You scrub it off. It comes back. You bleach it. It comes back. Mould that reappears within days or weeks of every clean isn't a cleaning problem. It's a moisture problem. Mould spores are in the air around us all the time; they only establish thriving colonies where they find what they need: warmth, poor light and, above all, persistent damp. If a corner of your bathroom keeps feeding them, something is supplying the moisture.
Here's how to work out what, and how to cut the supply for good.
Why it matters beyond the ugly
Recurring mould is worth taking seriously on two fronts. The health one: mould spreads by airborne spores, and elevated spore levels can aggravate asthma and trigger allergic reactions, a genuine air-quality issue, not just a stain. And the building one: the same persistent moisture that feeds mould is quietly working on everything around it: crumbling masonry, warping timber, lifting paint. Mould is often the visible tip of damage you can't see yet.
The four common moisture sources
1. Poor ventilation and high humidity. Every shower loads the air with moisture. Without decent airflow (a working exhaust fan, an open window, air circulating after use) that moisture condenses on cold surfaces and soaks into corners, day after day. This is the most common cause, and the cheapest to fix.
2. Leaking pipes and plumbing. A slow leak in a hard-to-reach spot keeps a wall or floor damp indefinitely. Dripping taps, weeping connections and worn seals around fittings all qualify, and they don't fix themselves.
3. Failed waterproofing. Compromised grout, perished silicone or a worn membrane lets shower and bath water seep behind the tiles with every use. The tell-tale: mould concentrated along grout lines, at wall junctions, or on the other side of a shower wall: the moisture is coming through, not settling on. This is a leaking shower (or balcony) problem wearing a mould costume.
4. Excessive condensation. Some window condensation is normal. When it's constant and pooling, the sealing around the frames may need renewing, and the humidity source (usually ventilation) needs addressing too.
Telling ventilation from ingress
The two big culprits behave differently, and a week of observation usually separates them:
- Ventilation mould spreads evenly across cold surfaces (ceilings, external walls, window reveals) and improves noticeably when you run the fan longer, crack a window and squeegee wet surfaces after showering.
- Ingress mould keeps returning to the same specific spots (a grout line, a junction, a patch behind or beside the shower) no matter how good the airflow gets. It often comes with friends: musty smell, crumbling grout, staining, bubbling paint.
Improve ventilation first; it's free to try. If the mould shrugs it off, you're looking at water getting in.
Fixing the supply, not the symptom
- Ventilation: run the exhaust fan during and well after showers, keep air moving, wipe down wet surfaces. Small habits, big difference.
- Plumbing: dripping taps and leaking pipes are a job for a licensed plumber, promptly, before the damp does lasting damage to timber and finishes.
- Failed seals and grout: rake out and regrout, renew the silicone and seal the surface. For showers this is exactly what professional sealing does, over your existing tiles, with the room back in use in 24 hours. Dense epoxy grout also resists mould far better than tired cement grout, so the fix doubles as prevention.
- Unclear source: when you can't tell where the moisture is coming from, leak detection (dye testing, thermal imaging, pressure testing) finds it with evidence, so you fix the right thing once.
Mould that won't quit? Send SmartSealed a photo and we'll tell you whether you're looking at a ventilation fix or a leak, and quote the repair for free. 30+ years across Greater Sydney.