Moldy or black caulk in the shower

Symptoms

• Dark spots or streaks along the caulk lines
• Musty smell in the bathroom
• Staining returns quickly after cleaning
• Caulk looks discolored even when dry

Likely causes

• CommonMold or mildew growing on caulk surfaceWarm, damp bathrooms with poor ventilation are ideal for mold growth on porous caulk.
• CommonPoor bathroom ventilationWithout an exhaust fan running during and after showers, humidity lingers and feeds mold.
• PossibleCaulk failing and trapping moisture underneathOnce caulk separates from the surface, water and soap scum collect behind it, worsening mold.
• Less commonHidden leak keeping the area constantly dampIf mold keeps returning fast after cleaning and recaulking, moisture may be coming from behind the wall.

What to do

1. Ventilate the bathroom well and put on gloves before cleaning mold.
2. Spray the caulk with a mix of one part bleach to ten parts water, or a commercial mildew remover, and let it sit 10–15 minutes.
3. Scrub gently with an old toothbrush and rinse thoroughly.
4. If staining returns or the caulk looks cracked or peeling, cut it out with a utility knife rather than trying to clean it again.
5. Clean the bare joint with rubbing alcohol and let it dry completely, ideally overnight.
6. Apply a new bead of mold-resistant silicone caulk in a continuous line, then smooth it with a wet finger or caulk tool.
7. Run the bathroom fan during and for 20–30 minutes after every shower going forward to slow mold return.

Tools and parts

• Utility knife
• Old toothbrush
• Caulk gun
• Rubbing alcohol

When to call a pro

Call a contractor if mold keeps returning within days after recaulking, since that can mean a leak behind the wall.

Safety