An overflow pipe dripping outside almost always means a valve inside is not shutting off, whether that is a WC cistern, a loft tank, or a pressure relief valve on a boiler or cylinder. Trace the pipe back to its source before assuming which one, and never simply cap it, as the drip is a safety feature doing its job.
Spotting a small trickle of water running down an external wall, or a steady drip from a pipe poking out under the eaves, is one of those things easy to notice and then put off dealing with. But an overflow pipe dripping outside is not decorative plumbing. It exists to carry away water that a valve inside your home has failed to stop, and every one of those pipes belongs to a specific tank or fitting with its own likely cause.
On this page
Why overflow pipes exist at all
Every overflow pipe is a deliberate safety feature. Cisterns, tanks and pressurised heating systems all have a valve that is meant to stop water or pressure rising past a set point. If that valve fails, the overflow gives the excess somewhere safe to go, outside the building, rather than letting it back up inside a cistern, tank or airing cupboard. A dripping overflow is the system telling you a valve has already failed or is close to it. It is a warning sign doing exactly its job, not a fault with the pipe itself.
WC cistern overflow
The most familiar overflow pipe usually runs from a toilet cistern, out through an external wall at low to mid height. If it drips, the ball valve or the modern diaphragm-style fill valve inside the cistern is not shutting off, letting water rise past its intended level and out through the overflow. Older syphon-flush cisterns and the ball valves that came with them are especially prone to this as washers age. A running or refilling sound from the toilet, even when nobody has used it, usually goes hand in hand with this kind of overflow.
Loft tank and F&E tank overflows
A pipe exiting higher up, often from the eaves or under a soffit, is usually the overflow for a cold water storage tank or a feed-and-expansion (F&E) tank in the loft, both of which are common in older vented plumbing systems across the region. The mechanism is the same as a WC cistern: a ball valve that should shut off at a set water level fails to, and the tank spills its excess out through the overflow instead of flooding the loft. We cover this specific fault in more depth in our guide to a loft tank leaking or overflowing, including how to tell a valve fault from an actual split in the tank.
Pressure relief valve discharge (not a tank at all)
Not every dripping pipe outside belongs to a tank. Combi boilers and unvented hot water cylinders have a pressure relief valve (PRV) that opens automatically if system pressure climbs too high, and its discharge pipe usually runs to an external wall, often low down near the boiler or cylinder's location. Typical cold system pressure sits around 1 to 1.5 bar, and a PRV that keeps dripping or discharging usually means pressure is creeping above the safe range, which is a different problem with a different fix from a simple worn tank valve. A continually dripping PRV discharge is worth having looked at by a qualified engineer rather than assumed to be "just the overflow".
| Where the pipe exits | Likely source | Usual cause |
|---|---|---|
| Low down, near a downstairs WC | Toilet cistern | Worn ball valve or fill valve washer |
| High up, near the eaves or roofline | Loft cold water tank or F&E tank | Ball valve not shutting off |
| Near the boiler or airing cupboard, narrow copper pipe | Pressure relief valve (PRV) | System pressure too high |
| Ground floor, near a utility room | Water softener or unvented cylinder overflow | Valve or expansion fault; check with an engineer |
Tracing the pipe back to its source
- Note exactly where the pipe exits the wall and roughly how high, since height and location narrow down the likely source quickly.
- Go inside and look for the nearest matching fitting: a downstairs WC, a loft tank, or the boiler or cylinder cupboard.
- Check whether it drips constantly or only after use, for example only after a toilet flush and refill, which points more clearly to that cistern.
- If you cannot place it, or more than one candidate seems possible, it is worth having a plumber trace it properly rather than guessing and fixing the wrong fitting.
Why you should not just ignore or cap it
It is tempting to see a drip as harmless, or worse, to simply block the end of the pipe to stop the nuisance. Neither is a good idea. Ignoring it means a valve fault typically gets worse, not better, and the volume of wasted water and the risk of external staining both increase over time. Capping the pipe removes the warning sign but does nothing to fix the valve, and it forces water to find another route out if the fault worsens, which is far more likely to mean an internal leak than another convenient external pipe.
If the underlying issue turns out to be more than a simple valve, or if you are unsure whether what you are seeing is an overflow drip or the sign of a hidden leak elsewhere, non-invasive plumbing leak detection can confirm what is actually happening before any work begins.
Overflow pipes on Cornwall and Devon properties
Coastal exposure and driving rain make external staining from a dripping overflow more visible, and often more damaging, on render, cob and soft local stone than on more weather-resistant modern brick. On granite cottages and older farmhouse conversions, we regularly find overflow pipes that discharge too close to the wall face rather than clear of it, which lets water track back into the masonry even when the valve fault itself is minor. Holiday lets are a particular blind spot: with nobody in residence between bookings, a dripping overflow can run for weeks, wasting water and staining walls, before anyone notices during a changeover clean.
Frequently asked questions
Why is my overflow pipe dripping outside?
Most often a valve inside a tank or cistern has failed to shut off fully once it reached its normal level, so the excess water is diverted safely outside through the overflow. It can also be a pressure relief valve on a combi boiler or unvented cylinder discharging because system pressure is too high.
How do I know which overflow pipe is dripping?
Trace the pipe back into the building at the point it exits the wall. A short run near ground floor often leads to a WC cistern, a pipe from the eaves usually serves a loft tank, and a narrow copper pipe near a boiler or airing cupboard is typically a pressure relief valve discharge, not a tank overflow at all.
Is a dripping overflow pipe an emergency?
Rarely an emergency in itself, but it should not be left. A dripping overflow wastes water continuously and often signals a valve that is getting worse, not better. A pressure relief valve discharge is more time-sensitive, since persistent high pressure can point to a developing fault worth checking sooner.
Can a dripping overflow pipe damage my house?
Yes. Constant dripping down an external wall can leave green or black staining, and in cob, render or soft brick it can encourage damp to track inward over time. Where the pipe discharges awkwardly close to the wall rather than clear of it, the risk of that staining and moisture ingress is higher.
Why does my overflow only drip sometimes?
Intermittent dripping usually means the valve is borderline rather than fully failed, closing most of the time but occasionally letting a trickle through, often linked to small changes in mains pressure or temperature. It is still worth having checked, since these faults tend to get more frequent rather than resolve on their own.
Should I just cap the overflow pipe to stop the drip?
No. The overflow is a safety feature designed to prevent an uncontrolled indoor flood if a valve fails. Blocking it does not fix the underlying valve fault, it just removes the visible warning sign and risks water finding another way out, which is usually inside the property rather than outside it.
Not sure which overflow is dripping, or why?
If you cannot trace a dripping overflow pipe to its source, or you suspect it is more than a simple valve fault, we can find and confirm the cause across Cornwall & Devon with minimal disruption.
Call Dickie on 07822 025 911 No Find, No Fee on residential leak detection (subject to terms)