Black spots creeping along the bottom of the shower or the back of the bath are one of the most common reasons people call us. The sealant may still be stuck down and watertight, but it looks dirty however hard you scrub.
Mould in sealant isn’t a sign that you’re not cleaning properly. It’s what happens when a joint stays damp, warm and coated in soap and skin residue for long enough. Once mould has grown into the bead, cleaning the surface won’t get it out. This guide explains why it happens, what you can try, and how to give a new bead the best chance of staying clean.
It also covers when black marks are a sign of something more serious, such as water getting behind the bath or shower tray, and what landlords and hotels can do to keep on top of it across several rooms.
What mould needs to grow
Mould needs a few things, and a bathroom provides all of them:
- Moisture. Sealant joints along baths and shower trays are wet several times a day, and water sits in the corners.
- Warmth. Warm, steamy air after a shower is ideal for mould.
- Food. Soap, shampoo, shower gel and skin cells leave a film on the sealant that mould feeds on.
- Poor air movement. If steam can’t escape, surfaces stay damp for hours.
Shower trays and the back edge of a bath are the worst spots, because water pools there and dries slowly. Take away one or two of these and mould struggles. Leave all four in place and even good anti-fungal silicone will eventually spot.
Why it gets into the silicone, not just onto it
Silicone is flexible and slightly soft. Mould that starts on the surface can grow into the bead, and the dark staining ends up below the surface where cleaning can’t reach. That’s why some black marks seem to come back days after you’ve scrubbed them.
Mould also gets behind sealant. If the bead has lifted at one edge, or new sealant was laid over old, water and soap get trapped underneath. You see black shadows through the sealant that no cleaner will touch. In a 1950s terrace in Three Bridges, Crawley, someone had sealed straight over failed sealant, and mould was growing underneath. Both layers had to come out before the bath could be resealed with Everbuild anti-fungal silicone.
Why bleach rarely fixes it
Bleach and mould sprays can lighten surface staining, sometimes very well. But if the mould has grown into the sealant or is trapped behind it, the colour usually comes back, because the source is still there. Repeated harsh cleaning can also roughen the surface of the bead, giving dirt and mould more to hold on to.
If you do try a mould remover, follow the label, ventilate the room, wear gloves, and never mix bleach with other cleaning products, especially acidic ones. Our guide to cleaning black mould out of silicone covers what’s worth trying.
When cleaning won’t work
It’s time to replace the sealant rather than clean it when:
- the black marks come back within days of cleaning;
- you can see dark staining under the surface or behind the bead;
- the sealant has lifted, split or shrunk;
- there are two layers of sealant;
- the mould has spread to the grout or the wall behind.
Replacing a mouldy bead properly means cutting all of it out, cleaning off the residue, treating the joint, letting it dry fully, and laying a new bead of anti-fungal sanitary silicone. That is what our bathroom resealing service does.
Need it done properly? Mr Mastic covers the South East, London and Scotland, with a one-year guarantee on every job. Call or email for a quick quote.
Does anti-fungal silicone stop mould?
Anti-fungal (sanitary) silicone contains fungicide that helps it resist mould. It makes a real difference compared with general-purpose sealant, and it’s what we use in wet areas. But it isn’t a force field. If the bathroom has poor ventilation and the joint stays coated in soap, mould can still grow on the surface film.
Think of it as one part of the answer. The other parts are good preparation when it goes in and good habits afterwards.
Ventilation: the part people forget
Most mould problems in bathrooms are ventilation problems. Some simple checks:
- Does the extractor fan actually pull air? Hold a sheet of toilet paper near it while it runs.
- Is it left on long enough after a shower to clear the steam?
- Is there a gap under the door, or a trickle vent, so fresh air can get in?
- Do towels dry, or stay damp all day?
If the walls are running with condensation every morning, a new bead of sealant will be fighting a losing battle. The UK government’s guidance on understanding and addressing the health risks of damp and mould is written for landlords but is useful reading for anyone dealing with mould at home.
How to stop mould coming back after a reseal
- Let the new sealant cure fully before it gets wet. We tell you the cure time for the product used; it’s usually around 24 hours.
- Run the extractor during every bath and shower, and for a while afterwards.
- Rinse soap and shampoo off the sealant and tray after use.
- Wipe or squeegee the joints dry, especially in the shower.
- Clean weekly with a mild, non-abrasive cleaner and a soft cloth.
- Deal with drips and leaks quickly, so water doesn’t get behind the bead.
Mould trapped under sealant: a leak warning
Black shadows behind a bead are more than a cosmetic problem. They mean water is getting into the joint, and it may be getting further. Look for:
- a damp or swollen bath panel or kitchen plinth;
- loose or hollow-sounding tiles near the shower tray;
- staining on the ceiling below the bathroom;
- a musty smell that doesn’t go after airing.
If you see these, the joint needs cutting out and the cause found. Our remedial sealant service is for exactly this kind of job.
Notes for landlords and hotels
Mouldy sealant is one of the most common complaints in rented homes and guest bathrooms. It looks unhygienic even when the room is clean. For landlords in Scotland, homes must meet the Repairing Standard, which includes being wind and watertight; in England, the government damp and mould guidance linked above sets out the health risks.
Checking sealant at each change of tenancy, fixing faulty extractor fans and resealing before mould takes hold will save repeated call-outs and complaints.
Quick answers
Why does my shower sealant keep going black?
Usually a mix of damp, warmth, soap residue and poor ventilation. Once mould grows into the silicone, cleaning only lightens the surface.
Can black mould be removed from silicone?
Surface staining can sometimes be lightened. Mould that has grown into or behind the bead won’t clean out, and the sealant needs replacing.
Is anti-fungal sealant worth it?
Yes, in wet areas. It helps resist mould, but it works best with good ventilation and regular cleaning.
Is mould behind sealant a sign of a leak?
Often. It means water is getting into the joint, so it’s worth having it cut out and checked.
Do you guarantee new sealant?
Every Mr Mastic job comes with a one-year guarantee.
Related services
More advice
- Can You Clean Black Mould Out of Silicone?
- Looking After New Sealant: Aftercare Tips
- Can You Put New Silicone Over Old Silicone?
- Sealant Maintenance for Landlords and Agents
- All advice
This guide is general information about sealant work. It isn’t legal, fire-safety or structural advice; for those, speak to a qualified professional.