The adhesive bond between a tile and its setting bed is fundamental to the durability and ease of maintenance of any installation. For dry area installations, 80% uniform adhesive contact is the minimum industry standard. For wet area applications, the minimum is 95% uniform coverage. In the case of food preparation or service areas, the minimum 95% adhesive bond also has an additional, very specific function: to minimize voids that could fill with moisture and harbor mildew, mold, and unwanted pathogens. A tile’s adhesive does more than just affix a tile to a surface. It can help (to a moderate extent) to improve a less-than-perfect setting bed surface, and it is the third barrier – after the tiles and grout – against moisture penetration.
Tiles may have a remarkably tough surface, but unless they are properly supported, they are prone to breaking and are anything but tough. On the other hand, I have been to tile installations so old that the tiles are almost completely worn away. Not broken or cracked or falling off the setting bed, but worn away after thousands of years of foot traffic. The key to such long life is a secure bond between the tile and the setting bed, and with beheading or amputation the reward for sloppy workmanship, it is no wonder quality was always “Job #1” in the old days. Unfortunately, a walk through many shopping malls reveals the most common example of poor adhesive application: broken tile corners. Often, especially when a grid of layout lines is used to position the tiles, some installers find it difficult to spread adhesive near close enough to the lines (especially in corners), and the broken tile corners are the result. For best results and longest life the 95% coverage minimum should be extended to all installations.
Dry-set thinset mortar, when mixed, applied, and cured properly, provides excellent performance when used over a stable base; the trouble is, today’s buildings are anything but stable. For this reason, I use dry-set mortar primarily as the manufacturer’s recommended support plane for tile backer boards and for some other installation products (Schluter-Systems’s KERDI sheet membrane, for example, calls for lamination with dry-set mortar). For all other uses, I prefer latex thinset for its increased flexibility and strength, and for its minimal curing requirements, but not all latex thinsets are alike. At first, latex meant a liquid that replaced the water used to mix dry-set thinset powders, but now, latex includes liquid latex (natural rubber) additives of various types and strengths as well as acrylic liquids (test tube rubber), dry polymers that are factory-added to thinset powders and require mixing with plain water, and ultimate-quality dry polymer thinsets that are mixed with a liquid acrylic.
Installers are always asking for a magic chart that will tell them what size notch to use, and I have tried to develop such charts in the past because many installation failures are caused, not by the wrong adhesive, but by not enough. For many installers, prying their fingers away from their beloved 1/4-inch notch trowel seems like an impossibility even while tile sizes get larger and larger.
Yes, a notched trowel is a great way to distribute an even layer of adhesive, but when the trowel is not used properly, or when the height of the notch approaches the thickness of the tile, those even rows of adhesive ridges can be the installer’s worst enemy. Instead of installing mosaic or thin tiles over ridges, use the following approach:
- Test for the right trowel, then spread an adhesive bed with a trowel one size larger than indicated by the test.
- Re-comb to ensure there are no excesses or short-shots, and then use the flat side of the trowel to gently flatten the adhesive ridges.
By back-buttering, I don’t mean slapping a blob of thinset on the back of the tile; I mean carefully applying an even layer of material on the tile back. This is best done with a mortar board to support the tile. Back-buttering is especially important for large tiles whose backs are configured with ridges or cavities, or when tiling in hot, windy conditions, and it should always be used as a companion to spreading adhesive on the setting bed. Regardless of whether the back of the tile will be covered with a thin skin or a thick layer of adhesive, the process begins by hard-troweling a thin layer of thinset on to the back of the tile, and building up that layer with the notched side of the trowel. As soon as the tile is prepared, it should be set into the setting bed’s adhesive layer: this provides the best possible adhesive bond.
Ultimately, it is the strength of the adhesive bond that determines an installation’s longevity. Grout can erode for many reasons but the adhesive must be able to endure. Eliminating voids in the adhesive layer is one of the keys to durability and performance.