Although we are now in a cooler season, this summer was unusually hot for long periods of time in many regions of the country. There is not much we can do about the weather, but the associated heat can wreak havoc on tile installations if the installer is not careful.
The installation of thinset mortar on a substrate under normal working conditions and temperatures does not generally experience problems. But as the temperature rises, so do the number of problems encountered. Most likely, the majority of tile installed during the summer season is completed in areas that are not air conditioned. The installer must work in hot conditions which are difficult both for the materials being used to install the tile and for the installer. Tile installers must work faster to get the tile into the mortar before it skins over due to the heat and air movement, while at the same time battling heat-related fatigue. The solution that many installers choose is to place a box fan nearby, which does a great job of moving large amounts of air throughout the room, hence a cooler feeling. The problem is that the air rushing across the setting material pulls moisture from the mortar and causes a skin or film to form on the surface of the recently troweled mortar. When this happens, the mortar does not transfer to the back of the tile for a proper bond, as you can see in the attached photo. Testing the mortar with your dry hand gently resting on the ridges of thinset will prove that the mortar is beyond workability and needs to be replaced with fresh mortar.
Another problem created by the box fan on the jobsite is the air movement across newly applied grout. Again, this air movement causes premature drying, which results in the grout curing several shades lighter than the sample selected by the decorator or consumer. There are only two cures for this illness: remove the discolored grout and re-grout or apply a grout colorant to return it to the selected color.
The next time tile is to be installed on a hot day, do yourself a big favor and leave the fan in the truck.