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Black Mould on Shower Caulk: Clean It or Replace It?
Short answer
If the mould wipes off with unscented dish soap and water and stays gone, cleaning is enough. If the stain stays after scrubbing, or the mould is back within weeks, the caulk has to come out and be replaced, and the moisture source needs fixing.
Why caulk stains from the inside
Mould on a tile surface sits on top and scrubs away. Caulk is softer, and a bead that has started to lift lets water and soap film get underneath it, where a cloth can't reach. That's why a bead can look clean for a week after scrubbing and then go dark again in the same place. At that point you're cleaning the symptom and the source is still sitting in the joint.
Skip the bleach
Bleach is the first thing most people reach for, and Health Canada's guide says plainly not to use it to clean up mould. For bathrooms, the guide recommends unscented dishwashing detergent and water. It also notes that painting over mould doesn't kill it and doesn't stop it coming back. Sealing fresh caulk over a mouldy bead has the same problem.
How big is too big
Health Canada treats an area of one square metre or less, in three patches or fewer, as small enough for a homeowner to clean. A mouldy caulk line almost always falls into that category. It recommends a professional for large areas, for mould that keeps returning after cleaning, and when mould is suspected inside walls or above ceilings. Strong musty smells with nothing visible are also worth a closer look.
Does the colour matter?
You can't tell what kind of mould you have by looking at it, and black staining on caulk doesn't tell you anything about how dangerous it is. Health Canada doesn't recommend routine air testing for mould. Remove it, dry the area and deal with the moisture.
What replacing it involves
The old bead comes out completely, including the film it leaves behind. The joint gets treated and dried, and a new bead of mould-resistant silicone goes on. That's the Caulk Control job in Kitchener, Waterloo and Cambridge, at a flat $225 per tub or shower plus HST, or $199 for bookings made by January 31, 2027. If we pull the old caulk and find a soft or rotted wall, we stop, show you, and you aren't charged.
Keeping it from coming back
New caulk won't help a bathroom that never dries. Use the exhaust fan when you shower and for at least 30 minutes afterwards, confirm it vents outside and not into the attic, and keep humidity between 30% and 50%.