Uncoupling membrane
Also called: Ditra, decoupling membrane, crack isolation membrane, tile underlayment membrane, uncoupling mat, Schluter membrane
A thin, dimpled polyethylene sheet bonded to the subfloor with thinset, with tile set on top, that lets the subfloor move slightly without transferring that movement to the tile. Ditra is the common brand; $1–$2 per square foot and the usual alternative to cement board.
Wood subfloors move: they expand and shrink with humidity, and they flex under load. Tile and grout don’t. Bond tile directly to plywood and the two fight until the grout cracks and tiles pop. The traditional answer was a thick mortar bed that floated free of the wood; the modern one is a membrane. The sheet’s underside is a fleece that thinset grips; the top is a grid of square dimples that the tile’s thinset keys into. Between them, the plastic layer lets the two sides shear a little relative to each other — “uncoupling” — so a subfloor’s seasonal movement or a hairline crack in a slab doesn’t telegraph up.
Installation is the whole trick. The subfloor still has to be stiff and flat — the membrane isolates movement, it doesn’t add strength or fix a hump. The sheet is bonded with the mortar the manufacturer specifies (unmodified thinset, for most), rolled out into a combed bed and pressed in with a float or a roller until the fleece is fully wetted. Tile goes on top with unmodified thinset again, because the membrane doesn’t breathe and modified mortars need air to cure; the newer “modified for membranes” products are the exception and say so on the bag. In a wet area, the seams get banded with the same maker’s tape and the membrane becomes the waterproofing.
Against cement board: the membrane is thinner (1/8 inch versus 1/4 or 1/2), faster to install, lighter to carry, and better at isolation; board is cheaper per square foot, adds a little stiffness, and doesn’t care which mortar you use. Either one is right over plywood. Neither is optional — the DIY guide puts it in the first step, and the cost page prices it on the floor tiers.