Overflow Pipe Dripping Outside? What It Means
That pipe dripping down your outside wall is a messenger: somewhere inside, a tank or cistern is overfilling, and its overflow is doing exactly its job — dumping the excess outside where you'll notice instead of through your ceiling. The message is always 'a float valve or similar has failed'; the urgency depends on which tank is talking. Identify the pipe's origin and you've identified the fault.
Which overflow is it?
Trace the pipe's exit point. High on the wall near the roofline: the loft cold-water storage tank, or the small central-heating feed-and-expansion tank beside it. Mid-height behind a bathroom or WC: a toilet cistern (though most modern toilets overflow internally into the pan instead — a modern toilet 'overflowing' shows as constant running into the pan). Near a cylinder cupboard on an unvented system: that's not an overflow but the pressure relief discharge pipe — a different and more pressing story.
Five minutes with a ladder-free look and a listen at each candidate (a hissing fill valve gives itself away) usually settles it.
Loft tank overflows: the standard case
The cold storage tank's float valve should close crisply at the waterline; a worn washer or diaphragm lets water creep up until the overflow takes it. It's rarely an emergency — the overflow is coping — but it's wasting water continuously and, in winter, painting your wall with a future ice sheet. Repair is a float valve washer or full valve replacement, £80–£150, with modern equilibrium valves the durable choice.
The small feed-and-expansion tank overflowing has an extra suspect worth knowing: if it overflows mainly when the heating runs, the coil in the cylinder or the system pressurisation may be pushing water back — one to describe carefully to the plumber, as it can indicate a failing coil.
Toilet cistern overflows
Older toilets with external overflows drip outside when the fill valve fails or the level is set too high — same fault family as the loft tank, same fix, £80–£150. Modern internal-overflow toilets send the same failure into the pan as constant running (see our running toilet guide) — so an outside drip from a bathroom wall generally means an older installation.
Either way, on a water meter this failure is a charged leak: hundreds of litres a day is routine. Treat 'just a drip outside' with the urgency of money leaving.
The one that isn't an overflow: unvented discharge
A pipe (often copper, terminating low near a drain, sometimes via a tundish visible in the cylinder cupboard) discharging water on an unvented hot water system is the pressure/temperature relief system operating — the cylinder's safety valves passing water. Occasional drips can be a tired expansion vessel; continuous or hot discharge means the safety controls are actively relieving.
This is G3-qualified engineer territory, promptly: the discharge is the safety system working, and the cause — expansion vessel charge, failed valve, thermostat fault — needs professional diagnosis. Don't cap, plug or ignore a discharging relief pipe, ever.
| Pipe location | Source | Urgency |
|---|---|---|
| High, near roofline | Loft tank float valve | Days — book repair |
| High, smaller pipe | F&E tank (heating) | Days — describe pattern |
| Behind WC, mid-height | Older toilet cistern | Days — metered = money |
| Low, near drain / via tundish | Unvented relief discharge | Prompt — G3 engineer |
After the emergency: the follow-up that matters
Once the immediate crisis is contained, three follow-ups protect you: photographs of everything for insurance (before cleanup, however tempting), a proper repair of whatever failed rather than a permanent-ised temporary fix, and an honest look at why it failed — because bursts, floods and failures usually have a preventable cause that will otherwise reoffend. Our prevention guides cover the systematic version.
Frequently asked questions
Is a dripping overflow an emergency?
Usually no — the overflow is successfully managing a fault. The exceptions: an unvented cylinder's relief pipe running (prompt professional attention), or an overflow flowing heavily rather than dripping, which means the fault is worsening and the overflow's capacity has limits.
Why does it only drip at night?
Mains pressure rises overnight as demand falls, pushing more past a marginal float valve washer. Night-only dripping is the same failure at an earlier, cheaper-to-fix stage.
Can I just bend the float arm down like my dad did?
On older ball valves, adjusting the float down does lower the shut-off level and can silence the drip — but a valve drifting upward is telling you its washer is worn. The adjustment buys weeks; the washer or valve swap fixes it.
The overflow froze and now water is coming through the ceiling — how?
A frozen or blocked overflow can't discharge, so an overfilling tank rises past it and finds the ceiling. It's the strongest argument for fixing 'harmless' drips before winter: the overflow is your safety net, and a dripping one iced shut isn't a net at all.
Where this guide's advice ends
Written guidance has a natural limit: it can tell you what usually causes a symptom, but only eyes on your actual pipework can confirm it. If what you're seeing doesn't quite match any pattern described here, or matches several at once, that ambiguity is itself diagnostic information worth giving an engineer. Our vetted local plumbers do this triage daily — and an honest one will tell you on the phone if your problem sounds like a cheap fix, a bigger job, or (occasionally) not a plumbing problem at all.