Large format tile requires a stable, clean, and level substrate or the installation will fail.

Suitable surfaces include cement backer board, concrete slabs, properly prepared exterior grade plywood over joists that meet stiffness requirements, and existing well-bonded tile when it is fully sound and properly primed. For concrete slabs remove paint, curing compounds, wax, oil, and efflorescence by grinding or using an appropriate chemical stripper, then vacuum and wipe with a damp mop to remove dust. With cement backer board fasten to manufacturer spacing, tape and thinset the seams, and ensure the board is flat with no deflection greater than one twenty-eighth of an inch over a twelve inch span, because large format tile magnifies any movement. When working over plywood confirm the subfloor assembly meets deflection limits of L over four hundred for tile and L over three hundred for porcelain large format pieces, add an underlayment or a second layer of recommended plywood if needed, and screw and glue the layers to eliminate bounce. Old ceramic or porcelain tile can be tiled over only if every tile is firmly bonded, grout joints are cleaned and profiled, and the surface is abraded to provide a keyed profile, then use a high quality modified thinset rated for large format tiles and follow the manufacturer open and working time. Always check moisture with a meter or perform a calcium chloride test on concrete and allow drying time or install a vapor mitigation system when moisture exceeds the tolerance specified by the adhesive manufacturer.

Common mistakes are installing over a contaminated surface, expecting adhesive to compensate for movement, and ignoring flatness tolerances. If the surface has wax, grease, soap, or dust the tile will not bond so strip and clean with appropriate cleaners, scrub and rinse, and allow to dry completely before tiling. If the substrate deflects or has structural movement do not rely on thicker adhesive to hide problems, reinforce the subfloor or install a decoupling membrane designed for large format tile to protect the tile from movement. If the substrate is out of level or has dips and high spots use a cementitious self leveling compound after priming the surface or spot level with a feather edge technique and then recheck flatness with a long straightedge before laying tile.

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