Is the black mould in my silicone or my grout, and can I clean it off?
The five-minute test that tells you whether mould is on the surface or grown into the material, why that decides everything, and what to do in each case.
- Difficulty
- beginner
- Time
- 45 min
- Read
- 5 min
- Safety
- caution
Short answer
Apply a bleach-based mould remover and leave it for the full stated time. Surface mould goes completely and stays gone once the moisture is controlled. Mould that lightens and leaves grey or black shadows has grown into the material — silicone that has to be cut out and replaced, or grout that has to be raked out and redone. No cleaner reaches into either.
The reason mould cleaning feels like a losing battle is that two completely different problems look identical. Mould sitting on a surface film of soap and body oil wipes off and stays off. Mould whose hyphae have grown into the body of a silicone bead, or down into porous cement grout, cannot be reached by anything you can safely put on it — because the cleaner works on contact and the growth is inside the material. Telling them apart takes one application and forty minutes of waiting.
Safety
What you'll need
- A bleach-based mould remover, or a household bleach solution
- Gloves and eye protection
- Kitchen roll or cotton wool, to hold the product against a vertical joint
- A stiff narrow brush
- A torch
Step by step
- First, work out which material you are actually looking at.Press the black line with a fingernail. Silicone is soft and gives. Grout is hard and does not. In a shower this matters enormously, because a corner joint should be silicone and a flat wall joint should be grout, and people frequently treat one as the other.
- Clean the surface first.Wash the area with detergent and a brush and rinse. A great deal of what looks like mould in a bathroom is a soap and body-oil film with mould growing on it, and removing the film removes most of the food supply. Let it dry.
- Run the test.Apply a bleach-based mould remover directly and leave it for the full time on the label, typically fifteen minutes to an hour. For a vertical joint, lay strips of kitchen roll or cotton wool along it and soak them so the product stays in contact instead of running off. Ventilate the room while you wait.
- Read the result.Rinse and dry, then look with a torch. If the joint is uniformly clean, the mould was on the surface and the material is fine. If it is lighter but still shows grey or black shadows, ghosting or speckles, the growth is inside the material and no further cleaning will change that.
- Do not repeat the treatment endlessly.A second application on genuinely penetrated silicone or grout gives a marginal improvement and then nothing. Repeated bleaching also degrades the silicone's surface, making it rougher and easier for the next lot of mould to colonise. One thorough test is enough to know.
- If it is in the silicone, replace it.There is no repair for mould growing inside a bead. Cut it out along both edges, peel it away, remove every trace of residue with a solvent silicone remover, clean with methylated spirit, let the joint dry completely, and reseal with a sanitary-grade silicone containing a fungicide.
- If it is in the grout, decide how deep it goes.Cement grout is porous and mould grows into it. Scrape a little with a fingernail: if clean grout appears immediately below the surface, cleaning plus sealing the grout may be enough. If it is black through, rake the joint out to at least 3mm and regrout.
- Check whether the joint should have been silicone in the first place.Every internal corner, and the junction where tile meets bath or tray, moves. Grout is rigid and will crack there and then grow mould in the crack. If you find blackened cracked grout in a corner, that joint should be sanitary silicone, not grout, and replacing like with like just repeats the problem.
- Then deal with why the mould is there.It is moisture. A bathroom that stays wet for hours after a shower grows mould in whatever it is given. Extractor running during and after the shower, door closed while it runs, screen and tiles squeegeed, towels dried elsewhere. Cleaning without changing that is a monthly chore forever.
If it doesn't work
The silicone goes clean and is black again within weeks
Cause: The growth is inside the bead and the surface cleaning only removed what was visible — Fix: Replace the bead. Repeated cleaning of penetrated silicone is a permanent chore with no end.
Grout is black only in the corners and in the tray joint
Cause: Those joints move, the grout has cracked, and mould is growing in the cracks — Fix: Rake the grout out of those joints entirely and replace with sanitary silicone. They are movement joints, not grout joints.
Black spots appear on the wall and ceiling as well as the joints
Cause: This is a room ventilation problem, not a sealant problem — Fix: Deal with the extraction and the humidity. Replacing the sealant will not stop mould that is growing on painted surfaces.
The silicone has gone black underneath, visible through it
Cause: Mould growing on the substrate behind a translucent or thin bead, often because the joint was sealed damp — Fix: Cut it out, dry the joint completely — a hairdryer or a day with a fan — treat the substrate, and reseal onto a dry surface.
Questions people ask
Does a mould-resistant silicone stop this?
It delays it. Sanitary silicones carry a fungicide that leaches out slowly over a few years, so they resist colonisation for longer. Nothing prevents mould in a joint that stays wet.
Can I paint over mouldy sealant?
No. Silicone cannot be painted at all, and painting over mould in any material simply hides it while it continues to grow.
Is bathroom mould dangerous?
For most people it is an irritant rather than a hazard at the quantities found on a sealant bead. It matters more for people with asthma or a compromised immune system, and it is worth removing properly rather than tolerating.