There doesn’t appear to be an industry definition for the category of “underlayments,” but this grouping can easily include products such as backer board, liquid and sheet membranes, uncoupling membranes, mortar beds, patching compounds and self-leveling products. No matter which products are being used on a project, the floor or wall surface must be flattened or possibly leveled to meet the industry standards. For the sake of this conversation, we will focus on patching compounds and self-leveling underlayments.

The underlayments used in concert with ceramic and stone tile installations may be, and most likely are, essential to the satisfactory outcome of a project. If the subfloor, be it wood or concrete, is not within the tile industry requirements, something must be done to correct the deficiency prior to starting the installation.  

In the past, floor preparation materials were not as readily available as they are today. When an inappropriate subfloor was encountered, it was up to the installer to find a way to make the floor flat enough to install the large-format tile of the day, the infamous 8 x 8, without one tile edge being higher than the adjacent tile. The use of the term “lippage” was not commonplace and may not have been yet created. 

Installers in those days needed to be creative in fixing low spots, especially in a wood subfloor.  Many times, multiple layers of plywood were used until the low spot disappeared. Then one of the early generation floor patch products was used to fill in the remaining spaces. This was a time consuming method and many times was not that effective in creating a flat floor.

The ANSI Specification, A108.02-, Subfloor Surfaces states: For tiles with all edges shorter than 15 inches (0.38 m), the maximum allowable variation is no more than 1/4 inch in 10 feet (6 mm in 3 m) and no more than 1/16 inch in 1 foot (1.6 mm in 0.3 m) from the required plane, when measured from the high points in the surface. For tiles with at least one edge 15 inches (0.38m) or longer, the maximum allowable variation is no more than 1/8 inch in 10 feet (3 mm in 3 m) and no more than 1/16 inch in 2 feet (1.6 mm in .6 m) from the required plane, when measured from the high points in the surface.

But getting the substrate to within these tolerances is the dilemma. How does an installer fix the floor making it flat enough to receive tile? Utilizing troweled-on patching materials and pourable self-leveling products will meet this challenge.

The National Tile Contractors Association (NTCA) Reference Manual lists this topic as “Underlayments, Trowelable and Poured.” These factory-prepared powdered materials usually fall into one of two categories: Gypsum-based underlayments and Cement-based underlayments.

Gypsum-based underlayments

Poured self-leveling underlayments are composed of various grades of gypsum along with a number of chemicals which control the setting time of the product and may be sanded or unsanded. Depending on the manufacturer’s recommendations, they may be mixed with potable water or a latex admixture. If using a latex additive, it is always wise to use the product provided by the manufacturer. These underlayments are only recommended for use in dry areas due to the water-sensitivity of gypsum-based products. They are most times used in conjunction with resilient flooring to patch small holes, cracks or to ramp up or down to an adjacent floor finish.  However, they may also be used to flatten the large floor areas of properly prepared wood or concrete.

Poured gypsum underlayments can solve subfloor issues such as out of flat and out of level, but they need to be installed per the manufacturer’s guidelines. Structurally sound concrete floors require that the gypsum underlayment meet a minimum compressive strength of 3,000 psi and be a minimum thickness of 1/2 inch. Structurally sound wood floors require a minimum compressive strength of 2,000 psi and a minimum thickness of  3/4 inch. All of these applications require that the design load of the structure meet the deflection requirement of L/360.

Once the underlayment has been placed by a certified poured gypsum installer, a drying time of three to 14 days, depending on the thickness, must be observed. Additionally, the site temperature and humidity may be a factor in achieving an adequately dry installation. When the gypsum has dried to the manufacturer’s specifications, the gypsum installer applies the required primer/sealer, which prevents the gypsum from pulling the moisture out of the tile setting mortar. This premature moisture removal from the mortar can compromise the bond of the tile to the gypsum.  

At this point, it is important that the tile installer obtains verification in writing, that the underlayment meets the minimum compressive strength, was allowed to dry out properly and had the appropriate primer/sealer applied in accordance with the manufacturer’s requirements.      

Cement-based underlayments

Trowelable underlayments contain cement, aggregate and are mixed with either potable water or a latex additive. Many manufacturers recommend applying a slurry coat of the product to the substrate, allowing it to dry until it becomes tacky. The recommended liquid and powder mix is then applied to the surface. Trowelable underlayments usually require sanding after it dries to remove trowel ridges and allow for an additional layer if needed.

Pourable self-leveling underlayments (SLU) contain cement, aggregate and chemical additives, which enhance their flowability and strength. They are mixed with either potable water or a manufacturer recommended latex additive. Substrates are usually treated with a latex primer which serves two functions. The first is to increase the bond strength between the substrate and the SLU. The second is to prime the substrate, which equalizes the absorption rate of the substrate, thus eliminating hot spots which can prematurely dry out the underlayment before it completes its designed curing process.

SLUs aid in correcting out of flat and out of level subfloors and can be installed from as thin as 1/16 inch up to 6 inches with compressive strengths ranging from 3,500 to over 7,000 psi depending on the manufacturer. All of these applications require that the design load of the structure meet the deflection requirement of L/360.

Once placed, SLUs normally require between two and 24 hours to dry before the tile can be installed. This timeframe will vary based on the thickness of the cement along with the site temperature and humidity. Normally, very minimal prep work is required prior to the installation of the tile. 

One of the newest entries to the market has been Thin Porcelain Tile (TPT). The installation of this product allows the installer only one attempt to set it correctly, since it is almost impossible to slide or lift it once it has been placed into fresh mortar. Here the requirement for a flat substrate, both wall and floor, is critical. Although neither a product nor an installation standard have been established for TPT, the substrate tolerances need to meet at least the 1/8 inch in 10 feet standard for tiles with at least one edge 15 inches (0.38 m) or longer, as noted in ANSI A08.02-  It is imperative that the air is expelled or burped out from under the tile and that the trowel ridges are collapsed. This lets little or no allowance for high or low spots in the surface to which the TPT is being applied. It must be corrected before the tile is installed.

Underlayments do a great job and the manufacturers are to be applauded for their new and innovative products that make the finished tile installation look the way it should — flat and lippage free.