Thinset on Back of Tile and Floor
If you look at many tile installations in progress, you may see the installer comb wet thinset on the floor in a defined area, usually marked off with chalk lines. The installer will then embed the tile into the thinset, give it twist, adjust for spacing, and call it good. In many cases, that’s all it takes to achieve a good bond.
However, with so many porcelain tiles on the market today, I’m finding that it’s increasingly important to apply thinset to the floor and to the back of each tile. This is referred to as “back-buttering.” Porcelain tile has a very low absorption rate (less than 1/2 percent) and as such does not easily lend itself to being embedded into thinset.
If you install tile for a living or are an accomplished do-it-yourselfer, you periodically check your coverage by embedding the tile in thinset, then pulling it up to check thinset coverage on the back of the tile. If you’re installing porcelain tile, upon pulling a tile up you may find inadequate coverage of thinset.
Of course, the obvious solution is to flat-trowel thinset on the back of each tile immediately before it’s set. Flat-troweling literally forces thinset into the pores on the back of the tile. This way, you have wet thinset on the floor and wet thinset on the tile, which gives you maximum coverage and bond.
Incidentally, a competent installer will also flat-trowel thinset onto the substrate as well before turning the trowel around and combing the thinset out with the notched side.
Here’s a couple of pictures from a recent job. My son is helping me on this one and I found out the day after this tile was installed that he failed to apply thinset to the back of these four tile. Of course they sounded a little hollow, indicating the lack of bond. They popped up very easily with a minimum amount of pressure from one edge. Note the complete lack of thinset on much of the tile backs.
The problem was repaired quickly and the customer is happy and satisfied – our ultimate goal was reached.