Cement backer board
Also called: cement board, backer board, HardieBacker, Durock, WonderBoard, CBU, tile backer, cementitious backer unit
A rigid sheet of cement and fiberglass mesh, 1/4 or 1/2 inch thick, screwed over a subfloor or wall studs as the surface tile is bonded to. It does not swell or rot when wet, but it is not waterproof — a shower needs a membrane over it. $12–$20 a sheet.
Tile needs a base that won’t move or degrade behind it. Drywall does both when it gets wet; plywood swells and flexes. Cement board — Durock, HardieBacker, WonderBoard and the generic CBU — is a sheet of Portland cement with mesh on the faces (or fiber-cement, in HardieBacker’s case) that’s dimensionally stable wet or dry and gives thinset something to bond to. It’s been the standard tile substrate for wet walls since the 1980s and remains the cheap, solid choice for floors.
On a floor: 1/4-inch board, laid over a bed of thinset combed onto the plywood (the mortar fills voids; it doesn’t add strength), screwed every 8 inches with backer-board screws, joints staggered and offset from the plywood seams, gaps at the walls, seams taped with alkali-resistant mesh and skimmed with thinset. On a wall: 1/2-inch board over studs 16 inches on center, screwed every 8 inches, same tape and mud. Cut it by scoring and snapping, or with a carbide blade outdoors — the dust is silica and needs a mask.
What it isn’t: waterproof. Cement board lets water through; it just doesn’t care. Behind a shower or a tub surround it needs a liquid or sheet membrane on its face, or a vapor barrier behind it, per the TCNA method — a board-and-tile shower with neither is the leak everyone’s plumber has seen. The alternative on floors is an uncoupling membrane, thinner and better at isolating movement; the alternative on shower walls is a foam board system that’s waterproof by itself. The cost page uses board on the bathroom floor and shower tiers.