A side view of tiles installed over concrete slab. Tile on left is properly adhered. Tile on right has insufficient thinset coverage at tile corner, plus voids beneath tile.

A side view of tiles installed over concrete slab. Tile on left is properly adhered. Tile on right has insufficient thinset coverage at tile corner, plus voids beneath tile.

All tile installations are composed of several basic elements including the tiles, the grout that surrounds them, the adhesive that secures the tiles, and the structure that supports the finished installation. Each of these components is important to the overall performance of the installation, but there is one that, well, cements the whole thing together: The tile adhesive. For tile installations there are two issues regarding adhesives. The first is the type of adhesive, and the second is how best to apply it. To understand how to choose and apply the most appropriate adhesive for each installation, it is important to know what functions are ascribed to tile adhesives in general.

Illustration #1shows a cross-section view of two tiles: one properly installed over a solid bed of adhesive, and another installed with minimal coverage over a void-filled bed of adhesive. For the best performance, a tile’s adhesive must thoroughly grip the setting bed and the tile with no gaps, omissions, or voids. Voids anywhere in the adhesive layer reduce an installation’s compressive strength, and can cause damage to occur to any tiles installed over the voided areas. Adhesive layer voids in wet area installations can harbor unwanted mold and bacteria, and make them virtually impossible to eradicate permanently. In cold climates, adhesive voids in exterior installations can fill with water and cause freeze/thaw damage to occur.

Apart from just attaching tiles to a setting bed, tile adhesive must also help to flatten the surface plane of the tile. Because of stresses that occur within the kiln, ceramic tiles are not manufactured perfectly flat, and, in fact, are sold with the understanding that there are allowances for flat and square that must be accounted for by a careful installation over an adhesive bed thick enough to tolerate the allowances.  Using the least amount of adhesive is the best way to produce a floor or wall whose surface is marked by excessive lippage. Many installers work hard to press each tile into the setting bed, and listen for the scraping of tile against setting bed, as an audible clue that the tile is properly installed. Quite the contrary, the best adhesion occurs when there is a minimum of 3/32-inch of adhesive (called a wet film layer) between the bottom of the tile and the top of the setting bed.

The main functions of the adhesive layer are: to support and affix the tile to the setting bed, to present the surface of each tile in a smooth plane, and to eliminate the presence of moisture between the tile and the setting bed. Not all tile adhesives are suitable for all tile installations. For example, organic tile adhesives (called mastics) are easy to prepare and apply, but most mastics cannot be used with porcelain tile because they lack the grab, most mastics cannot be subject to repeated immersion in water, many mastics are not compatible with stone tiles, and because organic mastics need exposure to air to cure and harden, most mastics should not be used for tiles over 10 inches. Water-based tile mastics also have considerably less compressive strength than thinset mortars: A major factor when deciding what adhesive to use on the floor.

Types of Adhesive

There are some dry interior areas where tile mastic can be used successfully, but in my opinion, thinset mortar provides greater strength, shorter curing and hardening times, and a more professional installation. For this reason, I have not included organic adhesives in this article. But not all thinset mortars are superior and unfortunately, the industry does not have any type of labeling system that would allow for quick comparisons. That said, there are several distinct types of thinset mortar.  At the bottom, in terms of overall performance (compressive strength, shear strength, and bond strength), is dryset thinset mortar, a combination of sand, cement, retarders and other dry powdered ingredients that are site-mixed with water to a spreadable paste. When properly damp cured (approximately 28 days), dryset thinset mortars are strong and unaffected by continuous exposure to water. They are also somewhat brittle and absorbent.

As a performance-enhancing additive for a wide variety of Portland cement products, latex (acrylic) is used, both wet and dry, in a variety of thinset mortars.

Latex thinset can be made by mixing dryset thinset powder with a recommended latex liquid. So-called polymer-modified thinset is a factory-made combination of sand, cement, retarders, dry-polymer latex and other dry ingredients that are site mixed with water. Additionally, there are a few modified thinsets that also use a liquid latex or acrylic component to replace the water used during site-mixing. Epoxy-emulsion thinset mortars use a base of sand and cement site-mixed with a resin/hardener combination that has special chemical resistance properties.

Another class of thinset mortars, called 100% solids epoxy grout and mortar involves the use of epoxy resins mixed with powder fillers, but this material does not use Portland cement as its base, and has its own particular texture, method of preparation and application.

There is not enough thinset mortar to properly support this tile.

Applying Thinset

Assuming a setting bed is perfectly flat, an installer’s first adhesive task is to determine what notched trowel provides the required minimum adhesive contact. The industry allows 80% minimum contact for interior dry-area installations, but it is a good idea to shoot for the wet-area minimum of 95% to maximize the effectiveness of the tiles and the installation.

This tile has good coverage across the entire underbody of the tile.

I have tried to develop charts that recommend what size to use, but chart results do not always account for actual jobsite conditions, so what I recommend for installers is to have a reasonable selection of notched trowels available; my job-site toolbox contains at least a half-dozen trowels for spreading thinset ranging from a 1/4-by-1/4-by-1/4 square-notch to a 3/4-by-3/4-by-1/4 U-notch; assess the surface conditions, and test the coverage on-site for each installation and setting bed surface.

Back buttering additional thinset may be required for hand-molded tiles.

Basing the size of the notch on only a small section of the floor is risky, so, because floors are rarely perfectly flat, I generally use a trowel that is one size larger. This gives me enough extra thinset to lift the tiles above any minor high spots, and results in a much smoother floor surface.


Selecting, mixing, and installing the most appropriate tile adhesive are critical steps for any tile application, but just because you have used the best materials and best application methods, the tiles may fail if the adhesive is not allowed to cure properly. Some thinset mortars will cure to a ready-to-walk-on hardness in an hour or two, but some may take as long as 72 hours or more before grout can be applied or the tiles are safe for traffic. Make sure you follow all the instructions for the materials you use to get the longest possible life out of the installation.