Black mould in shower silicone: clean it or replace the seal?

How to tell if the mould is on the surface or inside the seal

Surface mould sits on top of the silicone like a stain. Wipe it with a cloth dipped in diluted bleach or a proprietary mould spray and it usually lightens or comes off within a minute or two, leaving the silicone underneath its normal colour. If the black comes away and does not return within a day of cleaning, it was sitting on the surface.

Mould inside the seal looks different up close: grey-black threads or speckling that run through the body of the silicone rather than sitting on it, sometimes visible as a shadow under the translucent edge of the bead. Cleaning the surface does nothing to this, because the spray cannot reach the mould growing inside the material.

A simple test: press the silicone gently along the seam. If it has gone soft, spongy or has pulled slightly away from the tile or the tray, mould and moisture have likely worked their way underneath, and no amount of scrubbing will sort it out permanently.

What to try before cutting the seal out

If the mould is genuinely surface-level, start with a mould-specific spray or a mix of household bleach and water, left to sit for ten to fifteen minutes rather than wiped straight off. A soft toothbrush works better than a cloth for getting into the groove of the seal without scrubbing the silicone away.

Rinse thoroughly and dry the area with a towel, then leave the shower door or curtain open so air can get to the seal rather than trapping steam against it. Repeat after a day or two if any faint marks remain, since a single clean does not always shift mould that has had weeks to establish itself.

If the black returns within a few days looking exactly the same, in the same spots, that is a fairly reliable sign the mould is not sitting on the surface any more, and cleaning is now just a temporary fix rather than a solution.

When the old silicone needs cutting out

Cut the seal out and start again when the mould is visibly inside the silicone, when the bead has gone soft, discoloured all the way through, or has separated from the tile or tray leaving a gap that water can get behind. At that point cleaning cannot reach the problem and the gap itself will let more moisture in.

To do it properly: cut along both edges of the old bead with a sharp utility knife or a purpose-made silicone removal tool, peel it out, and clean the channel with isopropyl alcohol to remove every trace of old sealant and mould residue. Any leftover residue will show through the new silicone and can stop it bonding properly.

Let the surface dry fully, mask both edges with tape for a straight line, and apply a bathroom-grade silicone with mould inhibitor, tooling it smoothly with a wet finger or a silicone profile tool before removing the tape while it is still wet. This is a job we do as part of silicone sealing work, priced from 45 € per job, and it is also one of the smaller jobs we take on as ordinary hourly work from 39 € an hour if there is other bathroom maintenance to do at the same time.

• Mould visible inside the silicone, not just on top of it
• Bead has gone soft, spongy, or discoloured all the way through
• Silicone has pulled away from the tile or shower tray, leaving a gap
• Mould returns in the same spot within days of cleaning, repeatedly
• Musty smell from the seal even after it looks clean

If mould keeps coming back even after resealing

New silicone that grows mould again within a few months usually points to a ventilation problem rather than a sealant problem: a bathroom fan that is too weak for the room, one that only runs with the light, or no extraction at all in an older Tallinn flat. The seal will keep failing regardless of how well it was fitted if the room stays damp for hours after every shower.

It can also mean the shower enclosure itself has a design fault, such as a tray with a lip too shallow to clear water properly, or panels that do not close fully and let water run behind them onto the wall. In these cases resealing the same joint again will not solve anything until the underlying cause is dealt with.

If the enclosure is old, warped, or the frame has corroded behind the silicone, it is sometimes more sensible to replace the whole unit rather than keep resealing a structure that is past its best. Enclosure fitting is priced from 150 € per job depending on size and layout.

What it costs and whether doing it yourself is worth it

A tube of bathroom silicone and a removal tool from a Tallinn hardware shop costs relatively little, and for someone reasonably comfortable with a utility knife, resealing one shower is a manageable weekend job. The parts most people get wrong are not cleaning the channel fully before applying new silicone, and rushing the tooling so the bead ends up uneven or with air pockets that trap moisture again.

Having it done professionally as a silicone sealing job costs from 45 € and typically takes under an hour, with a clean, straight bead and a 24-month guarantee on the work itself. It is one of the more straightforward jobs on the price list, and worth booking alongside any other odd job in the bathroom to make the call-out worthwhile.

Where doing it yourself becomes a false economy is when the mould keeps returning: paying for silicone and time twice or three times a year costs more in the end than sorting the ventilation or the enclosure once and having a seal that actually lasts.