Overflow pipe dripping water outside
A dripping overflow pipe on the outside of your house is one of those problems that looks minor but shouldn't be ignored. It's actually your home's way of telling you that water somewhere inside isn't being managed properly — usually because a fill valve has stopped doing its job. Whether it's a toilet cistern, a cold water storage tank in the loft, or a hot water cylinder, the cause is almost always a valve that's stuck or worn out. The good news is that most of these faults are straightforward to diagnose and fix without calling anyone out. This guide walks through identifying which overflow is causing the problem and how to tackle it yourself, though if you're dealing with a hot water cylinder discharge, it's best to get a qualified plumber to check for overheating or pressure issues.
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Most likely cause & what to check
In the UK, an overflowing discharge pipe from the outside wall indicates that either a toilet cistern, cold water storage tank (loft), or hot water cylinder (indirect models) has a fill valve that is not shutting off properly.
Identify which overflow it is: toilet cistern (small pipe, usually at the back of the toilet or through the wall near cistern height), cold water storage tank (large pipe exiting high up through the eaves), or cylinder (overflow exits near the cylinder height).
For a toilet cistern overflow: remove the cistern lid and check the water level. If it is above the overflow outlet, the fill valve (ballcock) is not shutting off. Try bending the float arm downward, or replace the fill valve (£8–£20).
For a loft tank overflow: go into the loft and find the cold water storage tank. The ball valve arm may have jammed up, or the float may have sunk. Replace the washer or ball valve (£5–£15).
For a hot water cylinder overflow: if the indirect cylinder's temperature relief or cold feed ballvalve is discharging, call a plumber — this can indicate overheating or pressure issues.
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Frequently asked questions
How do I know which overflow pipe is causing the problem?
Location is the key clue. A small pipe at the back of the toilet or near cistern height points to a toilet problem; a large pipe exiting through the eaves high up suggests a cold water tank in the loft; and a pipe at mid-wall height usually indicates a hot water cylinder. Once you've identified which one, the steps to fix it become clear.
Is it expensive to fix an overflow pipe that's dripping?
Not at all — most repairs cost between £5 and £20 in parts alone. A toilet fill valve or loft tank ballcock are simple, affordable replacements available from any plumber's merchant. If you're confident doing it yourself, labour costs are zero.
Can I ignore a dripping overflow pipe, or will it cause damage?
Whilst a drip may seem harmless, it wastes water and signals that a valve isn't working properly. Ignoring it could lead to tank overflow inside your home or unnecessary water loss, so it's worth addressing quickly.
What should I do if the overflow is from my hot water cylinder?
Don't attempt a DIY fix on a cylinder overflow — it could indicate overheating, excessive pressure, or a faulty relief valve, all of which need a Gas Safe registered engineer to diagnose and repair safely.