You've taken a bleach-soaked toothbrush to the mildewed grout and applied new caulk where tile meets tub. Yet in no time, the mold is back, black, and gross all over. This could be the sign of a more serious problem lurking under the surface. Over time, water can get into voids between tiles and turn wallboard to mush, says Bart Bettiga, executive director of the National Tile Contractors Association
. It can also cause tiles to buckle. The only remedy is to remove the tiles and replace rotten wallboard with a waterproof material, such as fiber cement ("green board" is no longer recommended in wet areas). Be warned: Mold can be toxic and make you sick. Find out what's growing and do demo with caution, says Bettiga. It's also easy to break tiles when you pull them out, so plan to put in new ones or have matching replacements on hand.