Damaged subfloor undermines tile installation success and must be fixed before any adhesive or tile goes down.

Start by removing the laundry appliances and any loose flooring to inspect the full subfloor area for rot, soft spots, and deflection. Probe with a screwdriver every foot around suspected damage to map soft zones and mark where boards need replacement. For localized dents or holes under eight inches in diameter, cut back to sound wood and install a patch piece of plywood or oriented strand board of the same thickness, fastening with corrosion resistant screws every six inches along the edges and into joists. If boards are warped or water damaged over a larger area replace entire sheets rather than trying to shim; mismatched thickness creates telegraphing through grout joints. Scrape and remove all staples, nails and screws protruding above the surface, then sand or plane any high spots so the surface is flat within one eighth of an inch over a ten foot span. Clean thoroughly with a vacuum and a mop using a mild trisodium phosphate alternative, then allow complete drying for at least twenty four hours before installing a decoupling membrane or thinset; moisture maps and a moisture barrier under the membrane are critical in laundry rooms where occasional spills and humidity occur.

Three common issues are leftover fasteners causing ridges that telegraph through tile, inadequate replacement of damaged sheets so joints are unsupported, and ignoring moisture sources that re-damage repairs. Remove all fasteners and refasten edges to joists to eliminate ridges, then check flatness with a straight edge and correct high or low spots before tiling. Replace entire affected plywood or oriented strand board sheets and sister joists where needed so seams fall on solid framing and do not flex under load. For moisture, find and fix plumbing or venting leaks, install a vapor barrier appropriate for your subfloor type, and consider a cementitious membrane that resists moisture transfer to protect the tile assembly.

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