Hidden leaks in London flats: signs and leak detection

Not every leak announces itself with a drip. In a flat, a pipe can leak into a floor void, a riser or behind a kitchen unit for weeks before anything shows. By the time a stain appears on the ceiling below, the joists or the floor screed may have been wet for a long time.

This guide covers the signs, a simple test you can do yourself, and what proper leak detection involves.

Signs you might have a hidden leak

• Your water meter keeps turning when nothing is running.
• A damp patch, stain or bubbling paint that keeps coming back.
• Mould or a musty smell in one corner, often near a bathroom or kitchen wall.
• Floorboards or laminate lifting, or a soft spot in the floor.
• Lower water pressure than usual.
• Your neighbour below reports a stain or damp.
• A letter or email from Thames Water saying it has detected a leak at your property.
• Your boiler keeps losing pressure, if you have a sealed heating system.

The meter test

If your flat has its own water meter, this is the quickest way to know whether you have a leak.

1. Turn off all taps and anything that uses water, including the washing machine, dishwasher and any water softener.
2. Make sure nobody flushes the toilet.
3. Read the meter and write down every number.
4. Wait at least 30 minutes without using any water.
5. Read the meter again.

If the reading has changed, water is going somewhere after the meter. To narrow it down, close isolation valves one at a time. Under the toilet cistern, under the basin and behind the washing machine are common ones. If the meter stops when you close a particular valve, the leak is on that fitting.

Many flats in blocks do not have their own meter, or the meter is in a communal cupboard or in the pavement. If you are not sure where yours is, ask the managing agent or check your water bill.

Mansion blocks and risers

Across Westminster, 70.2% of households live in purpose built flats, according to the 2021 Census (Nomis, TS044). Many of those are mansion blocks in Marylebone, Maida Vale and St John’s Wood, where pipes run up through risers and along ceiling voids. A leak in a riser can show on several floors, or in a hallway well away from any bathroom.

In these buildings, hidden leaks are often a shared problem. The Leasehold Advisory Service notes that leaks can be hard to trace because a building may have many pipes and appliances, and water can travel through the structure. It suggests asking your landlord whether the buildings insurance includes cover for tracing the source of a leak.

What leak detection involves

Proper leak detection is about finding the source without pulling the flat apart. Depending on the job, a plumber or leak detection specialist may use:

Method	What it does	Good for
Visual checks and isolation	Closing valves and watching the meter	Narrowing down which fitting is leaking
Moisture meter	Measures damp in walls, floors and ceilings	Mapping where water has spread
Thermal camera	Shows warm or cold spots	Leaks on hot water or heating pipes
Acoustic listening	Hears water escaping under pressure	Supply pipes under floors and outside
Pressure testing	Tests a section of pipe for loss of pressure	Proving whether a run is leaking
Tracer gas	A safe gas pushed into the pipe and detected where it escapes	Very small or deep leaks

Not every job needs all of these. Many leaks in flats are found with valves, a moisture meter and a bit of floor lifting.

When the leak is outside

If the leak is on the supply pipe between your boundary and your home, it is still yours. Thames Water says the homeowner is responsible for the supply pipe from the boundary, including any part under a garden or path. In a block, the supply pipe is usually shared and the freeholder’s.

Once a leak on your property is confirmed, Thames Water says you must arrange a repair within four weeks, and you may be able to claim an allowance for the lost water if you do. Our guide to the Thames Water four week rule explains what happens next.

Hidden leaks and damp are not the same

Not every damp patch is a plumbing leak. Condensation, rain getting in through a roof or wall, or rising damp can look similar. The Leasehold Advisory Service notes that leaks caused by the structure, such as a leaking roof, are usually the freeholder’s responsibility. A plumber can usually tell you quickly whether a patch is from a pipe. If it is not, you will need a builder or surveyor instead.

Do not wait too long

A small hidden leak can cost more than a dramatic burst, because it runs for longer. If you have any of the signs above, do the meter test today. If it shows a leak, turn off the valve for the fitting involved, or the stop tap if you cannot find it, and get it traced. Our ceiling leak service covers leaks that are already showing below.