Black mould: what it may indicate (and what to do)

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.

Fig. 02

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)