The success of any tile-setting project is heavily dependent on the soundness of what’s beneath the mortar. Most installations demand at least one form of underlayment in place before mortar is put down. However, every installation stands to benefit from the heightened long-term stability that underlayment products provide. These can be mud beds, self-leveler, backerboard, exterior-grade plywood (EGP), patching compounds, waterproofing and/or uncoupling mats. An installation may only require one of these, but most will require a series to fall within industry specifications for a specific project.


Level Desired; Flat Required

Installations must have a substrate that meets the required flatness tolerance of ¼” in ten feet for standard size tile, and 1/8” in ten feet for large format tile. For uneven substrates, one solution is to grind it down to bring the high points flush with the low. This is extremely time-consuming, messy and expensive. The preferred remedy is to bring the low points flush with the high by using levelers or patching compounds.  

Products like Merkrete’s Pro Patch AL1 are excellent quick-setting, Portland-cement-based skim coating compounds for finishing and smoothing subfloors and wall surfaces. They are ideal for patching and leveling substrate defects from featheredge to 1”. And for larger floor surfaces that need to be smoothed quickly, there are flowable hydraulic cement underlayments like Merkrete’s Underlay SLU. These are pourable, pumpable and free-flowing Portland-cement-based underlayments designed to rapidly level substrates.  

If tile is to be installed over a wood substrate, the flatness tolerances remain the same. However, wood substrates must also meet deflection allowances of L/360 for ceramic tile installations and L/720 for natural stone. Failure to maintain these allowances can lead to cracked tile or grout joints. Adding an additional sheet of EGP underlayment can bring the deflection within standards while ensuring a flat surface, but it will raise the height of the finished floor. Therefore, adding a thin-mil membrane creates an optimal bondable surface for the EGP to receive tile without adding more height to the overall system.


Go for Stable, Not Groundbreaking

One underlayment that can aid any installation is a strong crack isolation membrane. Without one, any cracks or in-plane faults in the substrate can relay their energy through the tile system to fracture tiles and grout. Even on flawless concrete substrates without a crack isolation membrane in place, the chances of tile breakage increase significantly should fractures develop during the life of the installation. Therefore, it is best to look at crack isolation as insurance. Replacing cracked tiles and adding a crack isolation membrane after the fact are extremely costly, and matching existing tile and grout will likely prove very difficult.

Crack isolation products take several shapes. There are respectable sheet membranes and uncoupling mats on the market, but neither afford the combination of protection and flexibility of liquid-applied membranes. Liquid-applied products like Merkrete’s Fracture Guard FD are thin load-bearing membranes that bond to the substrate and keep cracks up to 1/8” from transferring to the surface. Applied at a wet thickness of 30 mil, these products will not raise the installation more than the thinness of a credit card. This trait is especially important when other flooring or surface types need to meet up with the tiled installation.


Underlayments that Hold Water

One last underlayment solution that builds on the protection of crack isolation products is the waterproofing product. Waterproofing products are indispensable to installations meant to hold water or allow it to drain properly. These include baths, showers, steam rooms, balconies and many other installations intended for occasional to heavy water contact. And like crack isolation, these are great insurance materials that can be applied under any tile or stone surface to prevent damage from planned or unexpected water exposure. Without this protection, water can move through the grout, mortar and substrate to cause damage to the building construction, adjacent spaces and property.

Also, like crack isolation membranes, there are several products available including waterproof boards, sheet membranes and liquid-applied membranes. Products like Merkrete’s Hydro Guard SP-1 are thin, load-bearing liquid-applied membranes that can be applied to fit virtually any shape. Liquid-applied membranes do not have seams that can leak like sheet membranes do. Instead, they form a completely monolithic barrier that can be applied by roller, brush, trowel or sprayer. Membranes like Hydro Guard SP-1 can be flood tested in as little as four to six hours. Their strength and elongation also lend crack isolation protection to the tile system.


Don’t Underestimate Underlayments

The lifespan of a tile installation is a testament to the workmanship of those who completed it. Often, the most important steps to a project need to be taken before the first tile is set. Without an adequately prepared surface and the underlayments to strengthen, protect and stabilize it, the quality of workmanship will be revealed to those tasked with repairing the installation. Therefore, we should never forget: it’s what’s underneath that counts.