If the subfloor sustains damage during old tile removal you must assess and repair it immediately to protect the new large format tile installation.

Start by removing all loose material and vacuum the area so you can clearly see the extent of the damage. For localized holes and cracks use a cementitious patching product compatible with your substrate; apply it in thin lifts and allow full cure according to manufacturer instructions so the patch attains full strength. When the damage is widespread or the subfloor is spongy tap the surface with a hammer to locate delaminated areas and cut out any sections that are structurally compromised then replace them with new plywood or cement board screwed at proper spacing to the joists. For coastal or moisture prone areas and for weak spots add a waterproofing membrane over the repaired area to prevent moisture migration that can weaken thin set and cause bond failure; apply the membrane per instructions and ensure seams are sealed. If you are installing large format tile and the surface has hairline fractures or poor mechanical key consider embedding a galvanized metal lath into a scratch coat of mortar to create a reinforced leveling base that resists crack transfer. Finally use a long straightedge and a polymer modified self leveling mortar or unmodified portland cement based floor patch to address flats and dips; check with a ten foot straightedge and shim until the substrate meets the flatness tolerance recommended for large format tile which is typically much tighter than for small tile.

Common mistakes include leaving loose or delaminated areas in place because they look small which later causes tile debonding; always remove to sound material and replace or reinforce. Another frequent error is skipping membrane or moisture mitigation over repaired zones which leads to future bond failure; apply an appropriate waterproofing or crack isolation membrane where moisture or movement is a concern. A third misstep is not verifying flatness and proceeding to set large format tile on a wavy surface which results in lippage and stress cracking; use a self leveling product and test with a long straightedge so the substrate meets the tighter flatness tolerances required.

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