Many times installers are asked to install exterior tile on a job that appears to be elementary. Unfortunately, some of them develop issues at a later date. Take for instance a job calling for a natural stone to be installed on the exterior wall of a new retail store which has a roof above it. The plans call for the stone tile to be installed over a waterproofing membrane coating the cementitious backer board on steel studs. It sounds routine and it should function well; which it did in this case, for about a year.
As time passed, the owner began to see a white powdery substance growing on the grout joints. Immediately the call goes out, “There is something wrong with the grout.” Upon investigation, the grout had not failed, but a naturally occurring mineral salt which is present in all Portland cement products (thin set mortars and cement grouts) had found its way to the surface. This salt deposit is called efflorescence. Normally, these residues can be washed away with a mild acid solution and a bristle brush while being careful not to harm the stone. In this case it worked…. for a little while. Continued scrubbing removed the salts, but it kept coming back. What was the cause?
For efflorescence to occur on the surface as seen in the attached photo, water must be present to carry the minerals to the surface, evaporate and leave the salts behind. You might guess, “It was the rainwater going through the grout joints.” The rain could have been the culprit, but not in this case. Remember, there was a roof over the tile. The problem was actually caused by the roofer who failed to properly install the metal cap flashing on the parapet wall above the roof. The water went through the defective flashing, down through the thin set mortar to the lower portion of the wall and blossomed on the grout joints.
Once the tile installer (now a forensic specialist) found the problem, the roofer made the necessary repairs, the tile installer washed away the salt deposits and magically the efflorescence disappeared. What is the key point? Don’t make guesses. Find the source of the problem and solve it.