- 5 Oct 2026
- 4 min read
Black mould on a bath edge looks like a cleaning job. It is not. The silicone has failed and water is already getting past it.
A bathroom, kitchen or wet room bead takes water every day. It wets, it dries and the bath or tray moves under you. Bleach and hard-water cleaner break the surface down. After years of that, the bead pulls off one face, splits or goes soft and black through the body. The black you can see is the late sign. The water behind the panel, under the tray or into the unit is the problem.
Look at the joint, not the bottle of mould spray.
- The bead has pulled away from the tile, the bath, the tray or the worktop
- It is cracked, split or missing in a section
- Black mould has gone through the silicone, not just sat on it
- The joint looks tidy and is soft, porous or breaking up underneath
- Water is escaping behind a bath panel or under a shower tray
Scrubbing the face off does not fix this. Once mould is in the body of the bead, it comes back. A new line of silicone over the old one does not fix it either. New sealant sticks to the old silicone, not to the tile or the tray. The old bead moves. The new one comes with it. You have two failed layers and a gap for water between them.
The repair is specific. Cut the failed bead out. Remove all of it. Do not leave a skin for the new silicone to sit on. That is the internal sealant replacement job.
Score both sides so the blade releases the bead without damaging the tile or the bath. Take the whole body of the old silicone out. Clean and degrease both faces so soap, limescale and residue are gone. If mould has reached the substrate, treat that surface before anything new goes on. Then run one continuous bead of sanitary-grade silicone, tooled tight so it sheds water instead of holding it. Sanitary-grade means a mould-resistant silicone made for constant wet use and cleaning products. A general tube from a DIY shelf or decorator's acrylic is the wrong product. It fails faster in a shower.
Do the full run. Joining a new piece to old silicone leaves a weak point and the rest of an aged bead is rarely far behind. Leave it to cure. Sanitary silicone typically needs 24 hours before you use the bath or shower.
That is the job on these joints:
- Bath to wall and bath to panel
- Shower tray to wall and the enclosure
- Sink to worktop and sink to wall
- Kitchen worktop to wall
- Window boards in bathrooms and wet rooms
- Wet room floor to wall
The same faults are on the bathroom and kitchen leaks page. It is not a tile repair, a grout job or plumbing. If the tray is cracked or the bath flexes enough that no bead will hold, Josh will say so. If water is showing in the ceiling below or well away from the fitting, the cause may be a waste, a pipe or a valve. That is a plumber. NSJ does not do general building, roofing or rendering. If the wet patch is not the silicone, you will be told.
If the bead has failed, it has failed. Waiting does not dry the floor behind the panel. A damp patch after rain on an outside wall is a different fault. That one is on the damp patches after rain post.
If the joint is clearly split, lifted or black through the body, that is the diagnosis. Book the cut-out and replacement. If damp is showing somewhere you cannot line up with the bead, get a building leak and damp inspection first. Diagnosis first. Repair second.
NSJ cuts out and replaces failed internal sealant across Bradford, Leeds, Harrogate, Ilkley, Otley, Skipton, Wetherby, Halifax, Saltaire and the surrounding towns. Speak to Josh direct. Call or WhatsApp 07892 854738 or use the contact page.