The three main types of damp
Understanding where the moisture comes from tells you what to fix. In British homes, the usual culprits are:
- Condensation – moisture from showers, cooking, drying clothes and breathing, settling on cold surfaces. The most common cause of mould on walls and ceilings.
- Penetrating damp – water getting in from outside, through leaking gutters, failed pointing, cracked render or damaged flashing. Often shows as patches that grow after rain.
- Rising damp – moisture coming up from the ground at the base of walls. Less common than often claimed, and frequently confused with other problems.
Signs the wall isn't ready to paint
Dark patches that feel cold or damp to the touch, tide marks, white powdery salts, bubbling or flaking paint and a musty smell all suggest moisture. If you see these, painting is the last step, not the first.
Fix the cause, then dry, then decorate
Condensation is usually tackled with better ventilation and heating – extractor fans that get used, windows opened, furniture moved away from cold external walls. Penetrating damp needs the leak or defect outside repairing. Suspected rising damp is worth getting looked at by an independent surveyor before agreeing to any treatment.
Once the cause is sorted, the wall needs time to dry out properly, which can take weeks or longer for solid walls.
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Dealing with mould
Black mould spots should be treated with a fungicidal wash, left for the time stated on the product and rinsed off – not simply painted over or wiped with a cloth, which can spread the spores. Then the area can be sealed with a stain-blocking primer if needed and painted, ideally with a mould-resistant paint in kitchens and bathrooms.
What a decorator can and can't do
A decorator can clean and treat mould, seal stains and repaint with suitable products, and a good one will tell you if they think a wall isn't ready. They can't cure damp by painting. If a decorator suggests painting over active damp without mentioning the cause, ask why.
