In your role as a tile installer, you've undoubtedly encountered job site conditions where patching the substrate (floors and walls) is necessary to make it "tile ready." Ensuring the substrate is sufficiently flat is paramount, especially when dealing with large-format tiles.
But when it comes to addressing irregularities, what's the best approach? Should you opt for patch or thin-set? Hopefully, your answer isn't thin-set.
The Objective of Substrate Preparation: Achieving a Flat Surface
Before diving into material choices, let's emphasize the importance of proper substrate preparation.
Ultimately, the goal is a flat surface. In the realm of tile installation, "flat" denotes a surface devoid of significant high or low spots, facilitating a smooth tile installation with minimal lippage between tile edges. This holds true for both floors and walls.
Lippage on Floors and Walls
The objective is to achieve a surface flat enough to mitigate lippage on floors and eliminate wall wash lighting issues.
Assessing Floor or Wall Flatness
To evaluate flatness, utilize a ten-foot straightedge or the longest straightedge that fits the tile area. Mark low and high spots to identify areas requiring attention to meet ANSI specifications.
Addressing Surface Flatness
To ensure surface flatness for your tile installation, adopt one of two approaches:
1. Grind down high spots cautiously, adhering to OSHA guidelines for dust control.
2. Fill low areas with the appropriate material, avoiding thin-set.
Utilize a cementitious patching compound or self-leveling underlayment (SLU) with the recommended primer to fill low spots and establish a flat surface.
Troweled-On Floor Patch for Low Areas
Consider using a troweled-on floor patch for low areas. These patch materials, available from various manufacturers, are designed to fill cracks, voids, rough surfaces, and low or depressed areas, ensuring a suitable substrate for tile installation.
Or Self-Leveling Underlayment
If job site conditions necessitate surface leveling, a self-leveling underlayment product, applied according to the manufacturer’s instructions, can yield favorable outcomes.
Steer Clear of Thin-Set at all Costs!
Unfortunately, patch material often ends up being leftover thin-set mortar from the truck. Avoid using thin-set mortar for patching under any circumstances!
Thin-set mortar is specifically formulated as a bonding agent for attaching tile to the substrate. It's not intended for patching, fixing, flattening, or repairing, and using it as such can lead to issues.
Thin-Set is Not a Filler
Manufacturers caution against using thin-set mortar as filler because it can shrink at varying rates during curing, resulting in irregular tile surfaces.
Recap: Opt for Proper Patching Materials
When preparing the substrate for tiling, prioritize proper patching materials over thin-set or large and heavy tile mortars. Whether filling low spots or feathering out high points, choose flash patch or self-leveling underlayment (with primer) for optimal results. For wall repairs, a high-quality flash patch should suffice.