Overflow pipes: why one is dripping and what it is telling you

A small plastic pipe poking out of a wall, dripping on to the path, is one of the most common plumbing puzzles in older UK houses. It is almost always an overflow, sometimes called a warning pipe. Its job is to make a fault obvious before a cistern floods the house. A drip is the warning working.

Which overflow is it?

Follow the pipe back, or work out which room it comes from.

WC cistern overflow. Older toilets often have an overflow pipe that runs from the cistern through the wall to the outside, usually low down near the toilet. Many modern cisterns have an internal overflow instead, which spills into the pan. You see a trickle down the back of the bowl rather than a drip outside.

Cold water storage cistern overflow. Houses with a tank in the loft have an overflow from it, usually leaving under the eaves or through a gable wall. It is often the highest small pipe on the house. See loft cold water tanks for how the tank itself works.

Feed and expansion cistern overflow. Older open vented heating systems have a second, smaller cistern in the loft that tops up the radiators. It has its own overflow. If this one runs, it is a heating question as well as a plumbing one; see below and our guide to feed and expansion tanks.

Bath overflow. The slotted or round opening under the bath taps. It is not a warning pipe. It joins the bath waste so the bath does not spill over the rim if you leave the taps on. A leak here usually shows as water under the bath, not a drip outside.

Unvented cylinder discharge. A copper pipe from a tundish (a small funnel with a visible gap) near a pressurised hot water cylinder. Water from here is a safety device discharging. It is not a DIY fix; see unvented cylinder basics.

What a dripping overflow usually means

For a WC or loft cistern, the cause is nearly always the same: the fill valve is not shutting off, so the water level rises past the overflow.

The usual reasons are:

• the float is set too high, or catches on something;
• the washer or diaphragm in the valve is worn or has grit in it;
• the float has a hole and has partly filled with water (older ball floats);
• on older side entry valves, the wrong type of valve or seat for the water pressure.

On a toilet, our running toilet guide walks through the checks, and toilet fill and flush valves covers choosing a replacement. A loft cistern's ballvalve works on the same principle, but it is bigger, harder to reach and sits above your ceilings, so take more care.

If a WC with an internal overflow trickles into the pan, check whether the water is going down the overflow tube or past the flush valve seal. The running toilet guide explains how to tell.

Checking a loft cistern safely

Only go into the loft if you can do it safely: boarded walkways or a crawl board, a light, and no stepping between the joists.

1. Look at the water level. If it is up at the overflow, the ballvalve is not closing.
2. Lift the float arm gently. If the flow stops, the valve can shut; the float is probably set too high or sticking.
3. If lifting the float does not stop it, the washer or valve is likely at fault.

Before taking a ballvalve apart, close the isolation valve on its supply pipe if there is one. If there is not, close the main stopcock. See stopcocks and water meters if you are not sure where it is. If you are not comfortable working at a cistern in the loft, this is a straightforward job for a plumber.

When the feed and expansion cistern overflows

If the smaller heating cistern in the loft is overflowing, its ballvalve may be at fault, just like the main cistern. But a heating cistern that keeps filling can also point to a problem inside the hot water cylinder or the heating system. Do not try to diagnose that from the loft. Ask a plumber or heating engineer to look at it. If anything involves a gas boiler, see when to call a Gas Safe engineer.

Why frost makes it urgent

A dripping overflow in winter can freeze at the outlet. Ice builds up, the pipe blocks, and the warning stops working. The cistern can then fill until it spills over into the loft or the bathroom.

So in cold weather:

• fix a dripping overflow promptly rather than waiting for spring;
• keep the loft cistern and its pipes insulated (see pipe lagging);
• if an overflow has frozen and the cistern is still filling, close the supply to the cistern.

Our frozen pipes guide covers safe thawing and what to do if a pipe has split.

Water you are paying for

On a water meter, a constantly running overflow is water you pay for. A slow one can add up over months. A toilet trickling into the pan is easy to miss, so check the back of the bowl occasionally.

Is a dripping overflow an emergency?

Not usually, but do not ignore it. It means a cistern is overfilling. Fix the fill valve or ballvalve soon, and sooner in freezing weather, when the outlet can ice up.

Which overflow belongs to which cistern?

Follow the pipe if you can. A low pipe near the bathroom is usually the toilet. A high pipe under the eaves is usually the loft cistern. Flushing the toilet or lifting a loft float and watching which one stops can confirm it.

Can I just cut the overflow pipe back?

No. The overflow is a warning. Without one, an overfilling cistern has nowhere safe to go. Fix the cause and leave the overflow in place.