Why mould keeps coming back in shower silicone and grout
Our old shower had a run of silicone along the bottom of the screen that went black no matter what I did. Scrub it, it looks clean for a week, then the grey speckle creeps back along the same line. If that is your shower, the reason is not that you are cleaning it wrong. It is where the mould is actually living.
Short answer: shower silicone and grout keep going black because the mould gets in below the surface: grout is porous and holds water, and silicone lets moisture creep into its edges and gaps. Wiping or bleaching the top lifts the stain but leaves the roots underneath, and a warm, wet, still shower recess plus spores in the air lets it grow straight back. What holds it back is keeping the recess dry, working the air, and replacing silicone that has gone black all the way through.
Why does mould keep coming back in shower silicone?
Silicone looks solid, but at the edges and in the tiny gaps where it meets tile or glass, moisture works its way in. Over time a film of mould establishes itself inside those gaps and into the surface of the silicone itself. Once it is in the silicone rather than on it, wiping the top does nothing, because you are not reaching where it lives. Old silicone that has gone black the whole way through is finished. No cleaner brings it back. It needs cutting out and re-sealing.
Why does grout go black and keep returning?
Grout is porous cement. It drinks water and holds it, which is exactly what mould needs. The black you see on the surface has roots sitting down in the pores below. This is why bleach is such a trap here: it pulls the colour out of the stain so the grout looks clean, while the living part below is untouched and the water that feeds it is still there. The US EPA is blunt about bleach as a routine fix. Verbatim: "The use of a chemical or biocide that kills organisms such as mold (chlorine bleach, for example) is not recommended as a routine practice." The mark fades, the mould stays, and a fortnight later it is back.
How to get it off silicone and grout without bleach
You can shift the surface growth without harsh chemistry. Clean the grout and silicone lines, let them dry properly, and keep them dry between showers. A squeegee on the screen and tiles after each shower does more than any spray, because it removes the water before the mould can use it. The one thing cleaning cannot fix is silicone that is black through: that is a re-seal job.
If you rent, that matters. Perished, mouldy silicone is usually a maintenance issue, not a cleaning failure on your part, so it is worth reporting to your agent in writing rather than scrubbing at it week after week. To see how the responsibility tends to fall, our renter mould data and state-by-state picture is a useful reference. What you can control day to day is the water and the air.
What each approach actually does
| What you do | What it targets | What happens |
|---|---|---|
| Bleach the line | The stain colour | Looks clean, mould and damp remain, returns |
| Scrub and dry | Surface growth | Helps, but returns if the recess stays wet |
| Cut out and re-seal dead silicone | Mould living inside the silicone | Removes it at the source for that seal |
| Squeegee and ventilate after every shower | The water mould needs | Starves the regrowth |
| Work the air side | Spores before they settle | Meets them before they land on wet grout |
The part that actually holds: the air side
Surface cleaning always leaves the same thing behind. This is how mould works from the air down: spores drift through the room and settle on the next damp surface, and a shower recess is the dampest one in the house. Sprays and scrubs stop at the surface you can see. The recurring bit is airborne.
That gap is the reason we made First Light to work on the air as well as the surface. On the surface, on direct contact, it is lab tested at a 99.2% reduction in black mould, using the BS EN 13697 method. In the air, in a 2022 UNSW Sydney lab-chamber study (Antibiotics), essential-oil vapour cut 72% of viable Aspergillus spores in 10 minutes. The limit is real: no spray dries a wet recess or replaces dead silicone. Keep the recess dry, keep the air working, and re-seal what needs it. That combination is what stops the grout going black again, which is really what keeping mould from coming back comes down to.
For the full prevention method, see our guide to how to stop mould.
Common questions
Why does the mould come back in the exact same spot? Because that spot stays wettest, and the roots or the spore source are right there. The pattern is the tell.
Do I have to replace the silicone? Only if it has gone black all the way through. Surface growth on sound silicone cleans off. Black-through silicone is a re-seal.
Can I stop it without renovating, in a rental? Yes, for the day-to-day: squeegee, ventilate, dry, and work the air. Report perished silicone to your agent as maintenance. As of 2026 that is still the realistic play for renters.
Related guides
- Black mould on a bathroom ceiling, and why it keeps coming back
- Why mould keeps coming back on window sills and frames
- How to stop mould in a bathroom with no window
For more rooms and surfaces, see our guide to tackling mould room by room. To keep cleaned grout and silicone clear for longer, a natural airborne preventative like First Light targets the spores that re-seed the mould.
Ben, founder of Aurala Naturals.
