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What are you seeing?

Black mould

Black mould is a visible sign that a surface has stayed damp long enough for mould to grow. The pattern and room conditions help identify why.

Diagnose this symptomCompare likely causes
Recognise the pattern

Does your black mould look like this?

Use several clues together. A single stain or reading rarely proves the cause.

01Dark speckling or patches
02Often around windows, ceilings or cold corners
03May return after cleaning
What could be behind it?

Compare the most likely cause families

The same visible symptom can have very different causes. Look for the clue that best fits timing and location.

Explore all problems
1

Condensation and high humidity

Compare this possibility with weather, room use, height and any nearby services before acting.

2

Cold bridging or weak ventilation

Compare this possibility with weather, room use, height and any nearby services before acting.

3

Leak or penetrating damp if highly localised

Compare this possibility with weather, room use, height and any nearby services before acting.

Do this before buying a fix

Three useful checks

1
Check indoor humidityAdd the result to the overall pattern rather than treating it as proof on its own.
2
Look for cold surfaces and poor airflowAdd the result to the overall pattern rather than treating it as proof on its own.
3
Compare after rain and after bathing/cookingAdd the result to the overall pattern rather than treating it as proof on its own.
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DampDeck symptom guide

Black mould: what to know

What it can mean

Usually indicates sustained surface moisture. Condensation is common around windows and cold corners, but leaks and thermal defects can also support growth.

Three useful questions

  • Is it on a cold external wall or around a window?
  • Does it worsen in winter?
  • Does it recur quickly after cleaning?

Do not choose treatment from the symptom alone

The same visible symptom can come from different moisture sources. Use timing, position and nearby building/plumbing details to narrow it down.

Evidence & reviewDampDeck reviewed this guidance on 3 September 2026. UK government guidance says damp and mould should be addressed by identifying and correcting the underlying moisture cause; surface mould removal alone does not prevent recurrence.UK Government source →
Continue the diagnosis

Related DampDeck guides

Black mouldVisible mould growth caused by persistent surface moisture; the moisture source matters more than the colour.Read next →CondensationMoist indoor air forming water on colder surfaces — often windows, external walls and cold corners.Read next →
Investigate before treating

Three questions that narrow it down

1

Is it on a cold external wall or around a window?

2

Does it worsen in winter?

3

Does it recur quickly after cleaning?

What can sit behind this symptom

Compare the likely moisture routes

The symptom tells you where to look. Timing and building context help identify why it is happening.

Condensation & humidityPossible causeCondensation & humidityMore likely when the surface is cold and the pattern is seasonal.Explore →Rain penetrationPossible causeRain penetrationMore likely when the patch changes during or after wet weather.Explore →Plumbing leakPossible causePlumbing leakMore likely around water services or where the weather makes no difference.Explore →
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Useful products for this stage

Only products that support the investigation or an appropriate fix are shown here. An active leak or building defect still needs repairing.

Digital hygrometerRelevant productDigital hygrometerBrowse on Amazon →Building moisture meterRelevant productBuilding moisture meterBrowse on Amazon →Protective cleaning glovesRelevant productProtective cleaning glovesBrowse on Amazon →
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Useful products for checking or controlling the conditions

Products can help measure, ventilate, dry or clean. They should not be presented as a substitute for fixing an active leak or building defect.

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Relevant to Black mould

Mould control

DampDeck only surfaces products that make sense for this stage of the investigation. Products do not replace diagnosis or repair of an active leak.

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Professional help

Find the right specialist once the likely cause is clearer.

Search locally by town or postcode instead of choosing a treatment from the symptom alone.

Find local specialists