Okay, here is a head scratcher for you tile guys out there:

About a year ago I did a kitchen job where we installed 18″x18″ porcelain floor tiles. The subfloor is 3/4″ CDX plywood, glued and screwed to the joists. On top of that, we installed an electrical radiant heat system (fiberglass mesh with embeded wires, stapled to the subfloor). On top of that, as per manufacturer’s instruction, the tile installer floated a layer of thinset (specially formulated for plywood subfloor application -” Versabond” brand.) About six months ago I got a call from the client, that a couple of the tiles were coming loose. Sure enough, the tiles sounded hollow, and came up with no resistance. The odd part was that they came up practically clean, with almost no thinset stuck to the back. The thinset on the floor was still strong and sound with no visible cracks or any other deterioration. I personally saw the tile installer butter the back of the tiles before installing them, so I was quite surprised to see the lack of bond. Now, almost all the tiles sound hollow and we have to pick them up and re-lay them. Could the radiant heat be the culprit? But then again, some tiles are not over the radiant heat area, but still coming off. Can anybody figure out what the problem might be?

Thanks

Kroozer