Remove all dust, thinset, and loose particles so the new tile will bond to a clean, stable surface.

Start by sweeping and vacuuming the entire subfloor to remove every bit of loose debris and dust; use a stiff brush and a shop vacuum with a HEPA filter for best results because even fine dust stops thinset from adhering. Next inspect the slab for levelness using a straightedge that is at least four feet long; mark any high or low spots and measure deviations greater than one quarter of an inch over eight feet which will require correction. For low spots, apply a cementitious patch or self leveling compound following the maker’s instructions and mix ratios, pour or trowel to within a hair below the tile thickness, allow full cure times which are commonly twenty four to forty eight hours but always follow product data sheets. For high spots, mechanically grind them down or chip them out, then clean the area again and feather a patch to smooth transitions; avoid thin veneers over major hollows because they will fail. If the surface is concrete and it is very porous or old with powdering known as laitance, scrub with a neutralizing acid wash or use a concrete primer compatible with your thinset to ensure adhesion, and always allow the substrate to dry completely to the touch before applying primer. Finally, do a dry layout by laying out a row of tile without adhesive to confirm cuts, centerlines, and that the pattern will not leave slivers at the walls, adjusting your starting lines as needed to keep end cuts sensible and consistent.

Common issues are inadequate cleaning which causes bond failure, uneven subfloor that causes lippage, and trapped moisture leading to mold or thinset breakdown. If the floor was not cleaned well, go back and scrub, vacuum, and wipe with a solvent recommended by the thinset maker, then allow full drying time before proceeding so the bond can form. When the slab is not level, do not try to hide dips with thicker adhesive; instead grind high areas and pour a qualified patch or self leveling product for a uniform plane so tiles sit flat. For moisture problems, test relative humidity or use a calcium chloride or modern moisture meter, and if readings are above manufacturer limits install a rated moisture mitigation membrane or use a waterproof uncoupling membrane before tiling.

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