Damp / mould / condensation / penetrating damp
Why does a damp patch appear after rain?
Start with the observable symptom. This page explains possible causes; it is not a diagnosis. It covers both “mould” and “mold” wording for the same intent.
A patch that appears or worsens after rain points more strongly to water entering from outside, such as a roof, gutter, wall, window or external joint. It still needs inspection because condensation and old water damage can overlap.
Start with the pattern.
A wall or ceiling becomes darker, wetter or more stained after rainfall.
Possible causes, not a diagnosis.
- Rainwater is entering through a roof, gutter, wall, window or masonry joint.
- An external crack, blocked outlet or failed seal is directing water into the building.
- An old leak is being re-wetted or the drying time is revealing the same damage.
Check what is safe to observe.
- 01
Map the patch and compare it with the outside wall, roof line, gutter and window above.
- 02
Check whether the patch changes after dry weather.
- 03
Look for cracked render, blocked gutters, missing roof tiles or failed sealant only from a safe position.
Take the smallest useful step.
- Photograph the internal and safely visible external areas.
- Protect belongings and keep the room ventilated while waiting for repair.
- Report the issue promptly if rented or if the patch is spreading.
- Do not climb onto a roof or work at height without competent equipment and training.
- Do not seal the inside wall and assume the water entry has stopped.
- Use a qualified roofer, builder, window professional or a landlord or housing provider to identify the entry point.
- Get urgent help for active leaks, ceiling sagging or water near electrics.
Professional requirements and rental responsibilities vary by location. Use the qualified local professional or housing authority that applies where you live.
UK example: GOV.UK repairs guidanceWhy this page says that.
This page separates an observable symptom from possible causes and keeps the recommended action within a safe household boundary.