Bathroom leaking into the flat below
Water from a bathroom reaching the flat below is almost never a failed membrane across the floor. It is nearly always an unsealed pipe penetration through the sunk slab, or a wall upturn that stopped below the splash line.
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
Pipe penetration through the sunk slab was never collared
The tell — The stain below sits directly under the WC, floor trap or shower drain position.
Bathroom Waterproofing → - 02common
Waterproofing upturn on the walls is too low
The tell — Damp appears on the neighbour's ceiling near the wall line rather than mid-room.
Bathroom Waterproofing → - 03common
Concealed supply or drain pipe leaking
The tell — The leak continues when the bathroom is unused for a day.
Injection Grouting → - 04less common
Sunk fill holds water with no drainage to the trap
The tell — Leakage lags use by many hours and continues long after the shower stops.
Bathroom Waterproofing →
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 stain below sits directly under the wc, floor trap or shower drain position.
- 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
Beyond the neighbour's ceiling, prolonged wetting corrodes the slab reinforcement between the two flats and turns a bathroom job into an RCC repair. It is also the most common cause of society disputes, because responsibility is contested until someone opens the floor.
Often seen alongside this
- 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.
- Damp patch on the ceiling below a bathroom
A stain on the ceiling below a wet area does not mark where the water entered.
- Cracked grout or lifting tiles in a wet area
Cracked grout in a bathroom is not itself a leak, but it removes the first line of defence.
Bathroom leaking into the flat below — questions we get asked
What is the most likely cause of bathroom leaking into the flat below?
Pipe penetration through the sunk slab was never collared. The sign that points to it is: the stain below sits directly under the wc, floor trap or shower drain position. 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?
Beyond the neighbour's ceiling, prolonged wetting corrodes the slab reinforcement between the two flats and turns a bathroom job into an RCC repair. It is also the most common cause of society disputes, because responsibility is contested until someone opens the floor.
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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