For most tiles, the recommended procedure for spreading adhesive is to use the flat side of the trowel to key the adhesive into the setting bed (Figure 1).


Figure 2: Use the notched side to apply a uniform amount of adhesive into the setting bed.

Selecting the right tile adhesive, and applying it carefully, are two keys to a durable, long-lasting installation whether you are working with ceramic, glass, or stone tiles. Grout is important because it protects tile edges, but the adhesive layer is even more important because it holds the tile in place, supports it from below, and eliminates voids.

Support is critical for all installations, especially those that have to withstand heavy loads or constant foot traffic. Voids in the adhesive layer can cause tiles to crack or collapse when loaded, and on exterior installations in cold climates, moisture can collect in voids and cause freeze-thaw damage.

On exterior or wet-area interior installations, voids that collect moisture make excellent breeding grounds for mold and mildew which are extremely difficult to remove. Selecting the wrong adhesive, or applying it incorrectly not only wastes the adhesive, but it can also cause damage to the tiles.

Organic Adhesives

There are two main types of tile adhesive: organic adhesives and thinset mortars. Organic adhesives, called mastic in the trade, are pre-mixed adhesives that, in my opinion, are better suited for non-functional installations such as lightweight mosaics installed on a dry, interior wall. When first entering the trade, I used solvent based adhesive for interior floor and wall work and found it to be adequate for most residential applications. As well, the solvent-based mastics I used were unaffected by moisture or water.

However, the trade-off for strength and ease of use was the industrial high that came with breathing in the fumes. Today, most organic adhesives made for use with tile are latex-based and produce little or no harmful fumes, but the trade-off is that latex-based adhesives are prone to re-emulsification: they soften or dissolve if exposed to water. For this reason alone, organic adhesives should never be used for exterior or wet-area interior applications. 80% uniform contact is the industry standard for adhesives used on dry-area floor or wall installations and to achieve that, a 1/4 x 1/2 x 1/4-inch (or larger) U-notch trowel must be used for tiles 10-inches or larger. When applied this thick, a latex-based organic adhesive may not completely cure, and, in fact, may remain soft enough that it offers little or no support for the tiles installed over it: latex-based organic adhesive should only be used on tiles less than 4-inches, and only on non-functional installations.

Figure 3: To help minimize adhesive squeezing up into the grout joints of thin tiles, I use the flat side of the trowel to gently flatten the notches.

Thinset Mortars

Forty years ago, there was only one type of thinset mortar called dry-set thinset mortar. It was, and still is, called dry-set because when it was used as the adhesive, tiles did not have to be soaked in water before they could be installed. Today, dry-set refers to Portland cement thinset mortar made without latex. Dry-set thinset mortar is quite strong compared to latex thinset mortars, but it needs to be cured carefully so that it does not completely dry out quickly – this can significantly reduce its strength. Dry-set thinset mortar is usually mixed with water, but it can also be mixed with a liquid latex for more strength and a simpler cure since latex tends to hold moisture longer than dry-set. This is a good thing, but it can work against you if your installation calls for porcelain tile. Latex thinset mortar is recommended for porcelain tiles, but since porcelain is impervious, latex thinset needs extra time to dry. As stated above, latex can be added as a liquid, or it can also be added – at the factory – as a powder called a dry polymer. Besides convenience, there is less weight to truck around and carry.

With large-format tiles more commonplace, thicker adhesive beds are required to provide the required 80% uniform coverage for dry-area installations, and 95% for exterior and wet-area interior installations. Generally speaking, when the adhesive layer needs to be thicker than ¼-inch (produced by a ½-inch notch trowel), a medium-bed thinset mortar should be used. As the name implies, medium bed thinset falls between thin and thick bed, and it differs from dry-set or latex thinset mortars because larger grains of sand are used to build strength and give the mortar body.

One thinset mortars that is not made with Portland cement is 100% solids epoxy mortar which is often used not just as an adhesive, but as grout. Depending on the manufacturer, epoxy thinset mortar has a wide range of properties unique to each brand. I use 100% solids epoxy mortar and grout solely to install some green marble and other stone tiles that react with water: the bodies of such tiles may warp or distort. When a non-reacting tile cannot be substituted, 100% solids epoxy adhesive should be used.

The latest development in thinset adhesives is crack isolation thinset mortar, which has two functions: adhesive and crack isolation. Crack isolation thinset mortar costs more than dry-set or latex thinset mortars, but in balance, it eliminates the need for a separate membrane and its installation costs. There may be a minimum thickness that must be applied to a setting bed to achieve the desired results, so check each manufacturer’s specific instructions before using.

Figure 4: When installing tiles 10-inches or larger, thinset mortar should be spread in one direction only to prevent air from being trapped between the tile and the setting bed.

Applying Adhesives

The success or failure of a tile installation can hinge on how well the adhesive has been applied. Tile industry standards require 80% uniform coverage for dry-area, interior applications, and 95% uniform coverage for wet-area and exterior applications. Uniform means that the adhesive covers the back of the tile evenly and without excessive voids that could cause cracks to appear in the tiles. To achieve a uniform layer of adhesive, notch trowels are used. A notch trowel selection chart may be found printed on the back of most sacks of thinset mortar, but the first step I use to determine the best trowel is to use this rough rule of thumb: ¼-inch square notch for tiles up to 4-inches, 3/8-inch square notch for tiles up to 10-inches, ½-inch U-notch for tiles up to 12-inches, and larger notches for tiles over 12-inches. Next, I try out a trowel on the job site setting bed with the adhesive and tiles selected for the installation, and use a larger or smaller notch to get the desired coverage.

Figure 5: When installing tiles with a patterned back, the cavities should be filled with thinset mortar prior to setting them in the mortar spread on the setting bed.

For most tiles, the recommended procedure for spreading adhesive is to use the flat side of the trowel to key the adhesive into the setting bed (Figure 1) and the notched side to apply a uniform amount (Figure 2). To help minimize adhesive squeezing up into the grout joints of thin tiles, I use the flat side of the trowel to gently flatten the notches (Figure 3). When installing tiles 10-inches or larger, thinset mortar should be spread in one direction only to prevent air from being trapped between the tile and the setting bed (Figure 4). When installing tiles with a patterned back, the cavities should be filled with thinset mortar prior to setting them in the mortar spread on the setting bed (Figure 5). This technique of back-buttering is not just useful for hollow-backed tiles – it increases the bond strength of any tile. With particularly large tiles that require more adhesive than usual, I use a notch trowel to increase the back-butter layer of adhesive. For best results for tiles installed in wet or dry areas, I aim for a minimum 95% coverage.