How to find a hidden water leak in a London flat, and what to do first

Water through a ceiling is alarming, but most leaks are slow and the damage is done over weeks, not minutes. Start by shutting off what you can. Every flat has an internal stopcock, usually under the kitchen sink, in a cupboard by the front door or in a utility cupboard on the landing. Turn it clockwise until it stops. If it is stiff or seized, do not force it with a wrench; a stopcock that snaps off is a far worse problem. Find the building's isolation valve instead, which the managing agent or concierge will know.

Then check whether the leak stops. If the dripping slows and stops over the next half hour, the source is on your mains-fed pipework: a supply pipe, a tap connection, a toilet fill valve, a washing machine hose. If it carries on regardless, the water is coming from somewhere else: the heating system, a waste pipe, a shower tray, rainwater, or the flat above.

If there is water near electrics, switch the circuit off at the consumer unit before you do anything else. And put a bucket under the drip, not a bowl; you will be surprised how much collects overnight.

A hidden leak leaves patterns, and the patterns narrow it down before anyone lifts a board.

• Wet only after a shower or bath points to a waste, a trap, a failed silicone seal or a cracked tray. Run the shower for ten minutes with the door open and watch the ceiling below.
• Wet only after the heating has run points to a radiator pipe or valve. Heating pipes are hot, so a warm patch on a floor or a wall with no radiator behind it is a strong clue.
• Wet all the time, worse overnight points to a mains supply pipe, because mains pressure is highest at night when nobody is using water.
• Wet after rain is not plumbing at all. Check the gutters, the flat roof, the window seals and the balcony drain.
• A boiler that needs topping up every few days with no visible leak is losing heating water somewhere, often under a floor.

Look at the stain itself. Brown rings that spread outwards from a centre are slow and long-standing. A fresh, clean wet patch is recent. Bubbling paint with dry plaster underneath usually means the leak has stopped and started before. Salt deposits on the surface mean the wall has been wet for a long time.

This is the question that makes leaks in flats different. Water travels sideways along joists, down pipe boxings and through service risers, and it can surface two rooms away from where it started, in a different flat.

The general position in most leases is that the flat where the leaking pipe or fitting is located is responsible for putting it right, and the block's buildings insurance normally covers the damage to the flat below, subject to the excess. That is worth knowing because it changes what you do next: if the stain is on your ceiling and your own stopcock made no difference, the conversation is with the flat above and the managing agent, not with a plumber for your flat.

Be polite and practical with the neighbour. Ask them to check under their bath panel, behind the washing machine and around the toilet, and to run each fitting while you watch your ceiling. A great many "mystery" leaks are a bath waste that only leaks when someone stands in the bath, or a shower screen seal that only fails when the water is aimed at it.

Keep a log with dates and photos. If the leak ends up as an insurance claim or a dispute over who pays, the record of what was wet, when, and what was tried is what settles it.

The meter test. If the flat has its own water meter, turn off every tap and appliance, write down the reading, wait two hours without using any water, and read it again. Any movement is a leak on the mains side. If there is no meter, listen at the stopcock in the quiet of the evening for a faint hiss.

The dye test. Put food colouring in the toilet cistern. If the bowl colours without flushing, the flush valve is passing and the toilet is running constantly. That is a leak into the drain rather than into the fabric, but it is a common cause of a high bill.

The bath test. Fill the bath, let it stand for twenty minutes (checks the bath itself and the overflow), then pull the plug and watch below (checks the waste and trap). Do the same with the shower, and separately splash water onto the screen and the tiled corners.

The heating test. Note the boiler pressure cold, run the heating for two hours, let it cool completely and read again. A drop of more than a few tenths of a bar overnight, repeated over three nights, is a heating leak.

If these narrow it to a fitting you can see, a plumber can fix it. If they tell you the water is going somewhere you cannot see, that is when proper detection saves money.

The point of leak detection is to find the source without destruction, so that only one board or one tile comes up rather than a whole floor. A detection visit uses several methods and cross-checks them:

• Thermal imaging shows temperature differences through plaster and floors. A leaking hot pipe shows as a warm plume; a cold mains leak shows as a cold patch as evaporation cools the surface.
• Moisture mapping with a pin or capacitance meter draws the outline of the wet area on the wall or floor, and the wettest point is usually closest to the source.
• Acoustic listening picks up the hiss of water escaping under pressure through floors and walls, and is the main tool for mains leaks.
• Tracer gas is used when nothing else works: the pipe is drained, filled with a harmless hydrogen-nitrogen mix, and a sniffer finds where the gas surfaces.
• Pressure testing each circuit in turn (hot, cold, heating) confirms which system is losing water before any tracing starts.

A good report gives you the location, the cause, photographs and the evidence, in a form an insurer will accept for a trace-and-access claim. From there the repair is usually a small job. The words to look for when you book are non-invasive leak detection, because "we will start lifting floors and see" is not detection, it is demolition.

Once the source is fixed, the fabric still has to dry. Plasterboard, timber joists and screed hold a surprising amount of water and take weeks to release it. Keep the room warm and ventilated, or hire a dehumidifier for a bad one, and do not redecorate until a moisture meter says the wall is dry. Paint over a damp wall and it will bubble again inside a month, and you will wonder whether the leak is back.

Check the ceiling below for sagging. Wet plasterboard loses its strength and a bulging ceiling is a ceiling about to come down; it needs propping or cutting out, not waiting. And if the leak ran for a long time, have someone look at the joists and any electrics that got wet before they are covered up again.