Hello everyone, I'm installing some 16x16 Armstrong vinyl tiles in a small area in a small condo. The flooring guy had a layer of 1/4 inch plywood in his bid, so I was going to do the same, (I'm a carpenter and have done some flooring, but not this kind). He was using the plywood to bring the tile up so it would plane out with some ceramic tile in the bathroom. Today, I tore up the carpet and discovered a concrete sub-floor. I'm guessing it's only 1 1/2 inches thick because it's a 2nd floor condo and there are some 2x4s buried in the concrete. It reminds me of a gypcrete subfloor except the building was constructed in the 70s...? The concrete is very uneven. Is there a good way to attach plywood to this or should I use cement backer or just forget about it and put in a sloped transition between the two levels?