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Why is my overflow pipe leaking?

An overflow pipe usually drips because a toilet cistern or water tank is filling past its set level, often due to a worn or badly adjusted fill valve. In Cheshire, find which tank the pipe serves first, then fix the valve or call a plumber.

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By the QuoteZila editorial team. Editorial policy.

What does an overflow pipe do?

If an overflow pipe is dripping outside your home in Cheshire, the cause is often simple. Whether you are in Crewe, Nantwich, Sandbach or elsewhere, the first step is the same: work out which tank or cistern the pipe comes from. Anyone in the UK can post or register on QuoteZila, and this page explains what to look for.

An overflow, sometimes called a warning pipe, is a safety outlet. If a cistern or tank keeps filling past its proper level, the extra water runs out through the overflow instead of flooding the room or the loft. Water coming out is the warning. Something inside is letting too much water in.

Which tank is the pipe coming from?

Follow the pipe back from where it ends. Common sources are:

  • A toilet cistern. The pipe often comes out of the wall near the toilet, sometimes low down outside. Some modern cisterns have an internal overflow that runs into the pan instead, so you see a trickle in the bowl rather than outside.
  • A cold water storage tank in the loft. The overflow usually comes out high up, under the eaves or through the fascia.
  • A small feed and expansion tank in the loft. This tops up an older open-vented heating system. Its overflow can also come out under the eaves.
  • The boiler. A copper pipe from the boiler to the outside may be a pressure relief pipe. That is a heating issue, not a tank overflow.

If you are not sure, flush each toilet and watch the pipe. If the drip gets worse just after the cistern refills, you have found it.

How do you fix a toilet cistern overflow?

Take the lid off the cistern and watch it fill after a flush. The water should stop below the overflow outlet. If it keeps rising and spills into the overflow, the fill valve is not shutting off.

Turn the water off at the small valve on the pipe to the cistern, usually with a flat-head screwdriver. Then check the fill valve:

  • Float on an arm (ballcock). Gently bend or adjust the arm so the float shuts the valve a little lower. Check whether the float has water inside it, which stops it rising properly.
  • Float on a vertical column. Most have an adjusting screw or clip to set a lower water level.
  • Worn washer or seal. If adjustment does not stop the flow, the washer or the whole valve may need replacing.

Turn the water back on slowly and watch a full refill. If the cistern still overfills, or the trickle into the pan continues, the flush valve seal at the bottom may be leaking instead. Removing a toilet flush valve explains that part. If you are not comfortable, a plumber can do both quickly.

What about a loft tank overflow?

A loft tank overflow works the same way: a float valve that does not shut off lets the tank fill past its line. Go into the loft only if it is safe, with boarding to stand on, good light and someone else at home. Keep your weight on the joists or boards. Never stand on the plasterboard between them.

Look at the float valve on the pipe feeding the tank. A sticking or waterlogged float, a worn washer, or a valve set too high are the usual causes. If the tank is old, cracked or has no lid, ask a plumber to look at the whole tank rather than just the valve. Our page on the header tank explains the small heating tank.

If you are unsure which tank is which, or the loft is hard to get into, call a plumber. A wet loft floor or a dripping ceiling below means the problem has gone past the overflow.

When should you call a plumber or engineer?

Call a plumber if the valve will not adjust, if parts are corroded or seized, if water is coming through a ceiling, or if more than one cistern seems to be overflowing. Call a Gas Safe registered heating engineer if the water comes from a pipe connected to the boiler. Do not open a gas boiler.

Water running down a wall can cause damp. GOV.UK guidance says damp can be caused by leaking water from pipes, overflowing baths or sinks, burst pipes or defective water storage vessels [S1]. It says fixing leaking pipes and other sources of water can help remove the cause [S1].

A like-for-like valve or cistern repair does not need building regulations approval. GOV.UK says most repairs and like-for-like replacements of toilets, basins and sinks do not need to be told to a building control body [S2].

When you ask for quotes, Citizens Advice says a quote is a fixed price and an estimate is just a rough guess [S3]. It says some traders charge for quotes, so ask about this first [S3]. Our plumbers page covers this kind of repair, and plumber call-out costs explains how a short visit is often priced.

Honest status: QuoteZila is new. We're starting in Cheshire, beginning with Crewe, Nantwich and Sandbach, and growing area by area. Anyone in the UK can still post a job or register a business. How fast you get quotes depends on how many local businesses have joined near you. Businesses can register and receive jobs straight away. We don't check insurance, qualifications or registrations, so please check these yourself before you hire.

Jobs you might post

These are examples, not taken from real job posts.

  • Overflow pipe outside the bathroom drips all day. Customer thinks it is the toilet cistern and would like the fill valve replaced.
  • Water pouring from an overflow under the eaves. Loft tank float valve may be stuck. Plumber to check the tank and the valve.
  • Copper pipe from the combi boiler is dripping outside. Heating engineer to check the boiler pressure and the relief valve.

Ask a plumber to fix it

Say where the overflow pipe comes out and which room is nearest, and add a photo. Your job goes to up to five matched local businesses [QZ2]. QuoteZila is free for customers and local businesses [QZ1].

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Questions about leaking overflow pipes

Is a dripping overflow pipe an emergency?

Usually not, but do not leave it. The pipe is doing its job by carrying spare water away instead of letting the tank flood. Water running constantly is wasted, and a pipe that drips down a wall can soak the brickwork. Fix the cause soon, and call a plumber the same week if you cannot.

Can a leaking overflow cause damp?

It can. GOV.UK guidance on damp and mould says damp can be caused by leaking water from pipes, overflowing baths or sinks, burst pipes or defective water storage vessels. It says repairing the source of the water, such as fixing leaking pipes, can help remove the cause of damp.

What if the pipe is a copper pipe near the boiler?

That may be the boiler's pressure relief pipe, not a tank overflow. Water from it can mean the heating system pressure is too high or a part needs attention. Do not open or adjust a gas boiler. Ask a Gas Safe registered heating engineer to look.

Do I need building regulations approval to replace a cistern valve?

No. GOV.UK says most repairs, replacements and maintenance work, and like-for-like replacements of baths, toilets, basins or sinks, do not need to be told to a building control body.

Does QuoteZila check plumbers?

No. We don't check insurance, qualifications or registrations, so please check these yourself before you hire. You can report a quote or a business from your job page.

This page is general information, not professional or legal advice. QuoteZila does not check Gas Safe, WaterSafe, NFRC, NICEIC or other registrations against the register on your behalf: look them up yourself.

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Sources

  1. QZ1: QuoteZila product fact. QuoteZila pricing line.
  2. QZ2: QuoteZila product fact. QuoteZila FAQ How does getting quotes work.
  3. S1: Understanding and addressing the health risks of damp and mould in the home, GOV.UK (MHCLG, DHSC and UK Health Security Agency). Accessed 2026-10-31. https://www.gov.uk/government/publications/damp-and-mould-understanding-and-addressing-the-health-risks-for-rented-housing-providers/understanding-and-addressing-the-health-risks-of-damp-and-mould-in-the-home--2
  4. S2: Building work, replacements and repairs to your home, GOV.UK (Ministry of Housing, Communities and Local Government). Accessed 2026-10-31. https://www.gov.uk/government/publications/building-work-replacements-and-repairs-to-your-home/building-work-replacements-and-repairs-to-your-home
  5. S3: Before you get building work done, Citizens Advice. Accessed 2026-10-31. https://www.citizensadvice.org.uk/consumer/getting-home-improvements-done/before-you-get-building-work-done/