A pipe dripping high on an outside wall can be an early warning that a water tank or toilet cistern is overfilling. But not every pipe that releases water outdoors is an overflow. Some carry discharge from a boiler safety device, and treating them as an ordinary leak can be dangerous. The first job is to identify where the pipe comes from without dismantling anything.
How to tell an overflow from a boiler discharge
A conventional overflow, sometimes called a warning pipe, normally allows excess water to escape visibly before a tank floods an internal space. It may be connected to a toilet cistern, an older cold-water storage tank in the loft, or a feed-and-expansion cistern for an open-vented heating system. A condensing boiler also has a condensate drain, which may normally release water while the appliance is operating. A separate safety or pressure-relief discharge should never be mistaken for that routine drainage.
Where a pipe emerges, whether the drip is intermittent or continuous, and which appliances were used shortly before it started can help a plumber trace its source. Appearance alone cannot reliably prove what a pipe carries, particularly where several pipes exit close together.
When the toilet cistern is the likely source
With a traditional toilet cistern, a float-controlled inlet valve should stop water entering once the desired level is reached. If the valve fails to close or the float becomes obstructed, the water level can rise to the overflow. A continuous trickle after nobody has flushed for some time is a useful clue. Some modern toilets route overflow water internally into the bowl, so an outside drip may point elsewhere.
Look for a toilet that refills repeatedly or has water running inside the bowl. Check whether the sound stops after the cistern finishes filling. Avoid forcing old plastic valve parts: a brittle fitting can turn a minor fault into an indoor leak.
Could the leak be coming from the loft?
Older UK homes sometimes have a cold-water storage cistern or a small heating feed-and-expansion tank in the loft. A faulty float valve can cause the cistern to fill above its intended level. In a heating system, unusual water movement or overheating may need professional assessment rather than a simple valve change.
There is no need to climb into an unsafe loft to investigate. From a safe position, note whether the outside pipe drips after hot-water demand, during heating, or throughout the day. If accessible, check for damp ceiling patches or staining below the suspected tank, but do not step between joists or move insulation around electrical fittings.
Red flags: hot water, boiler pressure and safety outlets
Water that is hot, a pipe connected to a pressure-relief assembly, a rapidly falling boiler pressure reading, or repeated pressure fluctuations all require caution. Do not cap, plug, bend or disconnect a pipe to stop water escaping: doing so may block a safety outlet. Never open a sealed boiler casing. A Gas Safe registered engineer should investigate gas-boiler issues, and a suitably qualified specialist should assess an unvented hot-water cylinder.
If a pipe is discharging very hot water or steam, keep people away and seek urgent professional help. A dripping pipe close to an electricity supply or a ceiling leak spreading towards lighting also warrants urgent action.
What to record before calling a plumber
- Photograph where the pipe exits and where water lands, without climbing.
- Note whether the water appears only during heating, toilet use or hot-water demand.
- Record whether the flow is one drop every few seconds, a constant trickle or a stream.
- Check for any damp patches indoors and whether the boiler displays an error.
- Explain whether the property has a loft tank, an unvented cylinder or a combi boiler, if known.
These details help the tradesperson choose appropriate parts and assess urgency. They are observations, not a substitute for identifying the source on site.
Can you leave a dripping overflow until next week?
A persistent overflow wastes water and may damage brickwork or foundations over time. Arrange an inspection promptly even if there is no visible indoor leak. Treat hot discharge, significant water flow, signs of structural damp or possible contact with electrical systems as urgent. The repair depends on the actual source; replacing a toilet fill valve will not address a boiler relief-valve fault.