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. The patch marks where the water finally surfaced, not where it entered.
Stable enough to schedule properly. Best resolved before the next monsoon rather than during it.
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
Hollow or cracked external plaster on the other side of the wall
The tell — The patch grows during and after rain, and the external plaster sounds hollow when tapped.
External Plaster & Painting → - 02common
Concealed plumbing leak inside the wall
The tell — The patch is constant regardless of weather and may feel warm on a hot-water line.
Injection Grouting → - 03common
Water tracking down from the slab above
The tell — Staining starts near the ceiling junction and works downward.
Terrace Waterproofing → - 04less common
Rising damp from the plinth
The tell — The patch is at the base of the wall with a fairly level tide mark and salt deposits.
Chemical Coating →
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: the patch grows during and after rain, and the external plaster sounds hollow when tapped.
- 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
Persistent damp destroys plaster and paint on a cycle, ruins anything stored against the wall, and in a shared wall becomes a dispute between two flats. Salts carried by the moisture also degrade the plaster itself, so each repaint fails faster than the last.
Often seen alongside this
- White powder or crystals forming on walls
The white deposit is efflorescence — salts carried out of the masonry by moving water and left behind as it evaporates.
- Paint peeling or blistering after the monsoon
Paint fails after the monsoon because it was applied over a substrate that was damp, hollow or unprimed.
- Water seeping through an external wall during rain
Water passing through an external wall means the render is no longer shedding it.
Damp patch spreading on an internal wall — questions we get asked
What is the most likely cause of damp patch spreading on an internal wall?
Hollow or cracked external plaster on the other side of the wall. The sign that points to it is: the patch grows during and after rain, and the external plaster sounds hollow when tapped. 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?
Persistent damp destroys plaster and paint on a cycle, ruins anything stored against the wall, and in a shared wall becomes a dispute between two flats. Salts carried by the moisture also degrade the plaster itself, so each repaint fails faster than the last.
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?
Plan it. Stable enough to schedule properly. Best resolved before the next monsoon rather than during it.
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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