Black mould on walls — what it usually means

Black spots or patches on walls almost always point to a moisture issue. The mould itself is a symptom — understanding the cause is what stops it returning.

Common causes

Condensation on cold surfaces, poor ventilation, leaks behind plaster, and cold bridging are the most common UK causes.

What to do first

Document with photos in good light, note when it appears, and avoid covering with paint until the cause is understood.

When to get help

If the patch grows, returns after cleaning, or there are health concerns in the household, request a specialist review.

Common questions

Is black mould always dangerous?: Most household black mould is not Stachybotrys. It can still affect breathing for some people — caution is sensible, panic is not.
Will bleach fix it?: Bleach can lighten the stain but does not address the moisture source, so the mould usually returns.

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