Hi. I'm wondering if someone can give us advice.

We are installing Kahrs wood flooring (floating) over concrete subfloor in our family room which is about 3ft below grade. This has not happened yet due to problems with the subfloor preparation -- it is covered with black adhesive that contains asbestos and the leveling compound did not stick to it very well. My contractor put FlexBond on it, followed by latex primer then LevelQuik self-leveling compound. While the floor is close to being level and flat, the leveling compound cracked and de-bonded from the concrete in some areas of the floor. We can start over again and use a different compound that might stick better to the adhesive (Kahrs and this message board seem to recommend Ardex), but the problem with this solution is it might disturb the asbestos when scraping the cracked compound out (it sticks in some areas); also there seems to be no guarantee it too will stick completely.

The other option is nail 1/8" or 1/4" plywood over what we now have (using Hilti gun) and use this as the new subfloor (floor is more level now and so we will just flattened with roofing paper). But there are probably disadvantages to this. Is this a good idea? Are there other solutions?

Any help is really appreciated.

I don't like the idea of using wood to shim up low spots on a below grade concrete slab. I think I'd try using asphalt roofing shingles glued with an asphalt mastic so they don't shift. The asphalt mastic should adhere well enough to the old adhesive and you can layer up the roofing shingles as needed, gluing as you go.

You have to use a portland cement, that is engineered to bond to cutback adhesive residue. ARDEX SD-F is one that I know will bond to cutback residue. By residue, they mean a very fine film where the concrete looks like stained concrete and not black. You must wet scrape the cutback to get it down to the fine film residue, before compatible portland cement can be used and bond.

Perry Wright (floorguy) is right