ASK THE EXPERTS QUESTION We found stress cracks in the floor several days after the tile was installed. I had never seen anything like this in my 40 years of being in the trade, and neither had my tile man. They showed up as slightly wavy white lines in the marble. There was a black line or crack you could feel. We removed a couple of tiles where two stress cracks intersected. The tile was well bonded to the 1/2” cement board below the tile. There was no intersection of sheets of cement board below the crack. The cement board looked sound. We removed the cement board to expose the tongue-and-groove ply- wood subfloor below. Where the cracks intersected in the floor was exactly where a sheet of plywood stopped and started.The cracks lined up with long 8’ run and one 4’ run. So, somehow, whatever movement took place in the sheet of plywood transferred up through the cement board to the tile. There were no cracks in the cement board and it was firmly bonded to the plywood. We took up all the tile and cement board. We checked the subfloor with a laser to see if the floor was showing any signs movement. The entire floor was within 1/8” of level everywhere we checked. There were no ridges in the plywood where the sheets butted together. The only suspi- cious thing I found was an area along the 8’ side of the plywood where the tongue of one sheet was not fully engaged in the groove of the other. This meant that there could be more flex in the plywood than there should have been. We removed the sheet of ply- wood and checked the framing. The new joists we sistered onto the existing joists were 2” x 8”. They were nailed soundly to the exist- ing 2”x 10” that were all over 2” thick. These joists spanned from the living room wall to the exterior wall. They also pass over the wood columns and beams that divide the stair well from the entry hall. The floor platform was really strong and robust. The plywood had been installed with construction adhe- sive and ring shank nails. It was fully bonded to the framing and very difficult to remove. We could not see any good reason why the tile came to show the stress cracks. The only reason that I could come up may have had to with using adhesive to bond the plywood to Sponsored by 20 TileLetter | February 2018