Penetrating damp: what to know
Penetrating damp becomes more likely when a patch worsens during or after rain, sits on an external-facing wall or ceiling, or lines up with a roof, gutter, chimney, window or masonry defect.
What to check first
- Inspect gutters and downpipes during rain if it is safe to do so.
- Check roof edges, flashings, chimneys, cracked render, failed pointing and window/door seals near the symptom.
- Compare the internal patch with the exterior position before treating the inside surface.
What not to assume
Waterproof paint on the inside does not repair an active external entry route.
Sensible next step
Repair the defect that is admitting water, allow the construction to dry, then make good damaged finishes.
The clues that matter most
Use several clues together rather than treating one mark, smell or meter reading as proof.
Inspect gutters and downpipes during rain if it is safe to do so.
Check roof edges, flashings, chimneys, cracked render, failed pointing and window/door seals near the symptom.
Compare the internal patch with the exterior position before treating the inside surface.
Waterproof paint on the inside does not repair an active external entry route.
Repair the defect that is admitting water, allow the construction to dry, then make good damaged finishes.
Could it be something else?
These routes help separate common causes that can create similar visible damage.
Useful products for this stage
Only products that support the investigation or an appropriate fix are shown here. An active leak or building defect still needs repairing.
