Originally Posted by Bud Cline

If the above is a reference to Custom's "Easy Mat" - Forget it.

Originally Posted by Daniel Holzman

This is an interesting, fairly common case. I used to do forensic investigations on structural damage, and I looked at about half a dozen tile failure problems in the southeast.

In cases where the slab has seriously cracked, you are most likely to get a large crack in the tile. Specifically, what you have appears to be a non-displaced hairline crack. The next size up in the world of cracks is a crack approximately the size of a credit card. You are nowhere close to that, so I discount the theory that the slab is significantly cracked. By the way, all concrete slabs crack, the issue is how much, and how significantly, they are cracked. At this point, I would say there is no evidence that your tile crack is due to slab failure.

I observed two cases where a "tenting" type failure, as noted by Bud Cline, had occurred. In both cases, there was an obvious hollow zone in the center of the floor. The tiles had literally detached from the thinset. In one case, the failure was traced to an improperly mixed batch of thinset, which had not properly adhered to the tile. In the other case, the failure was traced to dimensional changes in the house itself due to moisture changes, resulting in stress to the tiles because there was no relief gap along the edges of the tile. The result was failure of the tile at the center.

In one other case I examined, several tiles cracked because water had leaked onto the floor, and had remained there for several days. Apparently the water caused dimensional changes to the grout, leading the tile failure in a localized area. The apparent cause of the failure was that the grout was stronger than the tile, thus when the grout expanded, rather than cracking the grout, the tile cracked.

I don't see any evidence of slab failure in your case. Due to the length of the crack, and the linear nature, there may be a pipe of some sort embedded within the slab.

I like the use of Ditramat as a bond breaker between the slab and the tile, it certainly is a good product and will minimize the potential for future cracking, but may not eliminate the potential entirely. There are other products that purport to act as bond breakers, however I am not familiar with them, and cannot comment on their effectiveness.