We purchased a house that had a subfloor placed over a concrete slab. The previous owner had placed 2x4s flat on top of roofing tar and nailed 3/4" plywood on top of that for flooring. I want to remove the subfloor and install tile over the concrete. We have to heat the tar to take off as much as we can, it is very thick and uneven. Can we use thin set on top of the remaining tar? It is impossible to remove all of the tar, it is smeared across the entire slab.