What is penetrating damp?
Penetrating damp (or lateral damp) is moisture passing horizontally through the building envelope from outside. Unlike rising damp it can appear at any height, and unlike condensation it tracks the weather: patches typically darken during or after rain and fade in dry spells.
Common entry points
- Defective or eroded mortar pointing on exposed elevations
- Cracked, blown, or badly patched render
- Leaking or blocked gutters and downpipes soaking a wall
- Roof defects: slipped tiles, failed flashing, porous parapets
- Poorly sealed windows and door reveals
- Raised external ground levels driving moisture through below the DPC
Why London property is prone to it
Much of London's housing is solid-wall construction with no cavity to interrupt the moisture path, so once water gets into the brickwork it can travel straight through to internal finishes. South-west facing elevations take the prevailing weather and suffer most. Victorian brickwork that's been repointed in hard cement mortar (instead of breathable lime) traps moisture and makes things worse over time.
The fix is almost always external: repair the defect where the water gets in, then let the wall dry. That's why our surveys always include the outside of the building. Internal treatment alone just hides the symptom while the fabric stays wet. Where a leak or flood has caused sudden damage, a water damage survey maps the moisture spread and documents everything for your insurer.