Updated
Black Mould On Walls
Black speckling or patches on walls, ceilings, window reveals or behind furniture, often in corners and cold spots.
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What's happening?
Mould needs three things: spores, which are always present in air; something to grow on, which is the dust and finish on any wall; and moisture. Only the third is worth arguing about, which is why every effective response to mould is really a response to moisture. In UK homes the moisture is most often condensation, appearing on the coldest surfaces — external corners, behind wardrobes, at window reveals, at the ceiling-to-wall junction, and anywhere a cold bridge runs through the structure. That distribution is the diagnosis: mould in the cold spots and behind furniture points at condensation and ventilation, while mould in a defined patch, or in a band at the bottom of a wall, points at a leak, penetrating damp or rising damp underneath it.
What usually causes it?
- Condensation on cold surfaces, worst where furniture blocks air movement against an external wall
- Background ventilation missing or blocked — trickle vents shut or absent after a window replacement
- Extract ventilation in the kitchen or bathroom failed, disconnected, or discharging into the loft
- Cold bridging at lintels, at concrete window reveals, and at wall-to-ceiling junctions
- Moisture generation the ventilation cannot handle — indoor drying, an unvented tumble dryer, unlidded cooking
- An underlying leak, penetrating damp or rising damp keeping one area of the wall wet
- Intermittent heating leaving surfaces below dew point for long periods each day
Is it dangerous?
Yes, and this one is stated plainly in government guidance: damp and mould in the home present health risks, affecting the airways and lungs and also the eyes and skin, they can affect mental health, and some groups — people with certain health conditions, children and older adults — are at greater risk of more severe effects. The guidance is explicit that damp and mould should not be dismissed as a lifestyle choice and that the underlying causes, such as structural issues or inadequate ventilation, must be identified and put right. In rented housing there are legal duties on landlords and, in the social rented sector, statutory timeframes for investigating and making safe damp and mould hazards.
Can I fix it myself?
A good deal, if the cause is condensation. Open trickle vents and leave them open, run extract fans while cooking and bathing and afterwards with the door shut, dry laundry outside or in a vented dryer, pull furniture a few centimetres off external walls so air can move behind it, and keep heating steadier rather than in short bursts. Cleaning existing mould off hard surfaces is reasonable with the room ventilated and appropriate protection. Two limits: painting over mould with anti-mould paint without changing the moisture just delays it, and if anyone in the household has a respiratory condition, getting the cause diagnosed matters more than cleaning it.
When should you call a damp proofer?
Call a damp proofer where mould keeps returning after ventilation and heating have genuinely been addressed, where it is in a defined patch rather than the cold spots, or where there is a tide mark or staining underneath it suggesting damp in the wall. Call promptly where anyone is symptomatic. If you rent, report it to your landlord in writing — the guidance places the duty to identify and fix the underlying cause with them.
What will the damp proofer actually do?
- Map where the mould is and is not, which separates condensation from a localised water source
- Measure surface temperatures and humidity to identify cold bridging and dew point conditions
- Test extract and background ventilation performance, and confirm extracts discharge outdoors
- Investigate any underlying leak, penetrating damp or rising damp beneath the affected area
- Specify remedies that address the cause — ventilation provision, insulation to cold bridges, repair of the water source
- Treat and make good the affected surfaces once the moisture cause is resolved
Frequently asked questions
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