Short answer: if the black marks are on the surface of the caulk, cleaning can lift them. If the mould has grown into the caulk, which is what usually happens in a shower, cleaning will not reach it and the marks come back within weeks. The fix is to cut out the old caulk, clean and dry the joint, and apply fresh mould-resistant silicone.
Why mould grows in caulking
Shower corners and the joint along the tub stay wet longer than any other surface in the bathroom. Soap scum and skin oils give mould something to feed on. Over time the mould works its way into the soft caulk and under its edges, where no cleaner can reach. Cracked or peeling caulk is worse, because water gets behind it and into the wall.
How to tell surface mould from deep mould
- Surface mould: wipes lighter with a bathroom cleaner and a soft brush.
- Deep mould: black or pink spots stay after scrubbing, or return within weeks.
- Failed caulk: cracks, gaps, peeling edges or soft spots.
How caulking is properly replaced
- Cut out and strip all of the old caulk.
- Clean the joint and treat any mould on the surrounding tile.
- Let the joint dry completely.
- Apply new mould-resistant silicone in one smooth bead.
- Tool the bead for a clean, watertight finish.
- Let it cure before the shower is used. We tell you how long on the day.
Applying new caulk over old caulk traps the mould underneath and does not seal properly.
Keeping it clean afterwards
- Squeegee the shower walls after each use.
- Run the bathroom fan during and for 20 minutes after showers.
- Leave the shower door or curtain open to dry.
- Clean the joints weekly with a non-abrasive bathroom cleaner.
Silicone or acrylic caulk: which belongs in a shower
Showers and tubs need a flexible, waterproof sealant. 100% silicone with mould inhibitor is the usual choice for joints that get wet every day, such as the line where the tub meets the wall, inside corners and around the shower base. Acrylic latex caulk is easier to paint and tool but is less flexible and less water resistant, so it is better for trim and dry areas. Using the wrong product is one reason caulking fails early and turns black within months.
Bathroom habits that keep mould away
- Run the exhaust fan during every shower and for about 20 minutes after.
- Squeegee walls and glass after showering, and leave the door or curtain open to dry.
- Wipe the caulk lines along the tub and base with a dry towel.
- Wash bath mats and shower curtains often.
- Fix dripping taps and leaks, which keep corners wet all day.
When it is more than caulking
If tiles near the joint feel loose, the wall sounds hollow when tapped, grout is cracked or there are stains on the ceiling below the bathroom, water may already be behind the tile. New caulking will not fix that. A contractor should check the wall or floor before resealing. Catching it early is far cheaper than replacing a shower surround later.
Book shower recaulking
UltiClean removes old mouldy caulking, cleans and dries the joint, and applies new mould-resistant sealant. Call 647-676-0576 or see tile caulking and recaulking and tile and grout cleaning.
Mouldy Shower Caulking FAQs
Is the black in my shower the grout or the caulk?
Usually the caulk, in the corners and along the tub. Grout lines between tiles can often be cleaned. Read how to keep grout clean.
Does bleach kill mould in caulk?
It can lighten surface marks, but it does not reach mould that has grown into the caulk, and it can damage some grout colours.
How long does new caulk last?
Several years in a well-ventilated bathroom. Poor ventilation shortens it.
Can you replace caulking and clean the tile on the same visit?
Yes. Many customers book both. See tile caulking and tile and grout cleaning.
Is shower mould a health concern?
Mould can bother people with allergies or asthma. If you have health concerns, talk to your doctor.
Mouldy caulking in Toronto, Vaughan, Richmond Hill, Aurora or Newmarket? Call UltiClean at 647-676-0576.
Ready for a deep clean?
Carpets, air ducts and tile and grout across Richmond Hill, Aurora, Newmarket, Vaughan and Toronto.
Written by
Carpet, air duct and tile cleaning specialist at UltiClean
Seyedamir (Amir) Vafamand has 12 years of hands-on experience cleaning carpets, upholstery, air ducts, dryer vents and tile and grout in homes across Toronto, Vaughan, Richmond Hill, Aurora and Newmarket. This guide is based on that work.
- 12 years of experience
- 1,100+ jobs completed on Jiffy
- 98% approval rating
- $2M liability insurance
UltiClean Inc.
54 Limeridge St
Aurora, ON L4G 7X9
647-676-0576, open 24 hours, 7 days a week