Water leaking from the ceiling under a bathroom

A wet patch on the ceiling directly under a bathroom is one of the most common calls in London. It is also one of the easier ones to solve, because the list of suspects is short. The bath, the shower, the toilet, the basin and the pipes that feed them. The trick is working out which one, and doing it before the ceiling gets worse.

This guide explains how to narrow it down, what is safe to do yourself, and when you need a plumber to look at water leaking through the ceiling.

Make it safe first

If water is near a ceiling light, switch off the lighting circuit at the consumer unit. Do not flick the light switch to test it. If the ceiling is bulging, keep out from under it and put a bucket below the lowest point. Move furniture, rugs and electrical items out of the way.

Then stop the water reaching the ceiling. If the bathroom is yours, stop using it. If it is in the flat above, knock and ask them to stop using it. If nobody is in and the leak is getting worse, contact the managing agent or freeholder, who usually has a power under the lease to get access, according to the Leasehold Advisory Service.

Why bathrooms leak into the room below

In a lot of Lambeth conversions, the bathroom upstairs was added when the house was split into flats. In the 2021 Census, 20.6% of Lambeth households lived in part of a converted or shared house (Nomis, TS044). In a Brixton or Clapham house split into three flats, the upstairs bathroom may sit over your bedroom or hallway, not over another bathroom. The floor is timber boards on joists, and the ceiling below is often old lath and plaster that soaks up water before it drips.

That matters for two reasons. Water can travel along a joist before it drips, so the wet patch may be a little way off from the actual leak. And small, slow leaks can go unnoticed for weeks, because the floor and the ceiling absorb them.

The usual suspects

Source	When it leaks	What you might see
Bath or shower seal	Only when the bath or shower is used	Gaps or black mould in the silicone, damp floor beside the bath
Shower tray or waste	When the shower runs	Drip starts a few minutes into a shower
Bath waste or overflow	When the bath is full or draining	Leak when you pull the plug or fill above the overflow
Toilet pan connector	When the toilet is flushed	Damp floor behind the pan, a smell
Toilet cistern or valve	All the time or after flushing	Water on the back of the pan, a constant drip
Basin waste or trap	When the basin drains	Water in the cabinet under the basin
Supply pipe under the floor	All the time	Steady drip with nothing in use, meter keeps turning
Radiator or towel rail valve	All the time	Damp around the valve, stain on the floor

A simple test you can do

You can narrow down most bathroom leaks with a bowl, a torch and some patience. Ideally someone stands downstairs watching the ceiling while someone else works upstairs.

1. Leave everything off for an hour. If the ceiling keeps dripping, the leak is on a supply pipe or valve, not a waste. Check the water meter if you have one. If it moves with every tap off, there is a leak somewhere.
2. Flush the toilet a few times. Watch for a drip below. If it starts, the pan connector or the cistern is the likely cause.
3. Run the basin and let it drain. If it drips, the basin waste or trap is suspect.
4. Run the shower into the tray, not at the walls. If it drips, the waste or tray. Then spray the walls and the edge of the tray. If it only drips now, it is the seal or the grout.
5. Fill the bath, then drain it. A leak while filling above the overflow points to the overflow. A leak while draining points to the waste.

Stop each test as soon as the ceiling starts to drip. The aim is to find the source, not to soak the ceiling further.

What a plumber does next

Once the likely fitting is known, the plumber checks it properly. That can mean taking off a bath panel, lifting a floorboard beside the toilet or removing the shower trap. Many fixes are small: a new pan connector, a reseated waste, fresh silicone, or a new washer on a valve. Some are bigger, such as a cracked shower tray or a leaking pipe that runs under a tiled floor.

If the leak is in a supply pipe that has been running for a while, the plumber may also suggest checking the joists and ceiling. Long term leaks can soften timber and plaster.

Hard water and old seals

Thames Water says all the water in its region is hard. Limescale builds up on bath and shower wastes and around valve seals, and it can stop a washer sealing properly. Silicone also breaks down faster where it is always wet and scaled. If your bathroom is more than a few years old and the sealant is cracked or blackened, it is worth redoing even if it is not the cause this time.

Who pays

If it is your bathroom and your flat below, it is your repair. If the bathroom is in the flat above and leaked into yours, the Leasehold Advisory Service says each leaseholder is usually responsible for pipes and appliances serving their own flat. The repair to your ceiling is often claimed on the buildings insurance, which the freeholder arranges. If you rent, your landlord must keep sanitation installations in repair under section 11 of the Landlord and Tenant Act 1985, so report it in writing.

Whoever pays, take photos of the stain and the source and ask the plumber for a written note of the cause. For more on how responsibility works when the water is coming from upstairs, read our guide to a leak from the flat above.