Use a cementitious backer board or an uncoupling waterproof membrane depending on your subfloor and moisture risk.

For a wood subfloor use a half inch cement backer board installed over a thin layer of thinset mortar with screws placed every six inches along the studs and joints taped with alkali resistant mesh tape and thinset; this gives a rigid, stable surface that resists movement and prevents tile and grout from cracking. On concrete slabs consider a bonded waterproof membrane or an uncoupling membrane like Schluter Ditra installed directly over the slab to manage moisture vapor and prevent telegraphing of small cracks from the slab into the tile. If you need thermal or sound insulation add cement board over a layer of extruded foam board rated for tile backers and use an appropriate adhesive that bonds foam to the substrate then cement board to the foam; ensure the foam is compressive strength rated for floor loads. For wet areas and bathrooms prioritize a waterproof membrane either beneath the backer board or as a topical membrane over the backer board depending on the system manufacturer instructions; always follow the membrane cure times and embedment methods to avoid failures. When replacing existing damaged subfloor remove any soft or rotten plywood and sister joists if necessary so the new underlayment is supported on solid framing and fasten boards to minimize deflection to the long term industry standard of less than one sixty-fourth inch over six inches. Keep seams staggered, maintain manufacturer recommended expansion gaps at walls, and use a flexible grade thinset for final tile bedding to accommodate minor movement.

Three common mistakes are using regular drywall or standard plywood as underlayment which softens when wet and leads to tile failure, not replacing or reinforcing a squeaky or flexible subfloor so the installation exceeds deflection limits, and skipping a waterproofing layer in bathrooms which allows moisture to reach structural elements. The fix for drywall or plywood is to remove it and install cement backer board or a proper tile rated underlayment; for a flexible subfloor sister joist or add a layer of exterior grade plywood or an engineered panel to bring deflection within acceptable limits then install your underlayment. If waterproofing is skipped apply a bonded membrane or an uncoupling waterproof membrane and follow the flashing details at drains and transitions to ensure continuous protection.

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