Black mould in bathroom silicone: can it be cleaned or must it go?

Is it mould sitting on the surface or dirt in the silicone itself

Surface mould sits on top of the silicone. It usually appears first at the lowest point of the joint, where water pools and takes longest to dry, and it often wipes away or at least fades noticeably when you clean it with a household mould spray or diluted bleach. The silicone underneath stays smooth, flexible and stuck firmly to both tile and tub.

Staining that is inside the silicone is different. It looks like fine black threads or a grey-black tinge running through the material rather than sitting on top, and cleaning barely touches it because the mould has taken hold in the silicone itself, not just on its surface. This is the point where cleaning stops being a real fix.

What is worth trying yourself before deciding to replace it

If the discolouration is genuinely on the surface, start with a proper clean rather than jumping straight to replacement. A mould spray meant for bathroom sealant, left on for the time stated on the bottle rather than wiped off immediately, deals with most surface cases. For light staining, a paste of bicarbonate of soda and water worked into the joint with an old toothbrush is often enough.

Ventilation afterwards matters as much as the cleaning product. Wipe the joint dry after showering, leave the bathroom door or a window open for a while, and if the room has an extractor fan, run it for a good twenty minutes after bathing rather than switching it off straight away.

Give this a week or two. If the black returns within days in the same spot despite drying and ventilating properly, that is telling you the problem is not surface mould you can keep wiping off, but something underneath.

When redoing the sealant joint is the only real fix

Once mould has coloured the silicone right through, no cleaning product restores it, because the discolouration is part of the material now, not a layer on it. The same applies once the sealant has started to peel away from the tile or the tub edge, gone hard and cracked, or pulled away enough that you can see a gap or feel movement when you press it.

In either case the joint needs to be cut out completely, the surface cleaned back to bare tile and tub, and new silicone applied. Trying to caulk a fresh bead over old, discoloured or lifting sealant rarely holds, because the new silicone will not bond properly to what is already there.

This is straightforward, contained work, not a renovation. We offer silicone resealing from 45 € per job, and it is worth booking once you have established the discolouration is not coming off with cleaning.

Why bathroom sealant is so prone to this in the first place

Silicone around a bath or shower is warm, damp for long stretches of the day, and often not fully dried between uses, which is exactly the environment mould spores need. Cheaper sealants and older joints have no mould inhibitor left in them, if they ever had one, so once spores establish themselves there is little in the material itself resisting them.

Shampoo, soap and shower gel residue sitting in the joint gives mould something to feed on beyond moisture alone, which is why the lowest point of the joint, where product-laden water collects and evaporates slowest, is almost always where it starts.

• Rinse the joint with clean water after washing to remove soap and shampoo residue
• Squeegee or wipe the tub and tile edge dry rather than letting it air-dry
• Ventilate the bathroom properly after every shower or bath
• Reseal with a sanitary-grade silicone that includes a mould inhibitor
• Avoid leaving wet towels, bath mats or sponges resting against the joint

Could the problem run deeper than just the visible joint

A joint that keeps failing within months of being redone, rather than lasting a reasonable while, sometimes points to something behind it rather than the silicone itself. Grout that has cracked or gone soft nearby, a slight gap between tub and tile that lets water track behind rather than staying on the surface, or a shower tray that has begun to sit unevenly can all keep feeding moisture into the same spot.

If you notice the wall board or plaster near the joint feels soft, if tiles nearby sound hollow when tapped, or if the mould keeps returning at the same point no matter how carefully the joint is redone, the sealant is probably a symptom rather than the cause. That is worth having looked at properly rather than resealing the same joint a third time.

What replacing the sealant actually costs

Resealing a bath or shower tray joint is one of the more affordable jobs we do, and for a straightforward joint with no underlying damage it is priced from 45 € per job, covering removal of the old silicone, cleaning the surface and applying new sealant with a proper finish.

If the investigation turns up damp plasterboard, failed waterproofing under the tiles, or a shower tray that needs resetting, that is no longer a sealant job but a repair to the wall or floor build-up behind it. In that case we would give a fixed quote after seeing photos or visiting, rather than guessing at hourly time.

Where the bathroom as a whole is showing its age, with several of these issues stacking up together, a full renovation, which we price from 3500 € per job depending on size and scope, is often more sensible than fixing the same joint repeatedly over a couple of years.