How to Remove Mould from Silicone Caulk — What Works and What Doesn't

Black mould in the silicone caulk around a tub, shower, or sink is one of the most stubborn cleaning problems in a bathroom. The black discolouration is mould growing inside the porous silicone — not just on the surface — which is why spray cleaners that treat the surface don't work: the mould is embedded in the material itself.

Surface Mould vs. Embedded Mould

The first step is identifying which you have:

• Surface mould: appears as a hazy grey or pink film on the caulk surface, present for a few weeks; can often be removed with bleach spray and scrubbing
• Embedded black mould: the caulk is visibly black or dark grey through its entire thickness, and scrubbing doesn't change the colour; this is mould growing inside the silicone

Surface mould is treatable. Embedded mould requires recaulking — no amount of cleaning will fix it.

Treating Surface Mould

Method 1: Bleach Gel or Thick Bleach Paste

The challenge with liquid bleach spray on caulk is contact time — it runs off before it can work. The key is keeping concentrated bleach in contact with the caulk for an extended period.

1. Mix bleach with an equal part of baking soda to form a thick paste (the baking soda thickens the bleach so it stays put)
2. Apply the paste directly to the mouldy caulk using a small brush or old toothbrush
3. Cover with plastic wrap to keep the paste wet and in contact with the surface
4. Leave for 4–8 hours, or overnight for heavier staining
5. Remove the plastic, scrub with a toothbrush, and rinse thoroughly

Method 2: Hydrogen Peroxide (3%)

Undiluted 3% hydrogen peroxide (standard pharmacy concentration) is an effective mould treatment that doesn't require the ventilation precautions of bleach:

1. Pour onto a cloth and press against the mouldy caulk for 10 minutes
2. Scrub with a toothbrush and rinse

Both methods work for mild to moderate surface mould. If the caulk remains black after treatment, the mould is embedded and recaulking is needed.

When to Recaulk

Recaulk when:

• The caulk remains black after two cleaning attempts
• The caulk is cracked, pulling away from the tile, or shrinking
• Water is getting behind the caulk line (you'll see it lifting or bubbling)

Removing old caulk and applying new silicone takes about 1–2 hours and $15–20 in materials. It's the permanent solution; continued cleaning of embedded mould is a temporary fix at best.

Preventing Caulk Mould

• Run the bathroom exhaust fan during and for 15 minutes after every shower — this removes the humidity that feeds mould
• Squeegee or wipe down the caulk area after showering
• Use a daily shower spray (applied to surfaces after showering, no rinse needed) which inhibits mould growth between cleanings

Maidless's bathroom deep clean includes scrubbing caulk lines with a toothbrush. Book a Toronto deep clean — HST included.