How to Prep a Subfloor for Tile

Before any product goes down, the subfloor has to be stiff enough. The tile industry standard is L/360 deflection, meaning the floor should deflect no more than its span divided by 360 under load. A bouncy floor will crack tile no matter what membrane or board you add on top.

For a typical wood-framed floor that means at least 3/4 in plywood or OSB subfloor with joists at 16 in on center. If the floor flexes when you walk on it, add a second layer of plywood or sister the joists before you think about tile. Uncoupling membranes and backer board do not add structural stiffness.

Tile, especially large-format, needs a flat plane, ideally within 1/8 in over 10 ft. Check with a long straightedge and mark the low spots. Small dips can be filled with patch, but broad waviness is best fixed with a pourable self-leveler like LevelQuik RS or Henry 555 LevelPro.

Prime the substrate first, dam the perimeter and any penetrations, then mix small batches and pour, working fast because the pot life is short. Let it cure to the tile-ready window (about 12-16 hours for LevelQuik, 6 hours for Henry) before moving on.

Over a solid, flat plywood subfloor, cement backer board such as HardieBacker 1/4 in or Durock gives thinset a reliable surface. It must be set in a full bed of thinset and screwed on a tight grid, never just laid dry.

If you are tiling over a cracked slab, a seasonal or questionable subfloor, or you want waterproofing, use an uncoupling membrane like Schluter Ditra instead. It isolates tile from subfloor movement and, with sealed seams, keeps water out, all at only 1/8 in of added height.

Thinset is not one-size-fits-all. A polymer-modified mortar like Custom VersaBond (about $18-20 for 50 lb, roughly 2 sq ft per pound) bonds backer board to the subfloor and sets large-format tile on top of the board.

With Ditra the rule flips: use a modified thinset to bond the fleece side to the subfloor, but an unmodified thinset to set tile into the membrane's cavities, because the sandwiched modified mortar can't dry and cure properly. Always follow the membrane maker's thinset chart.

Backer board is water-durable but not waterproof, so showers, tub surrounds and curbless bathroom floors need a dedicated waterproofing layer. Roll on a liquid membrane like RedGard or install a sheet membrane over the board.

Ditra already provides waterproofing when its seams and perimeter are sealed with Kerdi-Band, which is why it doubles as both crack isolation and moisture control in a bathroom. Confirm coverage before you set a single tile.