Q: How can I be sure that I have proper thinset coverage for the tile I’m using?
A: Tom Meehan, co-author of Working With Tile (Taunton Press, 2011) and a second-generation tile installer from Harwich, Mass., responds: General recommendations for thinset coverage for tile are 80% for most interior work and 95% to 100% for exterior work. For shower areas, the recommendation is for at least 95% coverage.
Certain types of tile carry their own coverage recommendations as well. For natural stone, that number is 95% to 100%. Glass tile always requires 100%. And as a general rule, err on the side of greater coverage, especially when you’re dealing with large floor tile. I’ve seen kitchen floors where the tile seems just fine—until the appliance guys roll a refrigerator across the floor, and a tile cracks because of inadequate coverage. The most important reason for getting consistent coverage at the recommended levels is that the thinset needs to support the tile evenly over its entire area.
The first step is using a trowel with the proper-size teeth for the tile you are installing. I always go over an area with the flat side of the trowel first to ensure a good bond with the substrate, and then comb a layer of thinset with the teeth, keeping the ridges going in one direction. Swirling or mixing the direction of the ridges can create voids that result in uneven coverage and support.
When I set each tile in place, I push it down with a slight side-to-side motion. Then I tap the tile down with a rubber mallet to prevent high corners or lippage. It’s a good idea to check the coverage as you progress through a project, so after installing a few tiles, slip a margin trowel under the edge of a tile and lift it. If the tile comes up too easily, you probably haven’t achieved proper coverage. After lifting the tile, check to be sure the thinset is adhering to most of the tile surface.
Corners are particularly tough areas to achieve adequate coverage on, and they’re also the most vulnerable part of the tile for breaking. I drag the corner of the tile through the fresh thinset or butter the corner with thinset to ensure that it will be well supported.
Don’t forget to wipe tiles down after cutting them on a wet saw. Even if you’ve applied the thinset properly, a wet surface can keep you from getting the proper coverage.
Photos by Roe Osborn
