Quick answer
Stop grout going mouldy by controlling moisture, not by scrubbing harder. Mould grows where damp air lingers, so the real fixes are good ventilation (a decent extractor, used every shower), sealing the grout so water can’t soak in, and replacing failed silicone. Wipe walls down after showering and the mould has nowhere to take hold.
Why grout goes mouldy in the first place
It helps to understand what you’re actually fighting. The black spotting in grout lines isn’t dirt — it’s mould, a living thing that needs three conditions to thrive: moisture, warmth and something to feed on. A bathroom hands it all three. Grout is mildly porous, so it absorbs water; the room is warm; and soap scum and skin cells give the mould something to eat. Once it’s established in the surface of the grout, surface cleaning only buys you a week or two before it comes back.
This is why so many people feel they’re losing a battle they can’t win. They are — because they’re treating the symptom. The grout keeps going mouldy because the underlying conditions never change. Fix the conditions and the problem largely disappears.
The five things that actually work
In order of how much difference they make in a typical Plymouth bathroom:
- Ventilation — a properly sized, properly working extractor that runs during and after every shower clears the damp air before it can settle
- Seal the grout — a penetrating grout sealer stops water soaking in, so mould can’t get a foothold
- Wipe down after showering — thirty seconds with a squeegee or cloth removes the water mould needs
- Replace failed silicone — black, peeling silicone in corners and around the bath is a mould nursery; it should be cut out and re-run
- Leave the door open after showering to let the room dry
If your extractor is undersized, noisy enough that nobody uses it, or simply not there, that’s usually the root cause. See our note on whether you need an extractor fan.
Fresh, sealed grout and properly run silicone give mould nothing to cling to — the rest is good ventilation.
When grout is past saving
Sometimes the mould has gone too deep, or the grout was never sealed and has crumbled, and no amount of cleaning will bring it back. At that point the honest answer is to rake out the old grout and re-grout — a far smaller job than re-tiling, and one we do regularly as a quick upgrade. If the silicone seals have failed too, we replace those at the same time. A re-grout and re-seal can make a tired bathroom look years younger for a fraction of a refit.
If the grout is failing because the tiling or waterproofing behind it was never done right — water tracking behind tiles, movement cracking the joints — that’s a deeper issue, and we’d look at the wall properly rather than paper over it. Often it ties into how the bathroom was originally waterproofed before tiling.
Common questions
Does sealing grout really stop mould?
It makes a big difference. Sealing fills the porous surface so water can’t soak in and sit, which removes the moisture mould depends on. It’s not a substitute for ventilation, but the two together are very effective.
Why does the silicone keep going black?
Black silicone is mould growing in or under the seal, usually because it’s failed and water is getting behind it. Cleaning won’t fix it — it needs cutting out and re-running with a good anti-mould silicone. See how often to re-seal a bath or shower.
Is mouldy grout a sign of a bigger problem?
Sometimes. Persistent mould despite good ventilation can mean water is tracking behind the tiles, which points to a waterproofing fault. If cleaning, sealing and ventilation don’t hold it back, it’s worth having the wall looked at.
If your grout and seals are beyond a clean-up, our tiling & flooring service covers re-grouting, re-sealing and full re-tiles. More in our FAQs.
Beat the mould for good
Tired of scrubbing grout?
We’ll re-grout, re-seal and sort the ventilation so your bathroom stays clean between cleans — with a fixed price and no mess left behind.