You must create a flat, stable subfloor before laying tile in a kitchen to prevent tile cracking and grout failure.

Start by measuring and mapping the floor for flatness using a long straightedge or a four foot level and mark any gaps higher or lower than one quarter inch over the span of three feet. If the plywood is loose, secure it first by screwing every six to eight inches along edges and every eight to twelve inches in the field using corrosion resistant screws that are countersunk just below the surface. For low spots under one quarter inch, use a cement based floor leveling compound; prime the plywood with the manufacturer recommended primer, mix to the specified consistency, pour or trowel it into the depressions, and feather the edges so the compound blends with adjacent areas; allow the full cure time before proceeding. If the deflection or damage is greater than acceptable limits, remove and replace damaged plywood with exterior grade tongue and groove or CDX plywood at least five eighths inch thick or upgrade to three quarter inch for joist spacing wider than sixteen inches on center. For uneven high spots, plane or sand down raised areas or undercut with a circular saw on cross joints, then fill seams with an approved patching compound; never rely on thinset mortar to bridge more than one eighth inch. When plywood is sound but uneven, consider installing a cement backer board or a bonded uncoupling membrane system directly over properly prepared plywood to create a rigid, crack resistant surface; fasten per manufacturer instructions and tape joints with fiberglass mesh and thinset where required.

Common mistakes include ignoring subfloor deflection and assuming the existing plywood is sufficient, which leads to tile or grout cracking; check for bounce by walking the area and measure deflection with a straightedge and address by adding joist reinforcement or thicker plywood. Another frequent issue is using too much or too little leveling compound; follow mixing ratios, use primer, and do not apply more than the product maximum thickness in a single pour, instead build up in layers if needed. A third problem is skipping manufacturer instructions for cement backer board or membranes; always fasten with the recommended screws, use the correct thinset type and coverage, and let adhesives and mortars cure fully before grouting or loading the floor.

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