Black staining on bathroom caulk is one of the most common cleaning challenges in Toronto homes — and one of the most mishandled. The mould that grows in caulk is different from surface mould on tile or grout: it penetrates the porous silicone structure and can be impossible to remove without recaulking. Knowing when to clean versus when to replace saves time and frustration.
Surface Mould vs. Penetrated Mould: The Difference
Surface mould on caulk appears as black or dark grey speckling on the outer face of the caulk bead. It can often be removed with cleaning. Penetrated mould has worked into the silicone itself — the caulk appears discoloured through its entire thickness, not just on the surface. This cannot be cleaned out; the caulk must be removed and replaced.
How to tell: apply your cleaning solution, scrub, and rinse. If the discolouration is gone, it was surface mould. If the caulk is still dark after scrubbing, it's penetrated — plan to recaulk.
Method 1: Bleach-Based Cleaner (Most Effective for Surface Mould)
- Ensure the bathroom is ventilated — open a window or run the fan
- Mix 1 tablespoon of household bleach with 1 cup of water in a spray bottle, or use a commercial mould-and-mildew spray
- Apply directly to the mouldy caulk and let sit for 10–15 minutes
- Scrub with a stiff-bristle brush or old toothbrush
- Rinse thoroughly with water and dry
- Repeat if necessary for stubborn spots
For caulk around the bathtub, you can soak cotton balls or paper towels in the bleach solution, press them against the caulk, and leave for 30–60 minutes. This keeps the solution in contact with the surface longer.
Method 2: Hydrogen Peroxide (Milder Alternative)
3% hydrogen peroxide (available at any pharmacy) is antifungal and effective on surface mould without the harshness of bleach. Apply directly to the caulk, let sit for 10 minutes, scrub, and rinse. Less effective than bleach for established mould, but better for areas with coloured caulk where bleach might cause visible damage.
When to Recaulk Instead of Clean
Recaulk when:
- The mould is penetrated (doesn't come clean after scrubbing)
- The caulk is cracking, peeling, or pulling away from the surface
- The caulk has been cleaned multiple times and keeps moulding quickly — this indicates the caulk is old and no longer repelling water effectively
- The caulk is discoloured overall, not just in spots
Recaulking is a DIY task for most homeowners: score and remove the old caulk with a caulk removal tool, clean the surface thoroughly with rubbing alcohol, apply new silicone caulk, and smooth with a wet finger. The whole process takes about an hour and lasts several years.
Preventing Mould on Bathroom Caulk
- Run the bathroom fan during and for 20 minutes after showering — this removes the moisture that enables mould growth
- Squeegee the shower walls and door after each use
- Spray the caulk area with diluted white vinegar weekly and rinse — regular mild acid prevents mould from establishing
- Ensure the bathroom has adequate ventilation — if the fan doesn't clear steam within 20 minutes, it may need to be upgraded
Bathroom deep cleaning including caulk scrubbing is part of Maidless's deep cleaning service in Toronto. Book a Toronto bathroom deep clean — HST included.