Tile can be the least expensive or the most expensive finish, depending on the quality of the installation. The quality of the installation depends on proper substrate preparation. While surface preparation is necessary for any floor or wall tile installation, it is critical when dealing with large-format tile or Gauged Porcelain Tile (GPT) and Gauged Porcelain Tile Panels/Slabs. In addition to being very big, large-format and GPT frequently come with a very square or rectified edge. A tile that has been “rectified” has had all of its edges mechanically finished in order to achieve a more precise facial dimension. Another trend that accentuates the need for good preparation is rectangular and plank tiles, which are available in both traditional and GPT tile sizes. These square tile edges, in rectangular shapes with commonly requested tighter grout joints, can really accentuate substrate irregularity, potentially causing a perfect storm for lippage issues.

Traditional dust pressed tile (with a thickness of 3/8 inch or 10mm +>) with one edge greater than 15 inches is considered a large-format tile. GPT is tile less than 1 meter in size and Gauged Porcelain Tile Panels/Slabs are tiles greater than 1 meter and commonly as large as 1 x 3 meters (39 x 118 inches) in size as well. The gauged categories of tiles are typically 3 to 6 1/2 mm (1/8- to 1/4-inch) in thickness.

For thin-bed (thinset) tile installations when a cementitious bonding material will be used, including large and heavy tile mortar (medium bed), the maximum allowable variation in the tile substrates for tiles with all edges shorter than 15 inches is 1/4 inch in 10 feet from the required plane, with no more than 1/16 inch variation in 12 inches when measured from the high points in the surface. For tiles with at least one edge that is 15 inches in length, the maximum allowable variation is 1/8 inch in 10 feet from the required plane, with no more than 1/16 inch variation in 24 inches when measured from the high points in the surface. When an organic adhesive or epoxy adhesive will be used, the maximum allowable variation in the tile substrate is 1/16 inch in 3 feet with no abrupt irregularities greater than 1/32 inch. For thin-bed stone tile installations, the maximum allowable variation in the tile substrate is 1/8 inch in 10 feet.


Wall preparation is frequently overlooked, but critical when installing large-format tiles. Photos courtesy of MAPEI


Depending on whether it is a wall or floor installation, there are as many solutions, as there are  substrates that need correction. Wall preparation is frequently overlooked; most installers feel it can be done either by building up with a little thinset, or even worse, by spot bonding. Building with a minimal amount of thinset may be okay, if your coverage remains high enough and you don’t build it up so thick that there are shrinkage issues. The problem with spot bonding is it does not meet industry standards, mortars are not made for that purpose, it creates impact and moisture issues, and most importantly, it fails. It creates a very weak bond — any stress applied and it starts falling off. In the case of large tiles falling off, it is extremely dangerous. I saw spot bonding on a project in front of a new retail store with 18- x 30-inch cement tiles. I suggested to the general contractor to shut down the sidewalk until it was all removed because it was a major safety hazard, which they did do.

The best substrate preparation can be a mortar bed. However, most people in the industry do not have the skillset needed to properly mud a wall and it is frequently value (devalue) engineered out during bidding. If it is a mortar bed (mud), the method creates a great bondable surface that allows for truing of the substrate or framing. If framed with drywall (in dry areas only) or cement backer board is in place, rendering the wall with a patching is your only proper choice. Unfortunately, few manufacturers have recognized the need for this market and only produce and promote patching compounds for floors. Wall patches should set fast, have great workability when troweling vertically and be non-sag. Look for a manufacturer that has a suitable patching compound for walls that can be built up thick enough for your purpose and down to a minimal thickness. They are out there, but can be difficult to find because the distribution market does not understand the need. Ask your distributor to bring in this necessary wall patching material.

For floors, mortar bed installations are a traditional technique of creating a flat substrate with the use of a mortar applied 1 1/4 inch thick or thicker. It can also be used to create slope for drainage in commercial kitchens and baths. But, as stated earlier, it is frequently value engineered out because concrete contractors don’t want to recess the slab and limited installers have the skill needed to mud a large area. When used successfully in large or wet areas, the installer has control over the substrate and can prepare a very flat floor.


To get the value out of your tile installation, it must start with proper substrate preparation.


In addition to mortar beds, patching compounds are suitable for localized patches or filling “birdbaths” or dips in the floor. There are a variety of different patching compounds that are fast-setting, moisture-resistant and can be applied from skim coating to very thick, depending on the needs of your project.

When leveling large areas, floors can be prepared much easier and better than walls due to the self-leveling market. The self-leveling market has exploded because of the ease of installation. All installations of self-levelers require a good primer. Some manufacturers make a few different ones, so please check them out and select the one most appropriate for your project. The same can be said about self-levelers; there is a variety of different self-levelers, it just depends on your site requirements. Do you need fast-setting? Just a skim coat? One with minimal prep? How thick do you think you need to go or do you just need a competitive leveler? Please do not misunderstand me; I said they were easy, easy when compared to the alternative, a mortar bed. Self-levelers still require you to select the proper primer and self-leveler for your specific project. It is important that you carefully follow the directions, not exceed the maximum thickness recommendations, mix it properly and add the proper amount of water. Most manufacturers have a mixing kit that includes a large mixing pail, water buckets, proper mixing paddles, a gauge rake and other beneficial tools. Before attempting a self-leveling project, make sure you have the necessary tools. Self-leveling projects take good planning and the proper tools, and working as a team is best to keep a wet edge when pouring multiple pours in a large area.


spot bonding
Most installers feel preparation can be done either by building up with a little thinset, or even worse, by spot bonding (as shown above).


Tile requires mortar coverage to prevent bond failure, breakage and to get the expected life out of the tile installation. As a mortar manufacturer, I can assure you, thinset mortars work best when making contact with the materials needing to be bonded together. If the trowel you are using can provide the minimum thickness for thinset mortar 3/32 inch per ANSI A108.5 Installation of Ceramic Tile with Dry-Set Portland Cement Mortar or Latex-Portland Cement Mortar, but your wall has a large dip in it that is 1/8 inch, do the math; no mortar will be touching the tile without back-buttering. The minimum mortar requirements for GPT interior installations is 3/16 inch after embedding; that is to provide mortar contact, even when there is a 1/8 inch in 10 feet maximum allowable variation in the tile substrate. Generally, achieving 3/16 inch after beat in requires putting thinset mortar on both the GPT backing and the substrate.

To get the value out of your tile installation, it must start with proper substrate preparation. All of the other flooring trades are expected to skim and level their substrate, but the tile industry gets told to just put more thinset in that area. That is not acceptable. As we deal with larger, thinner, rectangular tiles with square-rectified edges, we must have better prepared substrates. You deserve to have a suitable canvas for your art. Use your industry standards to provide the references needed to verify how important proper surface preparation is for your large-format or GPT project so the owner, general contractor and you end up with the tile project that was contracted for and should be expected.