We hired a contractor to remodel our bathroom in February. We already had 6x6 white ceramic so that was torn out. The mud bed beneath this was kept for the new floor. Hardibacker was put down over the mud bed and screwed down at the seams every 6". The Hardibacker was put down in alternating pattern so that there were no continuing seams. I don't think the seams were taped. I don't think there was any mortar added to Hardibacker. The 8x16" travertine tile was laid down in a running bond pattern. Bathroom floor area is about 8x12 at widest points and extends into a small water closet room. The tile first cracked at the water closet door from, running from the frame diagnally across the tile end and opening up the epoxy grout (Laticrete). Then outside that door and in the main bathroom, we have noticed other tiles cracking, horizontally across tiles, all of which cover a span of about 2-3 months when we noticed the first crack and continuing on to now at a 6-month timeframe. We first heard "popping" tiles when we would walk on certain ones. It's almost as if there were not enough mortar put on the tiles (buttered), but that's just a guess. Before we call the contractor, we want an idea of what is causing the problem and to be prepared for a discussion with him. I expect it won't be pleasant. We have a lot of money in the tile, install, and certainly the expoxy grout. Any ideas and suggestions are greatly appreciated.

If you had a mud floor already why was the hardibacker installed? Was the origional mud floor uneven, or was residue left from the past floor that was installed on it and the backerboards helped remove this rough surface?
The only reason for this to crack is that something is moving under the hardibacker boards. Movement = cracks.

The boards should have been properly set into the floor with a cement thinset base to "glue" them down. If the boards were set without any mortar under them there is most likely gaps under the boards which are allowing this movement to take place.

The biggest issue here is that the floor will require removal to fix properly. Unless you have a way to inject some sort of apoxy through the boards where the tile was when removed that cracked. This may fill the few voids under the backer boards and stop any future movement in that area. But it may be a lot of work as other cracks may develop in other areas at time moves on.

Hi, and thanks for your reply. First I must correct some of the information I provided. The mud base was removed when the ceramic tile was taken up. There are 2, 1/2" plywood boards, then the Hardibacker is screwed into the plywood. Next is the layer of thin set that looks to be about 1/4". This morning we identified 9 tiles that are cracking--so far. I had assumed there is some movement below the tile, but there is no bounce--thus we just don't know what is moving. I also expect, as you say, that more tile will crack. The plywood is the existing subfloor that was put down when the house was built. The Hardibacker is 1/4". Thus, with my correction of the information, I think the point is that something is moving to cause the problem. I don't know if there was any glue put down between the Hardibacker and the plywood before screwing everything down. How do we assess the problem when we call the contractor back? Would it be reasonable to take up a few tiles just to look (at what I don't know), or is it obvious that the entire floor needs to be taken up and replaced? We believe that the problem is probably improper install and that we should not have to pay any more. Just as another note, we had 500-600 sq. feet of travertine installed in our kitchen, laundry, and back hall area using a 5-piece pattern (Versailles) with very large tiles. There is no cracking in any tiles, and there is much more traffic in that area. Thanks again for your help and any advice.