Penetrating damp is moisture forcing its way horizontally through walls, roofs or joinery from outside — the patch that darkens when it rains, the stain below a window, the wet corner behind a blocked gutter. Unlike rising damp it can appear at any height, and unlike condensation it tracks the weather. Here is how to recognise it, what causes it, and how to fix the cause rather than the symptom.
Signs of penetrating damp
- Damp patches that grow during or after rain and dry in fine spells
- Staining at any height — not just the metre-high tide mark of rising damp
- Blistering plaster, salt deposits and black mould localised to one wall
- Musty smell in one room while the rest of the house is fine
Common causes in London homes
Defective gutters and downpipes; failed pointing or cracked render on solid Victorian walls; bridged or debris-filled cavities; porous brickwork on exposed elevations; leaking roof junctions and flashings; and raised ground or paving bridging the damp-proof course. On terraces, water can even track through a shared party wall from a neighbour’s defect.
Penetrating damp vs rising damp vs condensation
Rising damp climbs from the ground to about 1–1.2 m with tide marks; condensation favours cold corners, windows and poor ventilation and worsens in winter regardless of rain; penetrating damp maps to a specific external defect. Diagnosis matters: injecting a chemical DPC into a wall suffering penetrating damp wastes money and fixes nothing.
How to fix it — in order
1) Fix the source: gutters, pointing, render, flashings. 2) Let the wall dry — a solid wall dries roughly 25 mm per month. 3) Only then repair internal plaster. An independent damp survey (£200–£400 in London) identifies the true cause before you spend on works — see our full damp survey cost guide.
Worried about a damp wall?
Our independent surveyors diagnose damp with no treatment products to sell. Call 0204 579 8270 or book a damp survey.
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