If your subfloor is tile-ready after demolition, you can confirm it by checking flatness, soundness, cleanliness, and moisture compatibility.

Start by inspecting for residual thinset or mortar and remove any high spots with a chisel and hammer until the surface is even with the surrounding subfloor. Run a long level or straightedge across the floor in multiple directions and note gaps; most flooring manufactures require flatness within one quarter of an inch over ten feet, so measure and mark areas that exceed that tolerance. Tap the subfloor with a mallet to listen for hollow or loose spots and fasten squeaky or loose sections with screws every six inches along edges and every eight to twelve inches in the field for plywood; if joist spacing is wider than recommended, add blocking or adhesive-backed underlayment to stiffen the surface. Check for residual thinset that might prevent bonding by feeling for rough patches and removing stubborn mortar with a grinder fitted with a cup wheel while wearing proper respiratory protection. Test moisture with a calcium chloride test for concrete subfloors or a reliable moisture meter for wood; if moisture exceeds manufacturer limits, address the source and use an appropriate moisture mitigation system. Finally, vacuum thoroughly and wipe with a damp mop to remove dust, then apply a primer or bonding promoter if specified by the new flooring manufacturer before installing underlayment or the finished floor.

Common problems are uneven high spots left by old mortar, loose or squeaky subfloor panels, and subfloor moisture issues. For high spots grind them down and feather with a cementitious or polymer-modified leveling compound after cleaning and priming so the compound bonds; follow mixing and cure times on the product label. For loose panels drive ring-shank screws into the joists and add adhesive beneath panels to eliminate movement, then recheck flatness and screw spacing to prevent future creaks. For moisture concerns identify and repair the source such as leaks or vapor intrusion, allow the subfloor to dry to acceptable levels, and use a moisture barrier or remediation system if readings remain above the flooring manufacturer guidelines.

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