Overflow Pipe Dripping Outside? What It Means and What to Do

That thin plastic pipe sticking out under the eaves, or poking through the bathroom wall, does nothing for years. Then one morning there is a drip onto the path or a green streak down the brickwork. The pipe is an overflow, sometimes called a warning pipe, and the drip is it doing exactly the job it was put there for: telling you a valve somewhere inside is no longer closing.

Quick answer: find which pipe it is, then shut off the supply to whatever feeds it. For a toilet, close the small isolating valve on the cistern’s water pipe. For a loft tank, gently lift the float arm and prop it up so the valve shuts. If it is hot water, a copper pipe near the boiler or cylinder, or a steady stream, close the stopcock and call or WhatsApp 07861 933265.

Work out which pipe is dripping

Most London homes have more than one pipe that can drip outside, and each points at a different fault. Follow the pipe back to where it goes through the wall.

• At bathroom height, near the toilet: an older cistern with a side overflow. Lift the lid; if the water sits right up at the overflow outlet, the fill valve is letting by.
• High up, under the eaves or near the gutter: the overflow from the cold water storage tank in the loft. Common in Victorian terraces and in the 1930s semis of the M25 towns.
• High up, but only while the heating runs: the feed and expansion tank, the little companion tank beside the main one on open-vented heating systems.
• A copper pipe near the boiler, often with a bend that turns back to the wall: the boiler’s pressure relief discharge. It drips when the heating system pressure has gone too high.
• A copper pipe from an airing cupboard or utility room, sometimes running to a gully: the discharge from an unvented hot water cylinder’s safety valves.
• A white plastic pipe from the boiler to a drain: that is the condensate pipe. Water from it while the boiler runs is normal.

For toilets, flush and watch the outside pipe as the cistern refills. If the drip starts soon after, it is the cistern; if nothing changes, look upstairs.

What to do today

A toilet cistern. Close the isolating valve on the pipe feeding the cistern, usually a small slotted screw you turn a quarter with a flat screwdriver. The toilet will still flush once. Our guide to a toilet making noise when not in use explains how a worn fill valve or a float set too high causes this, and which you can adjust yourself.

The loft tank. Take a torch, stand only on boards or joists, and look into the tank. If the water is up to the overflow outlet, raise the float arm and wedge a piece of batten across the tank under it, or close the valve on the pipe coming into the float valve if one is fitted. If there is no safe way into the loft, turn off the stopcock: the tank stops filling and, although the bathroom taps will slowly run dry, nothing overflows.

The heating tank. Turn the heating off and let it cool. If the drip stops, you have confirmed the cause, and the system can run for short periods while you arrange a repair, as long as you keep an eye on the pipe.

Boiler or cylinder discharge. Leave the valves alone. For a boiler, look at the pressure gauge; our page on boiler pressure that is too high explains what the reading means. For an unvented cylinder that is discharging hot water, switch off the immersion heater and the boiler, then call us.

Why it happens

Inside every tank and cistern is a float valve. As the water rises, the float lifts and pushes a small washer or diaphragm against the inlet. London’s hard water leaves scale on that seat, rubber washers perish with age, and grit flushed through after mains repairs in the street can lodge in the valve. Any of these leaves the inlet slightly open, so the water keeps creeping up until it finds the overflow.

On sealed heating systems the cause is different: pressure climbs because a filling loop has been left open or the expansion vessel has lost its charge, and the relief valve opens to protect the boiler.

What not to do

• Do not push a cork, a bag or tape into the end of the pipe to stop the drip. That turns a wet patch on the patio into water through a bedroom ceiling.
• Do not bend the arm of a modern plastic float valve; most have an adjusting screw or clip for the water level instead.
• Do not ignore it into winter. In a hard frost the drip freezes at the open end and seals the pipe, and the tank then spills into the loft. If water is already showing on a ceiling, see our page on water coming through the ceiling.
• Do not tamper with the safety valves on an unvented cylinder or try to cap its discharge pipe.

Flats, conversions and mansion blocks

In a house split into flats, the tank in the loft usually serves only the top flat, but its overflow may drip onto the garden or path of the flat below. In purpose-built blocks and mansion blocks the storage tanks are often on the roof or in a plant room, and an overflow running down the building is a matter for the managing agent. Report it in writing either way.

What the repair involves

Most dripping overflows are a short job. The engineer finds which tank or cistern is overflowing, cleans or replaces the washer, or swaps the whole float valve if the body is scaled or cracked, and sets the water level so it shuts off well below the overflow. On a heating tank, the level is also reset so there is room for expansion. If the tank itself is old galvanised steel, rusting or missing a lid, we will tell you and you can decide; our cold water tank page covers replacements and upgrades. Cylinder safety valves and expansion vessels are booked heating work at the same rate.

There is no charge for coming out. Labour is £110 for the first hour on site and £45 for each half hour after that, plus parts, whether you call at 8am or after midnight. Send a photo of where the pipe leaves the wall on WhatsApp to 07861 933265 so the engineer knows which valve to expect.

Keeping it from coming back

Once a year, after the heating has run for a while, look up at the outside wall. A dry overflow means every float valve is shutting. Make sure the loft tank has a lid and insulation, and that the overflow pipe runs clear to the outside rather than being cut short inside the loft.