Overflow Pipe Dripping Outside? What It Means and How to Fix It

You step outside and notice water trickling from a short pipe poking through the wall, usually up near the eaves or below a bedroom window. It has left a green stain down the brickwork, and the drip never quite stops. That pipe is an overflow, and a dripping overflow is one of the most common plumbing calls we get across London. The good news is that it is almost never an emergency in the sense of flooding your home in minutes. The less good news is that it is telling you something inside has failed, it is quietly wasting water every hour of every day, and it will not fix itself.

This guide explains what an external overflow pipe actually is, the handful of reasons it starts to drip, how to identify which tank or cistern is at fault, what you can safely do yourself, and the point at which it makes sense to bring in a plumber. It is written for London homes, where a mix of older tank-fed systems and newer sealed systems means the answer genuinely depends on your property.

What an external overflow pipe is for

An overflow pipe is a safety device. Any part of your plumbing that holds water and refills automatically needs a way to get rid of excess water if the refilling mechanism fails. Rather than let the tank or cistern spill inside the loft or bathroom, the overflow carries that water to the outside of the building, where you will see and hear it. In other words, the drip is the system doing exactly what it was designed to do. It is a visible alarm, deliberately placed where you cannot easily ignore it.

In a typical London house there are usually two or three things that can feed an overflow pipe:

• The toilet cistern. Modern toilets often have an internal overflow that discharges into the pan, so you see a constant trickle in the bowl rather than outside. Many older cisterns, though, still have an overflow pipe that runs out through the wall.
• The cold water storage tank. This is the large tank in the loft that feeds your taps and, in many homes, the hot water cylinder. It has its own overflow pipe, usually a fairly fat one, exiting high on the gable or under the eaves.
• The feed and expansion tank. This is a smaller tank, also usually in the loft, that tops up and gives room for expansion in an open-vented central heating system. It has its own overflow too.

If your home has a combi boiler and no loft tanks, the overflow you are looking at is very likely coming from a toilet cistern. If you have a hot water cylinder in an airing cupboard and tanks in the loft, any of the three could be the culprit. Working out which is the first real job.

Why an overflow pipe starts to drip

Nearly every dripping overflow comes down to the same underlying fault: the valve that is supposed to stop the water when the tank is full has stopped stopping it. Water keeps trickling in, the level creeps above where it should sit, and the surplus runs out of the overflow. There are a few specific ways this happens.

A faulty float valve or ballcock

Every filling tank has a float valve, often still called a ballcock after the traditional brass arm and floating ball. As the water rises, the float rises with it and gradually closes the valve. When the tank is full, the valve should shut completely. If the valve is worn, sticky, or full of limescale, it never fully closes, and a slow feed keeps the level topped up past the overflow. In London's hard water, limescale is the single biggest reason these valves fail early.

A worn washer or valve seat

Inside older piston-type float valves there is a small rubber washer that presses against a seat to seal off the flow. Washers perish, harden, and split with age and hard water. A worn washer is one of the cheapest possible faults to fix, but the symptom, a dripping overflow, looks identical to far more serious problems, which is why diagnosis matters.

An incorrectly set water level

Sometimes nothing has actually broken. If a float has been knocked, bent, or set too high, the tank simply fills higher than the overflow and spills. This is common after someone has been working in the loft or has recently replaced a part and not adjusted it correctly. Modern valves usually have an adjustment screw or clip that sets the shut-off height.

A waterlogged or split float

The float relies on being buoyant. If a plastic float cracks and takes on water, or an old metal ball corrodes and floods, it sinks instead of rising. A sunken float never tells the valve to close, so the tank overfills continuously. You can often spot this by lifting the float by hand and seeing whether the water stops.

A failing part in an older system

Tank-fed systems in Victorian and Edwardian London houses can be decades old. Brass arms seize, valve bodies corrode, and parts that are no longer made get bodged with mismatched replacements. In these systems a dripping overflow is sometimes the first sign that the whole valve assembly, or occasionally the tank itself, is reaching the end of its life.

How to identify which tank or cistern is at fault

Before you can fix anything, you need to know where the water is coming from. The overflow pipe on the outside wall gives you a clue by its position, but the reliable way is to trace it back inside. Here is a simple, methodical approach.

Once you are at the suspected source, the confirmation is straightforward. Take the lid off the cistern or look into the loft tank. The water should sit below the mouth of the overflow, typically around 25mm below it. If the water is right up at the overflow or visibly running into it, you have found your culprit. Gently lift the float arm by hand. If the trickle of incoming water stops, the valve and washer are the issue rather than the float being waterlogged. If lifting the float does nothing, the valve itself is stuck open.

One more useful test: touch the overflow water or watch the drip rate at different times. A feed and expansion tank that only overflows when the heating is running behaves differently from a cold storage tank that drips constantly. If the drip gets noticeably worse when the central heating fires up, suspect the feed and expansion tank and, occasionally, a more serious fault where the hot water cylinder coil is leaking and pushing extra water into that tank. That particular scenario is one to get looked at properly rather than left.

Temporary steps you can take safely

You do not have to leave the tap running down your wall while you sort out a permanent fix. There are a few sensible holding measures, and the honest consensus you will find on forums such as r/DIYUK and DIYnot is the same: these buy you time, they do not cure the fault.

• Tie up the float. The classic short-term trick for a loft tank or cistern is to rest the float arm on a length of timber or tie it up with string so the valve stays shut. This stops the overflow but also stops that tank refilling, so you may lose the water supply it feeds until you release it.
• Turn off the supply to that tank. Most float valves have an isolation valve or a nearby stopcock on the pipe feeding them. Turning it off stops the drip immediately. Again, remember which fixtures rely on that tank before you leave it off overnight.
• Gently work the valve. Sometimes flicking the float up and down a few times dislodges a small piece of limescale and the valve seats properly again. This is a temporary reprieve at best in a hard water area, but it can stop a drip long enough to plan a repair.
• Check for an obvious high setting. If you can see an adjustment screw and the level is only slightly high, a small tweak downwards may bring it below the overflow. Do not force old brass fittings.

What the forums also make clear, and what we would echo, is to be cautious about improvised fixes on very old or fragile systems. Cranking on a seized brass ballcock or over-tightening a corroded fitting can turn a slow drip into a burst pipe in a cold loft. If anything feels stuck or brittle, stop and get it looked at.

Why leaving it is a false economy

A dripping overflow is easy to ignore because it is outside, it is not damaging anything you can see, and it never seems to get worse. All three of those are reasons people leave it for months. Here is why that is a mistake.

It wastes a surprising amount of water. A continuous trickle adds up. Over weeks and months a dripping overflow can waste a meaningful volume of clean, treated water, and if your London home is on a water meter you are paying for every litre that runs down the brickwork. Even unmetered, it is waste for no benefit.

It damages the building. Water running down a wall does not stay on the surface. It keeps brick and render permanently damp, encourages moss and algae, works into mortar joints, and in freezing weather can get into small cracks and expand. Over a winter or two, a persistent overflow can cause real pointing and damp problems that cost far more to put right than the original valve.

It can mask a bigger fault. As mentioned above, an overflow from the feed and expansion tank that worsens with the heating can indicate a failing cylinder coil, not just a tired valve. A cold tank that overflows intermittently might be hiding a valve that is about to fail completely and dump water into your loft. The drip is information, and ignoring information is how a cheap job becomes an expensive one.

You can read more about the wider picture in our guides to water leak repair in London and, for cistern-specific problems, fixing a running or leaking toilet and what it costs.

When you need a plumber

Plenty of dripping overflows are genuinely within reach of a confident homeowner, particularly a modern toilet cistern where the float valve is a cheap, easy swap. There is no shame in the opposite either. Here is where we would say it is worth getting a professional in.

• The tank or valve is old, brass, and seized. If parts do not move freely, forcing them risks a burst. A plumber will have the right replacement valve and the experience to fit it without cracking a decades-old fitting.
• You cannot work out which tank is overflowing. If tracing the pipe leaves you unsure, or more than one thing seems to be involved, a proper diagnosis saves you buying the wrong parts twice.
• The overflow worsens with the heating on. This points at the feed and expansion side and possibly the hot water cylinder, which is not a DIY job.
• The drip has become a flow, or you see water in the loft. That is the point where a slow problem is turning into an urgent one, and it is worth calling an emergency plumber in London rather than waiting.
• You are simply not comfortable working in a loft or with the water off. That is a perfectly good reason on its own.

What a repair typically costs

Costs vary with the property, the part, and access, so treat the figures below as typical UK trade cost-guide ranges rather than a fixed quote. A straightforward float valve or washer replacement on an accessible cistern or tank is usually one of the cheaper plumbing jobs. Where the tank is awkward to reach, the valve is an obsolete pattern, or the fault turns out to involve the cylinder or heating system, the cost rises accordingly because more time and more parts are involved.

However your job falls out, the way we work is simple and up front. We give you an honest arrival window rather than a vague all-day wait, and we agree the price with you before we travel, so there is no surprise on the doorstep. If the diagnosis on site changes the scope, we tell you before doing the work, not after.

The bottom line

A dripping external overflow pipe is your plumbing raising its hand. In most London homes it means a float valve, a washer, or a water level has stopped doing its job in a toilet cistern, a cold water storage tank, or a feed and expansion tank. Trace the pipe back to its source, check whether the level is sitting too high, and you will usually find the fault quickly. Simple resets and valve swaps are within reach for many people, while seized old fittings, heating-related overflows, and anything that has turned from a drip into a flow are worth handing to a plumber. Whatever you do, do not leave it running for months. It wastes water, marks and damages your walls, and occasionally hides a fault that is far cheaper to catch early than to meet head on when a loft tank finally lets go.