Are You Patching Your Tile Substrate With Thin Set?

We've all encountered subfloors that needed to be patched. This, of course, is especially important if you're working with large format tile. Ultimately it's critical all around to have a flat substrate if you want a quality tile installation. Knowing that, the next question becomes, "What should I use to fix areas that need patching?" **Spoiler Alert: Your answer should not be thin set.** Let's start with the goal of substrate preparation. The end goal is a flat surface. According to CTEF, "In the tile world, the word 'flat' means that the surface substrate does not have any significant high or low spots and which will yield a smooth tile installation with no lippage between tile edges. Flat means no significant high or low spots."

FIRST: Check the Floor or Wall Flatness

According to CTEF, "Determining the flatness of the floor or wall is relatively easy by using a ten-foot straightedge or the longest straightedge the tile area will accommodate. Simply mark the substrate with some method such as circling the low spots and placing an X on the high spots to quickly show where work is needed to meet the ANSI specification."

SECOND: Fix the Substrate

You can ensure that the surface is flat using two different methods:
Now back to our original question, what should you use to fill the low points? You should use either a self-leveling underlayment (SLU) or a cementitious patching compound. If you use a self-leveling underlayment make sure to use the recommended primer from the manufacturer to fill the low spots and provide a flat surface. As the CTEF points out, "Floor patch materials come from a variety of manufacturers and are used to obtain the required plane for tile installations. This includes filling cracks, voids, rough surfaces and low or depressed areas. Most of these products are latex-modified and require mixing with potable water. These patch products are designed to tenaciously bond to the properly prepared surface and fill low areas."

Why Not Use Thin Set For Patching?

Thin set is meant to attach tile to the substrate, not repair the substrate. Oftentimes, it would require a much thicker application of thin set to properly accomplish the repair and that is a bad idea. As the CTEF points out, "When used as filler, especially when it is thicker than the recommended thickness after the tile is embedded, the mortar can shrink at different rates during its curing process. When this takes place, some of the tile surface that looked perfect when installed will look extremely irregular in a couple of days." You can find everything you need for successful tile installation in our South Florida stores or online in tile installation materials. Here are a few more resources you might enjoy: Have You Adequately Prepped the Substrate for Tile? A Flat Floor vs. a Level Floor: What's the Difference? Is Your Floor or Wall Flat Enough for Large Format Tile