhesion problems.

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