Shower stall walls. Well-constructed tile shower walls consist of individual tiles glued to waterproof backer board, with grout used to fill gaps between the tiles. Since grout is not waterproof, some water penetrates the grout to run down the face of the backer board behind the tile to the shower pan or tub. Correctly installing the waterproof backer board before the tiles are attached is the key to preventing leaks.
Since waterproof backer board is more expensive than either drywall or mildew-resistant wallboard, some builders and tile contractors do not use it. This common shortcut creates a potential leak problem when water gets through the tile to the board. Wet board also creates a potential mold and mildew problem, as the grout between the tiles is kept continuously wet from the damp board behind it. Replacing the grout without fixing the damp board problem may not fix the long-term problem. New grout, exposed to the same mold, mildew, and dampness that discolored the old, may soon discolor.
