Welcome KP,

I'd guess the subfloor. Tile definitely benefits from having a really solid subfloor. It's extra trouble and expense, but the use of concrete board under tile really helps. I'd guess they laid tile right on the plywood.

Is this a brand new house? If so there's probably a warranty. Hopefully someone else will come out with better ideas, but I'd say it might have to be re-done....good luck.

-k.

Had the exact problem in my home (new construction. The builder refused to do anything about the problem-had to sue him. We were told by the independent flooring expert that we hired that the underlayment was inadequate both in material and depth;the mortar used was not mixed with the proper latex additive; the underlayment was improperly affixed to the subfloor (they used staples instead of screws). We had the entire floor removed 600 sq.ft. and replaced. The good news was that it was so poorly installed we did not damage the subfloor when it was torn up. Hope you do not have the same luck that we had with our builder. A reputable builder/installer would have fixed the problem at their expense.

Interesting, LCP.... I'd heard this before--that this is a rather common builder shortcut. It's unfortunate, as there's really not much hope of having it hold up for an extended period of time. It's such a waste, too--a waste of good tile, and a waste of time to deal with it later. Not to mention court time... Hopefully they used concrete board when they fixed it. A good tile job really should hold up for years--if not centuries!