- 19 Oct 2026
- 5 min read
Short answer: if the black is on the surface of a bead that is still stuck down on both sides, clean it and deal with the moisture. If the black has gone into the body of the silicone, or the bead has split or lifted, cleaning will not shift it. The bead needs cutting out and replacing. Condensation explains why it shows up in winter. It does not fix a failed seal.
Why it gets worse when the weather turns
Condensation forms when warm, moist air meets a cold surface. In winter the tiles, walls and window boards in a bathroom are colder, the window stays shut and more washing gets dried indoors. The Centre for Sustainable Energy lists poor ventilation, extra moisture from cooking and drying clothes, cold spots and too little heating as the usual causes.
After a shower, the silicone at the bottom of the tiles is one of the last places to dry. A bead that stays damp for hours every day is where mould gets a hold. That is why it so often shows up in the colder months.
Clean it or replace it?
Look at the bead, not the bottle of mould spray.
Cleaning may be enough if:
- The black sits on the surface and wipes lighter
- The bead is still firm and stuck to both faces
- There are no gaps at the corners or the ends
It needs replacing if:
- Black spots stay after cleaning because they are in the silicone, not on it
- The bead has pulled away from the tile, the bath, the tray or the worktop
- It is split, cracked, soft or missing in a section
- Water is getting behind the bath panel or under the shower tray
Once mould is in the body of the bead, it comes back through any amount of cleaning. A new line of silicone over the top does not fix it either. It sticks to the old silicone, not to the tile, and lifts with it. The repair is a full cut-out and reseal with sanitary-grade silicone. That is our internal sealant replacement job, and the process is set out in cut out failed bathroom silicone, do not bead over it.
Is it condensation or a failed seal?
- Condensation: droplets on the window and mirror, and mould in cold corners, on the ceiling and around the window as well as on the silicone. Worse after showers and in cold spells. The silicone may be sound underneath.
- Failed seal: mould only along the bead, gaps you can see, staining on the bath panel, a soft floor by the tray, or a mark on the ceiling below that gets worse after showers.
It can be both. Condensation from everyday humidity or poor ventilation is not something NSJ fixes. An extractor fan that works, an open window after a shower and steady heating all help, and the RICS damp and mould consumer guide is a sensible place to start. If the silicone has failed as well, we cut it out and replace it. Our page on damp, condensation or a leak covers the wider question.
Shower leaking through the ceiling?
Note when the mark appears. If it only gets worse after someone has used the shower, the tray, the waste or the seal around them are the usual suspects. If it gets worse when nobody has used the bathroom, suspect a pipe or a valve. That is a plumber, not a sealant job. Keep clear of a ceiling that sags and of any wet light fittings.
If the tray is cracked or moves underfoot, no bead will hold and we will say so. If the wet patch is not the silicone, you will be told. Where damp is showing somewhere you cannot line up with the bead, a building leak and water ingress inspection comes first. More on the signs is on the bathroom and kitchen leaks page.
Resealing in winter
Winter is fine for resealing a bath or shower in a heated house. Silicone cures more slowly when it is cold. Manufacturers' data sheets give an application range starting at +5°C, quote cure rates measured at room temperature and say low temperatures and low humidity slow curing down. On the day:
- Keep the heating on in the bathroom
- Keep the room ventilated
- Leave it at least 24 hours before using the bath or shower, longer if the room is cold
We will advise based on the product used.
Recent resealing jobs
Before and after: failed, mouldy shower tray silicone cut out and replaced with sanitary-grade sealant in Harrogate and Baildon. More photos are on our work page.
What it costs
Internal sealant replacement is £24 per linear metre, minimum £100. A bath perimeter is typically 5 to 8 linear metres, from £120. A shower tray perimeter is typically 4 to 7 linear metres, from £100. That covers full removal of the old bead, cleaning, mould treatment where needed and a new bead in sanitary-grade silicone. It is not tile repair, grout replacement or plumbing. The full breakdown is in how much does sealant replacement cost.
Getting a price
Send a photo of the joint and your town on WhatsApp to 07892 854738, or use the contact page. Quotes are generally free, but sometimes an inspection is needed first (inspections from £125). We cover West and North Yorkshire, including Bradford, Leeds, Harrogate, Ilkley, Otley, Shipley and Baildon.
Questions people ask
- Why does bathroom silicone go black in winter?
- Colder rooms, shut windows and washing dried indoors mean more condensation, so the silicone stays damp for longer after each shower and mould gets a hold. If the bead is still sound, cleaning and better ventilation may be enough. If the mould is in the body of the silicone, it needs replacing.
- Can I clean black mould off silicone?
- Sometimes. Surface mould on a sound bead can often be cleaned. If black spots stay after cleaning, the mould is in the silicone and will come back. The fix is to cut the old bead out completely and reseal with sanitary-grade silicone.
- Is my shower leaking through the ceiling because of the silicone?
- It can be. If the mark only grows after the shower has been used, the tray, the waste or the seal around them are the usual suspects. If it grows when nobody has used the bathroom, suspect a pipe or valve and call a plumber. We will tell you if it is not a sealant job.
- Can silicone be replaced in winter?
- Yes, in a heated room. Silicone cures more slowly in the cold, so keep the heating on, keep the room ventilated and leave it at least 24 hours before using the bath or shower, longer if the room is cold.
- How much does it cost to reseal a bath or shower?
- NSJ charges £24 per linear metre with a minimum of £100. A bath perimeter is typically 5 to 8 linear metres, from £120. A shower tray perimeter is typically 4 to 7 linear metres, from £100.
- Does NSJ fix condensation?
- Not condensation from everyday humidity or poor ventilation. We replace failed silicone. For condensation, the RICS damp and mould consumer guide is a sensible place to start.
Sources
These are plain references so you can check them yourself. They are general guidance, not NSJ policy, and not a promise about any particular property.
- Centre for Sustainable Energy, Condensation, damp and mould (cse.org.uk/advice/condensation-damp-and-mould/)
- RICS, Damp and mould consumer guide (rics.org/consumer-guides/damp-and-mould)
- Bostik, S31 Silicone technical data sheet TDS02191, May 2023 (application temperature +5°C to +40°C, curing 2 to 3mm per 24 hours)
- Henkel, Ceresit CS15 sanitary silicone technical data sheet, December 2025 (low temperature, low humidity or large joints need longer curing)
- Checkatrade, How to fix a shower leak (a leak only when the shower runs usually points to a seal, tray or drain rather than a supply pipe)