Substrate Preparation and Evaluation
Substrate Preparation and Evaluation
Large format tile (LFT), typically defined as tile with any edge longer than 15 inches, demands a substrate that is flat, sound, clean, and properly cured. The most common flatness standard for LFT is 1/8 inch in 10 feet (or 1/16 inch in 2 feet) when using a 1/2-inch notch trowel and a medium-bed mortar. Substrates that are not flat will cause lippage and bond failure. Always check flatness with a 10-foot straightedge or a laser level, and map high and low spots. Concrete must be fully cured (28 days for standard concrete), free of curing compounds, sealers, and contaminants, and have a moisture vapor emission rate (MVER) within the adhesive manufacturer's limits (often 3–5 lbs/1000 sq ft/24 hr). Use a calcium chloride test or in-situ relative humidity probe. Wood subfloors must be structurally sound, with deflection not exceeding L/360 for ceramic or L/720 for stone, and should be at least 1-1/8 inches thick (or two layers of plywood). Cement backer board must be embedded in thinset and fastened per manufacturer. Traps: assuming a concrete slab is flat without checking; forgetting to test moisture; using plywood directly under LFT without an underlayment; and ignoring deflection. Worked example: If a 10-foot straightedge shows a 1/4-inch gap under the center, the floor is out of tolerance by 1/8 inch. You must grind high spots or use a self-leveling underlayment to bring it within 1/8 inch in 10 feet before tiling.