Why mould always returns to the same spot

You clean the mould, it disappears, and weeks later it's back in exactly the same spot. That isn't coincidence or bad luck: that point on the wall meets the conditions mould needs, and cleaning it changes none of them.

What you see is half the problem

The black patch is the visible part of the fungus. Underneath is a network of filaments that penetrates the plaster, the paint or the silicone.

When you scrub the surface you remove what's visible, but the filaments stay inside the material. As soon as the conditions return — moisture and something to feed on — the patch reappears at the exact same point.

That's why it always comes back to the same place. It never left.

Why bleach doesn't solve it

Bleach is the usual remedy, and it half works: it bleaches the stain, so it looks as though it's gone.

The trouble is twofold. First, bleach is essentially water with chlorine — and the chlorine evaporates quickly while the water soaks into the material. On porous surfaces like plaster, you're adding moisture exactly where the fungus needs it.

Second, it discolours the filaments without eliminating them. The wall looks clean and the fungus is still alive inside.

The three conditions mould needs

Mould doesn't appear because a surface is dirty. It appears where three things coincide:

• Moisture. Above roughly 60% relative humidity, or condensation on a cold surface
• Food. Organic matter: soap residue, skin oils, dust, cellulose from plaster or wallpaper
• A porous surface. Where spores can settle and filaments can penetrate

Removing the stain eliminates none of the three. That's why it returns.

Why it always appears in the same places

Corners, window frames and north-facing external walls share one characteristic: they're colder than the rest of the room.

Warm air holds more water vapour than cold air. When it touches a cold surface it cools suddenly and releases the water it can no longer hold — condensation. It's the same effect that fogs a cold glass in summer.

Those cold spots are thermal bridges, and they explain why mould always draws the same map in your house.

What to do, in order

1. Reduce the moisture. This is the most important step and the cheapest. Ventilating for five minutes a day with windows wide open removes more moisture than a window left ajar all day. Extractor fan in the shower, lids on pans, and don't dry laundry indoors without ventilation.

2. Remove the food source. This is where enzymatic cleaning makes a difference: enzymes break down the organic residue — soap, skin oils, grease — rather than simply bleaching it. Without food, recolonisation is far slower.

3. Change the surface. A treatment that reduces porosity and repels water leaves the surface much less hospitable. It kills nothing: it stops the spot meeting the conditions.

An honest caveat

No cleaning product solves a structural damp problem. If you have leaks, rising damp, a pipe weeping inside the wall or poor insulation, the mould will return however well you clean.

Signs the problem is structural: paint bubbling or peeling, stains spreading upward from the skirting, or mould in several rooms at once. That needs a professional, not a cleaner.

And a warning

If the affected area is larger than roughly one square metre, or if anyone in the house has asthma or respiratory allergies, don't treat it yourself. Scrubbing dry mould releases spores into the air. Wear a mask, ventilate well, and consider calling a specialist.