To install tile over an existing surface, whether it is wood or concrete, often requires “something” to be placed on that surface to allow for a quality tile installation. There may be an inappropriate subfloor, an unbondable surface, an out-of-flat or out-of-level floor or wall, and the list goes on. To meet the ever-growing and demanding jobsite requirements, that “something” most times is called an underlayment. However, the selection of an underlayment can be a daunting task, considering the number of product categories in this sector as well as the number of products within each category.

Underlayments may take the form of backer board, trowel-applied patch, self-leveling, uncoupling membranes and even a mortar bed. These products function well, but one crucial issue needs to be addressed. If the surface to be tiled is deficient, most times the tile installer is called on to “fix” the issue before it becomes a problem. The problem occurs when the tile placer (I won’t call them installers or mechanics) uses foolish judgment and doesn’t correct the problem while it is a non-issue. Done correctly, everyone is happy. Done poorly, the tile industry gets another black eye in the court of public opinion. 

We will look at each category separately to better find the optimum choice for the job.

Backer board

Most of the products in this group are available in ¼- and ½-inch thicknesses and serve as an excellent bonding surface. Both thicknesses may be used on interior floors, while wall applications always require a ½-inch board. However, this is where the similarities cease. Only cement backer board may be used in an exterior wall application (TCNA Handbook method W244E-12). Cement backer board and Cementitious-Coated Extruded Foam backer board are the only two in this group that can be placed into the mortar bed of a stall shower as seen in B415-12 and B426-12 respectively. Fiber-cement backer board is also listed under B415-12, but it must be held up out of the mortar bed with a gap between the two. Coated Glass Mat Water-Resistant Gypsum backer board (B420-12) and Fiber-Reinforced Water-Resistant Gypsum Backer Board (B431-12) must be held up out of the mortar bed and have flexible sealant applied between the board and the mortar bed.

Many times the floor area to be covered with backer board is irregular or wavy. The only feasible way to correct this deficiency is to flatten the offending area with a trowel-applied patching product. This is extremely important since the patch needs to be under the board, not on top.

Trowel-applied patch

Trowel-applied products solve many issues encountered by installers trying to fill cracks, voids, rough surfaces and low or depressed areas. They also work well to flatten high spots by feathering them gradually to meet the required tolerance of the tile being used. For tiles with all edges shorter than 15 inches, the maximum allowable variation is ¼ inch in 10 feet from the required plane, with no more than ¹⁄16 inch variation in 12 inches when measured with a metal straight edge from the high points of the surface. For tiles with at least one edge 15 inches in length, maximum allowable variation is ¹⁄8 inch in 10 feet with no more than ¹⁄16 inch variation in 24 inches. They may be formulated either as a cement base or a gypsum base, and each has their own installation criteria. Caution needs to be exercised here, since some of these products are not for use on exteriors. The level at which these products may be placed varies from a feather edge to ½ or even 1 inch in a single application. With the addition of the proper aggregate, some products may be installed up to 5 inches thick.

Self-Leveling Underlayments (SLU)

Read more about underlayments here

The extremely fast-paced schedules of today’s jobsites demand products that can fix floor surfaces quickly, efficiently and in a cost-effective fashion. These surface coatings are sometimes called on to correct extreme variations in plane — while drying quickly and ready to receive tile in short order. Self-levelers or SLUs, as they are known, meet these conditions and function well. They may be formulated in either a cement base or a gypsum base and are generally mixed with potable water. Caution again needs to be exercised here, since mixing times and the amount of water used to mix SLUs are critical. Also, be certain to determine if the product is rated for exterior use before installing. As always, follow the manufacturer’s recommendations. 

Many times, the manufacturer may require that a primer be applied with a roller, broom or spray to the surface prior to placing the SLU. The function here is to provide a bondable surface that controls the amount of moisture being pulled out of the mix which, if premature, may be detrimental to the final cure. These manufactured goods, when mixed according to package directions, are poured onto the prepared surface and assisted into place with a gauged rake or paddle. Many of these SLUs can also be pumped in place, thereby eliminating carrying multiple buckets to the work site. Once in place, they dry quickly and can be ready for tile in as little as four hours. SLUs also provide a quick and easy way to encapsulate in-floor heat hydronic tubing as well as electric cables, which are extremely popular.

Uncoupling underlayments

Uncoupling membranes are generally a plastic product that has some sort of bonding scrim or mesh applied to the back of the product to allow it to be bonded to the wood or concrete floor.  The face of product may have a geometrical configuration or a mesh to promote bond to the back of the tile. Once in place, their function is to disconnect the tile surface from the substrate to help eliminate cracked tile. Uncoupling membranes may also act as a waterproofing membrane and function well in equalizing vapor pressure conditions encountered in young or newly placed concrete.

Mortar beds

For many years, mortar beds have been the grandfather of underlayments that solved many of the problems with floor and wall irregularities. When the floor was deemed to be substandard, using a mortar bed most times solved the problem of it being out of flat and/or out of level. Using a mortar bed allowed the tile mechanic to correct the issues with a floor that was otherwise unacceptable. Likewise on wall installations, the surface can be made perfectly plumb and flat, which eliminates lippage. Mortar beds do a great job in this capacity, but times have changed, and unfortunately the mortar bed has lost some of its favor. The reason being is that a good quality mortar bed takes time to produce, which jobsite schedules may not allow. Mortar beds can also add significant weight to the structure, which may not be tolerated in today’s tightly engineered buildings.

The rest of the story

Given the demands of fast-track projects and the get-it-done-yesterday mentality of today’s installations, mortar beds have become less prevalent. These changes have required the industry to make adjustments as well. Somehow, manufacturers had to devise ways to make the unacceptable floor acceptable for a thin-bed installation — and do it quickly. Hence, new products and methods were born.

Manufacturers of these products have met the challenge and provide a wide array of floor and wall patching, flattening and leveling materials. The only issue left is to decide which one will best meet the requirements of your next project.