How to Remove Mould from Bathroom Caulk — Toronto Home Guide

Black mould on bathroom caulk is extremely common — caulk around the tub, shower, and sink is constantly wet, poorly ventilated, and provides a porous surface for mould spores to establish. Surface mould responds well to bleach treatment; mould that has penetrated deep into the caulk requires recaulking.

Surface Mould — Bleach Method

1. Ensure the bathroom is well-ventilated — open windows and turn on the exhaust fan. Wear gloves
2. Apply undiluted household bleach directly to the mouldy caulk using a cotton ball, old toothbrush, or tissue soaked in bleach — the aim is to keep the bleach in contact with the caulk rather than running off
3. For caulk on horizontal surfaces (tub rim), lay strips of bleach-soaked paper towel directly on the caulk and leave for 2–4 hours
4. Remove and scrub remaining mould with an old toothbrush
5. Rinse thoroughly with water

Hydrogen Peroxide Alternative

For those who prefer to avoid bleach fumes or have coloured caulk:

1. Spray 3% hydrogen peroxide directly onto the mouldy caulk
2. Let sit 15–30 minutes
3. Scrub with an old toothbrush and rinse
4. Repeat if needed — peroxide is less aggressive than bleach but safer for coloured surfaces

When to Recaulk

If the black discolouration remains after bleach treatment, the mould has penetrated through the caulk to the substrate — cleaning the surface won't resolve it. The caulk needs to be removed and replaced:

• Score the caulk with a utility knife along both edges
• Pull off the old caulk in strips — a caulk removal tool helps
• Clean the surface with bleach, let dry completely — 24–48 hours
• Apply fresh mould-resistant silicone caulk and smooth with a wet finger
• Allow to cure 24 hours before getting wet

Prevention

• Run the exhaust fan during and for 15 minutes after every shower
• Wipe down wet caulk with a dry cloth after showering
• Use mould-resistant caulk when recaulking — it contains antimicrobial additives