How do I fix caulk that is turning black?

Black caulk is caused by mold and mildew growth and needs to be completely removed and replaced - cleaning alone won't solve the problem permanently.

The black discoloration you're seeing is mold that has penetrated into the caulk material itself. This happens when moisture gets trapped behind or within the caulk, creating the perfect environment for mold growth. Simply scrubbing the surface might temporarily improve the appearance, but the mold will return quickly since it's embedded in the caulk.

Complete removal and replacement is the only effective solution. Start by cutting out all the old caulk using a utility knife or caulk removal tool. Clean the area thoroughly with a bleach solution (1 part bleach to 10 parts water) to kill any remaining mold spores on the surfaces. Allow everything to dry completely - this is crucial because applying new caulk over damp surfaces will just recreate the same problem.

In Ottawa's climate, proper ventilation is essential to prevent this issue from recurring. Our humid summers and temperature fluctuations create ideal conditions for mold growth in bathrooms. Make sure your bathroom fan runs during and after showers, and consider upgrading to a more powerful fan if moisture problems persist.

When applying new caulk, choose a high-quality mold-resistant silicone caulk specifically designed for bathrooms. These products contain antimicrobial additives that help prevent mold growth. Apply the caulk in a continuous bead, smooth it with your finger or a caulk tool, and allow it to cure for 24 hours before exposing it to water.

For extensive mold problems or if this keeps happening, it may indicate a larger moisture issue that requires professional attention. A bathroom renovation contractor can assess whether there are ventilation problems, leaks behind the walls, or other issues contributing to excessive moisture buildup.

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