Bathroom mold, explained
Surface Spots or Something Behind the Tile in Your San Mateo Bathroom?
Bathroom calls split into two groups. Some of it is surface growth on grout lines and caulk, feeding on body oils and soap scum, and a good cleaning handles it. The harder cases are growing out of sight, in the wall behind the tub surround or in the subfloor, kept damp by a cracked shower pan liner, a dripping valve body or a toilet seal that's been seeping for a long time.
People fight caulk mold for years without winning. Once mold roots into the silicone itself, bleach only lightens the color for a few weeks, so the bead has to be cut out, the joint dried and cleaned, and fresh mildew-resistant silicone laid in. Ceilings are the other hot spot: steam from long showers condenses on cold drywall above the tub when the exhaust fan is undersized, isn't run long enough, or vents into the attic instead of outdoors.
Older tub surrounds often hide paper-faced drywall behind the tile, and paper is mold food once water gets past the grout. When the cause is a leak, the remediator and a plumber usually have to coordinate.