Water seeping through an external wall during rain
Water passing through an external wall means the render is no longer shedding it. Either the plaster has debonded and is holding water against the masonry, or cracks have opened a direct route through the wall.
Not an emergency, but it will get worse and more expensive with every monsoon it survives.
Probable causes, ranked
Ordered by how often we find each one to be the actual source. The 'tell' is what distinguishes it from the others.
- 01most common
Plaster has debonded and is holding water against the wall
The tell — Large areas sound hollow when tapped, and internal damp follows within hours of driving rain.
External Plaster & Painting → - 02common
Cracks in the render, particularly at junctions and around openings
The tell — Internal staining lines up with a visible external crack or a window edge.
Elastomeric Coating → - 03common
Failed sealant around window and door frames
The tell — Damp appears at the reveal or sill rather than in the wall field.
Expansion Joint Treatment → - 04less common
Chajja or drip mould missing, so water runs back onto the wall
The tell — Staining forms a fan shape below a projection.
External Plaster Work →
Sixty seconds, before you call anyone
- Photograph it while it is actively wet. A dried stain tells a surveyor far less than a running one.
- Note whether it worsens with rain or with usage. Rain-linked points to the building envelope; usage-linked points to plumbing. This single distinction settles most arguments about responsibility.
- Check the area directly above and beside the damage, not only below it. Water travels along a slab before it drops.
- Look specifically for the tell of the most likely cause: large areas sound hollow when tapped, and internal damp follows within hours of driving rain.
- Do not chip into saturated plaster or paint over a damp patch. The first risks bringing material down; the second destroys the evidence needed to trace the path.
What it becomes if you leave it
A wall that stays wet loses its plaster on a cycle and eventually its reinforcement. On sea-facing elevations the water also carries chloride into the cover concrete, which accelerates corrosion well beyond the wet area itself.
Often seen alongside this
- Damp patch spreading on an internal wall
A damp patch on an internal wall means water is reaching the wall from somewhere you cannot see — usually the external face, a concealed pipe, or the slab above.
- Plaster falling off an external wall
Plaster falling from a facade is a safety issue before it is a building one — falling material from height injures people.
- Water entering around a window frame
Water at a window is usually entering at the junction between frame and wall rather than through the window itself.
Water seeping through an external wall during rain — questions we get asked
What is the most likely cause of water seeping through an external wall during rain?
Plaster has debonded and is holding water against the wall. The sign that points to it is: large areas sound hollow when tapped, and internal damp follows within hours of driving rain. That said, the ranking here is a starting point — the actual source is confirmed by survey, because more than one of these can be true at once.
What happens if I leave it?
A wall that stays wet loses its plaster on a cycle and eventually its reinforcement. On sea-facing elevations the water also carries chloride into the cover concrete, which accelerates corrosion well beyond the wet area itself.
Can I diagnose this myself before calling anyone?
Partly, and it is worth doing. Photograph it while it is actively wet, note whether it worsens with rain or with usage, and check the area directly above and beside the damage. That single rain-versus-usage distinction narrows the cause more than anything else you can do in sixty seconds.
How urgent is this?
Act this season. Not an emergency, but it will get worse and more expensive with every monsoon it survives.
Send us a photograph and we will tell you what we think it is
WhatsApp a photo taken while it is wet, with a note on whether it tracks with rain or with usage. That is usually enough for a first opinion, and the site inspection that follows is free.
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