Damp on a wall: condensation, plumbing leak or rainwater?
Look for useful clues without assuming that every damp patch needs injected damp proofing.
Applies to: UK homes; damp diagnosis may need a competent survey and sometimes invasive checks.
Last checked: 16 October 2026
Key points
• Condensation, plumbing leaks and penetrating water can look similar.
• Find the source of moisture before paying for cosmetic decoration or chemical treatments.
• Persistent damp and mould can affect health and should be dealt with promptly.
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Why it matters
Patterns are clues, not diagnoses. Localised staining near pipe routes, problems after rain and widespread cold-surface condensation call for different investigations.
Checklist before you start
• Note when the stain appears or worsens, especially after rain, showers or heating changes.
• Check accessible gutters, roof details, overflows and visible plumbing without opening dangerous areas.
• Photograph the pattern and any mould, blistering or salt deposits.
• Ask the surveyor to explain evidence for the diagnosis and what work will remove the cause.
What to do next
Do not simply paint over active damp. If mould is extensive, recurring or affecting vulnerable occupants, seek appropriate housing and health advice as well as addressing the building defect.
Official sources
Use these official sources for the current detail that applies to your job, contract or location.
BuildPair provides general information and signposting, not legal, structural, electrical, gas or building-control approval. Requirements can change and the right answer depends on the work, contract, location and facts.
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