QUESTION

Cracked Grout Joints - I just had tile installed and I live in a raised house. I had family members say they knew how to install and didn't think into it much but I think they did it wrong. We put it in the kitchen and we got the wonder board that is cement about half inch thick (we bought the thicker stuff at home depot) and we got the thin set stuff and the grout. Now the grout is cracking (looks like only in the spots where the wonderboard meets) I really hope we didn't screw this up too much because we already installed cabinets over the tile. Is there anything we can do about this? I think they left a gap at the corners but we never went back to fill the gaps in the corners so I hope that has nothing to do with it.

ANSWER

ANSWER - It is hard to make any definitive conclusions without inspecting your floor thoroughly.  When there is a problem it is normally not for one single thing that has gone wrong, but for compounding reasons.  I would guess that the problem is the subfloor may not be adequately supported and may have too much deflection within it.  Plus the Wonder Board Cementitious Backer Board (CBU) is probably not properly installed.  It should be nailed every 6" to 8" with the proper fasteners, and each board placed in a staggered position to the adjacent board.  The joints of the board filled with thin-set.  Since it is likely cracking at the CBU joints all of those conditions could contribute to that problem.  Since the grout is cracking and not the tile, then hopefully the tile is substantially attached to the floor.

The only way to properly fix the problem is to remove, repair, and replace.  Although since only the grout is cracking, you can try to fix the symptom of the problem, but there is no guarantee it will work.  Just replace the cracked grout with a pure silicone or urethane sealant in a complementary color that meets ASTM C920 and has a Shore A hardness of 35 or greater.  Do not use one of those latex or acrylic caulking, because it won't last.  Make sure you read the instructions on the sealant.  Remove the grout and clean the joint thoroughly, and then put masking tape along the tile edges.  Fill the joint with sealant, tool it, and pull the tape right away.   Good Luck.