To prevent bleed-up when installing thin tiles, gently flatten the adhesive ridges with the smooth edge of the trowel.

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.

The coverage on this tile is uniform; unfortunately the ridge marks clearly indicate that not enough adhesive was spread to properly support the tile. This level of coverage ensures freeze/thaw damage, and problems with mold and mildew.

Tile adhesives are available in a variety of forms including organic adhesives or mastics, pre-mixed thinset, site-mixed Portland cement thinset mortar, latex thinset mortars, and epoxy thinset, which, unlike the others can also be used as a grout joint filler. I regard organic adhesive as a craft material useful for decorative, non-functional, dry-are installations, but inappropriate for most functional, wet-area applications since it does not form an adhesive bond with porcelain tiles, it cannot be used in thick cross-sections required for large tiles, and because, unlike older, solvent-based mastics, today’s latex-based mastics have a reputation for re-emulsifying when used in wet areas. I reserve the use of epoxy adhesive to those installations needing special chemical-resistance.

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.

With no voids to absorb moisture, this layer of latex thinset provides complete support for the tile, high adhesion, with a very low probability of freeze/thaw damage.

Overall, it is possible to arrange a performance ladder with dry-set mortars hovering at the bottom and the hyper-thinsets at the top, but just because a particular thinset falls into a certain category does not necessarily mean that it can offer performance expected of that category. The performance of each material must be judged on its own merits. When selecting mortar for a single installation, the extra cost between an OK thinset and the best is relatively insignificant; for an entire housing project, though, the extra cost can be significant. On the other hand, some premium thinsets offer more than just adhesion. For example, Custom Building Product’s MEGA LITE is rated for use with porcelain and glass, and it can also be used as a crack isolation mortar over floor cracks less than 1/8-inch. As an added bonus, this mix, which is packed in traditional 50-pound sacks, weighs only 30 pounds. The built-in carrying straps make it easy to carry two sacks at a time.

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.

By adjusting the angle, a single notch trowel is capable of spreading a range of thicknesses: the key to spreading is to hold the trowel at a consistent angle.

Instead of a chart, which can really only provide a rough guide to trowel selection, the right size notch can only be determined by an actual test of the tile against its intended setting bed using the specified adhesive. Many installers insist on using the least amount of thinset possible, but because surfaces are rarely uniform, its good practice, after the test, to select the next larger size trowel. Remember, the industry standard calls for a minimum 3/32-inch layer of adhesive beneath a tile after it has been set in place. Applying an excessive amount can cause the adhesive to ooze up into the grout joints. This is a problem with any size tile, but a cleaning disaster with mosaic tiles.

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.
It does take a bit of practice, but this technique provides a generous bed of adhesive without filling the grout joints with unwanted material or leaving a time-consuming mess to clean up.

This mortar board was assembled with plywood, scrap backer board, shims, and some screws, and it allows the installer to exert considerable down-force on the trowel. Hard-troweling this skim coat is one of the keys to achieving the highest adhesive grip.

Trowel selection is determined by the size of the tile, the type or brand of adhesive, and the condition of the setting bed. Small mosaic tiles are the most forgiving when it comes to the flatness of the setting bed surface while large tiles are the least. The industry standard of flat/level/plumb to within 1/4-inch in 10-feet works well with tiles up to 10-inches, but for attaining a smooth, lippage-free surface when larger tiles are installed, the setting bed surface should be incrementally flatter. For tiles over 16-inches, I always specify a self-leveling underlayment and back-buttering to improve adhesion.

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.

To minimize voids, select a trowel deep enough for the bonding task, spread in one direction only, hinge the tile down into the adhesive bed so that air beneath the tile has a route of escape, then move the tile sideways and back a half-inch. This tested method has shown to considerably improve adhesion and compressive strength.

Finally, thinset mortar has to cure properly for its anticipated properties to be achieved. This means keeping the tiles from direct sunlight and temperature extremes for the entire amount indicated by the adhesive’s manufacturer. Temperatures in the 70s (F) are ideal. Freezing temps cause ice crystals to form in fresh thinset; the accompanying expansion of the material can reduce the compressive strength to virtually nothing. High temps, and especially direct exposure to sunlight, cause moisture to be driven from freshly applied thinset mortar. With no moisture, the chemical process that changes powder to stone stops, and the thinset – and grout or other Portland cement product – never has the opportunity to achieve its anticipated strength. In the southwest, surface temperatures can quickly reach 120-degrees or more. At this temperature, even when installed under impervious porcelain tiles, exposure for an hour can completely strip all water from thinset mortar, the same being true for sheet membranes. Less direct exposure is required to ruin self-leveling underlayments, trowel-applied membranes and grout.

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.