What goes behind shower tile — cement board, waterproofing membrane, then tile

Tile and grout are not waterproof — the membrane behind them is. A shower wall is framing → cement board (never standard drywall) → A118.10 waterproofing membrane → thin-set at 95% coverage → tile.

A cross-section through a shower wall shows the layers from the dry side to the shower side: wood studs, a cement backer board, an ANSI A118.10 waterproofing membrane, thin-set mortar at 95 percent coverage, and tile with grout. Because tile and grout are not waterproof on their own, water that passes the tile is stopped by the membrane behind it. Standard drywall is prohibited behind shower and tub tile because it wicks water and rots; code requires cement board or a coated glass-mat panel. — Source: ANSI A118.9 / A118.10, TCNA B415, IRC R702.4.2

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A cross-section through a shower wall shows the layers from the dry side to the shower side: wood studs, a cement backer board, an ANSI A118.10 waterproofing membrane, thin-set mortar at 95 percent coverage, and tile with grout. Because tile and grout are not waterproof on their own, water that passes the tile is stopped by the membrane behind it. Standard drywall is prohibited behind shower and tub tile because it wicks water and rots; code requires cement board or a coated glass-mat panel.

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