Damp patch on the ceiling below a bathroom
A stain on the ceiling below a wet area does not mark where the water entered. Water spreads along the underside of the slab before it drops, so the entry point is often a metre or more from the visible patch.
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
Unsealed pipe penetration in the sunk slab above
The tell — The patch darkens within hours of the bathroom being used.
Bathroom Waterproofing → - 02common
Failed waterproofing in the sunk portion
The tell — A broad, slowly spreading patch rather than a defined point.
Bathroom Waterproofing → - 03common
Leaking concealed pipe in the floor above
The tell — Constant regardless of whether the bathroom is used.
Injection Grouting → - 04less common
Water tracking from an adjacent shaft
The tell — The patch is nearest the shaft wall rather than the bathroom.
Injection Grouting →
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 darkens within hours of the bathroom being used.
- 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
Prolonged wetting corrodes the slab reinforcement and eventually causes the plaster and then the concrete cover to fall. Treating the stain rather than tracing the source is why these leaks reappear after every repair.
Often seen alongside this
- Bathroom leaking into the flat below
Water from a bathroom reaching the flat below is almost never a failed membrane across the floor.
- Toilet leaking into the flat below
A WC leaking downward is almost always the soil pipe penetration through the sunk slab, or the floor trap.
- Leakage in a service duct or shaft
A shaft collects water from every floor it passes, so a leak there affects many flats at once.
Damp patch on the ceiling below a bathroom — questions we get asked
What is the most likely cause of damp patch on the ceiling below a bathroom?
Unsealed pipe penetration in the sunk slab above. The sign that points to it is: the patch darkens within hours of the bathroom being used. 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?
Prolonged wetting corrodes the slab reinforcement and eventually causes the plaster and then the concrete cover to fall. Treating the stain rather than tracing the source is why these leaks reappear after every repair.
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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