Waterproofing membrane
Also called: shower membrane, Kerdi, RedGard, Hydro Ban, sheet membrane, liquid waterproofing, bonded membrane
The continuous barrier behind and under shower tile that keeps water out of the walls and floor: a bonded sheet (Kerdi, Wedi), a foam board system, or a liquid-applied coating (RedGard, Hydro Ban). Sold as a system with its own drain and seam tape; mixing brands is where leaks start.
Cement board is not waterproof, tile is not waterproof, and grout is not waterproof. What keeps a tiled shower from soaking the framing is the membrane applied over the backer board and the pan before tile goes on — the layer nobody sees and the one that decides whether the shower lasts thirty years or five.
Three families. Bonded sheet membranes (Schluter-Kerdi, Laticrete Hydro Barrier sheet) are a thin polyethylene fabric set into thinset over drywall or cement board, overlapped at seams and connected to a matched drain flange; tile is set directly on the sheet. Foam board systems (Wedi, Kerdi-Board, Durock Shower System) are waterproof boards that replace the cement board entirely, with sealed seams and their own pans and drains. Liquid-applied membranes (RedGard, Hydro Ban, AquaDefense) are painted on cement board in two or three coats to a specified thickness, with fabric at corners and seams — the cheapest and the easiest to under-apply.
Rules that matter: use one manufacturer’s system end to end, including the drain, corners, pipe seals and seam tape; follow the coverage and cure times on the label; wrap the curb and every niche and bench; and flood-test the pan for 24 hours before tiling. A contractor’s quote for a tile shower should name the system; a DIY tile shower should be built from one manual, not from three videos.