Problems
Black mould: what it may indicate (and what to do)
Black mould is a sign — not a diagnosis. The pattern tells you far more than the colour. Here is a calm read on what it may mean and the right first step.
What black mould actually is
Most household 'black mould' is a mix of common species — typically Cladosporium, Aspergillus or, less often, Stachybotrys chartarum. Despite the headlines, the species matters less than the moisture source feeding it.
Mould needs three things: moisture, a food source (paint, paper, dust) and time. Remove the moisture and the mould stops growing.
Why it appears
- Condensation on cold surfaces (single-glazed windows, external corners, behind furniture on outside walls)
- Leaks from plumbing, roofs or guttering
- Cold bridging where insulation is missing or interrupted
- Poor ventilation — extractor fans missing, blocked vents, sealed trickle vents
How to read the pattern
- Ceiling corners and behind furniture on cold walls → usually condensation
- Horizontal tide mark at the base of walls → possible rising damp
- Patches that worsen after rain, often near external walls → possible penetrating damp
- Around windows or in bathrooms → usually condensation and ventilation
What NOT to do
- Don't paint over it — paint hides the surface but not the moisture source
- Don't rely on neat bleach — it bleaches the colour but rarely kills the root growth
- Don't disturb large areas (over A3 size) without protection — spores can spread
- Don't ignore it because it's small — small patches that return are telling you something
When to escalate
Speak to a professional, your landlord or your GP if any of these apply:
- Anyone in the home has a respiratory condition, is very young, elderly or immunocompromised
- The affected area is larger than an A3 sheet of paper
- Mould keeps returning within weeks of cleaning
- You're renting and the cause is structural (leak, ventilation, insulation)
Start your evidence report