Tile is rigid and brittle. It fails when what is behind it moves, gets wet, or both. Choosing the substrate is the most consequential decision in the job.
Wall substrates
Cement backer board (HardieBacker, Durock, WonderBoard) — cement and fibreglass mesh. Unaffected by water, though not waterproof — water passes through it. Needs a membrane over it in wet areas. Heavy, hard to cut, and the standard for showers.
Foam backer board (Kerdi-Board, GoBoard, Wedi) — extruded polystyrene with a waterproof facing. Lightweight, cuts with a knife, and waterproof in itself when the seams are treated. More expensive, and it is what most tile setters now use in showers.
Greenboard / moisture-resistant drywall — paper-faced gypsum with a water-resistant core. Acceptable behind tile in dry and damp areas; not permitted in showers or tub surrounds by current code. It was permitted decades ago, which is why so many older showers fail.
Standard drywall — fine behind a kitchen backsplash and other dry-area tile. Nowhere near water.
The line is simple: anything that gets directly wet needs cement or foam board plus a membrane. Anything that only sees humidity can take drywall.
Floor substrates
Floors fail through deflection — flex under load that cracks tile and grout.
- Ceramic and porcelain: subfloor deflection no worse than L/360 of the span
- Natural stone: L/720, twice as stiff, because stone is far less tolerant of movement
For a 12 ft joist span, L/360 allows 0.4 inch of deflection under design load. L/720 allows 0.2.
Meeting it usually means 3/4 in tongue-and-groove plywood subfloor plus 1/2 in cement board, or an uncoupling membrane. Adding a second layer of plywood stiffens a bouncy floor more effectively than adding backer board does — backer board is a bonding surface, not a structural stiffener.
Uncoupling membranes
Ditra and its equivalents sit between substrate and tile and allow the two to move independently. They do not stiffen the floor; they absorb in-plane movement so a shrinking subfloor does not drag the tile with it.
Particularly valuable over plywood, over new concrete that is still curing, and over in-floor heating.
Note the mortar requirement: most uncoupling membranes want unmodified thinset above them, because a polymer-modified mortar sandwiched between two non-porous layers cannot cure properly. Read the manufacturer's instruction — this is a common and expensive mistake.
Fastening
Backer board is screwed with corrosion-resistant screws at the manufacturer's spacing — usually 8 inches — and the seams are taped with alkali-resistant mesh tape and filled with thinset, not with drywall compound.
Drywall compound in a tile seam is a failure waiting for the first cycle of humidity.