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Damp Patch Appearing After Decorating
A dark, cold or spreading patch has come up on a wall you have just had painted or papered.
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What's happening?
A patch that arrives after decorating is telling you the wall was not ready, or that there is a moisture source the decoration has now made visible. Three things commonly sit behind it. New plaster that had not finished drying, then sealed under a coat the moisture cannot pass through. A solid or older wall coated with a modern film-forming paint that has stopped it breathing, so moisture that used to evaporate now builds behind the film. Or a genuine building defect — penetrating damp through a failed pointing joint or gutter, rising damp, or a leaking pipe — that the fresh light-coloured finish has simply made obvious. Which of the three it is decides who you need, and a painter cannot cure the third.
What usually causes it?
- New plaster painted before it had dried out, typically needing weeks rather than days depending on thickness and ventilation
- A non-breathable emulsion or vinyl paper applied to a solid wall that needs to lose moisture through its face
- Condensation forming repeatedly on a cold spot — a lintel, a wall tie or an uninsulated reveal
- Penetrating damp through failed pointing, a cracked render or an overflowing gutter outside the same spot
- Rising damp where a damp-proof course is bridged, absent or breached
- A live plumbing or heating leak in the wall or the floor above
- A blocked or undersized extract fan leaving bathroom and kitchen moisture in the room
Is it dangerous?
Persistent damp is a health issue, not just a decorating one. The joint government, UKHSA and MHCLG guidance is explicit that damp and mould in the home are not a lifestyle choice and that the underlying cause — structural defect or inadequate ventilation — has to be identified and fixed, because prolonged exposure carries real respiratory risk, most severely for children, older adults and people with existing conditions. Repainting over it is not a fix.
Can I fix it myself?
You can narrow down the cause. Note whether the patch worsens after rain (points outside), after baths and cooking (points at ventilation), or stays constant regardless (points at a leak or rising damp). Check the gutter, downpipe and pointing directly outside the patch. Keep the room ventilated and heated. What you should not do is seal the patch with a waterproof coating or hang new paper over it — that moves the moisture sideways into sound plaster.
When should you call a painter and decorator?
Call a painter and decorator when the wall is dry and the problem was the coating or the timing — they will strip back, let it dry and recoat in a breathable system. Call a damp proofer or a builder instead when the patch tracks with rainfall, when there is a tide mark low on the wall, or when a moisture meter reads high with no obvious plumbing source: that is a building defect and no decoration will hold on it.
What will the painter and decorator actually do?
- Take comparative moisture readings across the patch and a sound area of the same wall
- Establish whether the substrate is new plaster still drying, a sealed breathable wall, or wet from an external source
- Report honestly when the cause is a building defect, and hold off decorating until it is repaired and dried
- Strip the failed decoration back and allow the wall to dry before any coating goes on
- Recoat in a breathable, moisture-tolerant system where the wall needs to lose moisture through its face
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