HOW-TO GUIDES
How to fix a leaking overflow pipe
Updated 06/26
By: Olivia Scammell
Reviewed: Kim Staples
Is water dripping from a small pipe on the side of your house? That's most likely your overflow pipe - and if it's leaking excessively, it may need a repair.
Fast Facts
Most overflow pipe leaks are due to problems with the float valve
You may be able to fix it yourself, but it's best to call in a plumber
This guide will help you identify where the leak is coming from, and what's causing it
This guide is here to help you identify your specific leak and find the solution to fix it. However, some issues that can cause an overflow pipe to leak require skilled, professional help.
Don’t want to tackle the problem on your own? Call in a trusted plumber who will be able to assist.
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What is an overflow pipe?
An overflow pipe is a safety pipe that removes excess water, routing it safely outside when water levels become too high.
If it wasn’t for overflow pipes, things like water tanks, boilers, and toilets would end up overfilling and therefore flooding rooms. Overflow pipes are there to remove any excess water and 'send' it to the drainage system.
An overflow pipe is fitted to the following water supply fixtures to avoid them overflowing:
Toilet cisterns, cold water tanks, central heating water tanks and combi boilers all have overflow pipes that are connected to the exterior of the house
A hot water cylinder has an overflow pipe that's also known as an expansion or vent pipe. This pipe runs from the cylinder into the cold water feed and expansion tank
Sinks, baths, and basins all have overflow pipes that lead the water into the drain rather than outdoors
So weirdly enough, a leaking overflow pipe is a sign of the pipe doing its job. But excessive or unusual leaking usually indicates a problem - and that's what we're here to solve.
Why is my overflow pipe leaking?
There are a number of reasons why water might be dripping from the overflow pipe outside your house. The kind of leak you're seeing can indicate what the problem is...
Constant dripping? It's most likely a faulty float valve.
Water running continuously? The water level is likely too high.
Hot water flowing out? There's an issue with your boiler or hot water.
Overflow only running occasionally? There may be an intermittent valve fault.
Diagnosing the issue will also depend on where the overflow pipe is located, and which fixture or system it's connected to. If the pipe is...
Behind your toilet, you're looking at a toilet cistern overflow.
Coming from the loft, it's likely a cold water tank overflow.
Near your boiler, it's most likely the boiler pressure relief pipe.
Connected to a hot water tank, the problem is probably the tank's expansion or vent pipe.
What is a float valve?
All that said, when you discover a dripping or running overflow pipe outside, the most common problem is the float valve, which is found in toilet cisterns, cold water tanks and central heating feed and expansion tanks.
It's that strange-looking plastic arm and ball that goes up and down with the level of the water.
When the water in the tank is used, the level of the water goes down, and so does the float valve. The movement from this then turns the cold water feed on so that the tank refills. As the water level rises, the ball also rises to switch the water feed off again when the tank is full.
However, if the float valve isn't working properly, the cold water feed might not switch off and could cause the water level to continue to rise above the overflow pipe. Voila! You have your leak!
Can I fix a faulty overflow pipe myself?
If you're handy and the issue is simple, then yes - you can fix an overflow pipe yourself with basic DIY skills.
In most cases, the problem is down to the float valve, which has a DIY fix.
However, it's best to call in a plumber if:
You can't identify the source of the leak
The leak is coming from a boiler or water tank
The leak keeps returning
Components need replacing
You're not confident with plumbing
In these cases, search Checkatrade to find a plumber working in your area and book them in to help you out.
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What if I'm renting?
If you're a renter, all plumbing repairs are the responsibility of your landlord.
You may be able to quickly fix a float valve if you're handy. But generally speaking, it's always best to contact your landlord so they can deal with the issue properly, and hire a trusted plumber if they need to.
How to fix a leaking overflow pipe
Whether it's a cracked pipe or you have boiler leaking water from your overflow pipe, keep reading to find out how to fix a leaking overflow pipe.
Step one: Find the leak
Firstly, you need to identify what tank or cistern is overflowing by working out what the pipe is attached to.
If it's at ground or first-floor level, then it's probably going to be your bathroom overflow pipe dripping. This could be coming from the bath, basin or toilet cistern.
However, if the leaking overflow pipe is coming from the loft, then it could be your cold water or central heating feed that is leaking.
If you’re still unsure and need help solving the puzzle, call in a plumber. They will be able to check the water level in each of these places and find the culprit.
Step two: Check the float valve
If you've found the culprit and water is running continuously into the tank, check that the float valve isn't being held down by an obstruction.
If it’s moving with no problems, the plastic ball could be punctured and filled with water. It will need to be replaced if so.
Still haven't found the problem, and the water is still running? The washer on the valve could be worn and also need replacing. It's time to speak to a trusted plumber for help.
Step three: Replace the faulty part
A problem with your float valve? Then you’ll need to replace it or replace the washer, as mentioned above.
But first, you’ll need to stop the flow of new water into the problematic tank and drain it so you can work.
How to stop the flow of new water in different tanks:
Toilet cistern: Use the isolation valve to turn the water feed off. The valve is located on the water pipe that leads into the cistern. You will need a flathead screwdriver to turn the screw, then flush the toilet to drain the water.
Water tank: Stop the water feed by turning the mains water off at the stopcock. Then drain it by turning on your bathroom taps.
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How to fix a leaking overflow pipe from the toilet
The toilet overflow pipe is pretty obvious as it's directly behind the toilet and runs outside. If it's leaking or dripping, it means the water supply to the toilet cistern isn't turning off. So, here's what you can do:
Check the float valve isn't obstructed
Adjust the position of the float, as it might be set too high to turn the water off
Check the water supply pressure isn't too high, causing the overflow pipe to leak. Lower it by turning the isolation valve on the water supply pipe
Replace the washer in the float valve
Replace the float valve itself if it's punctured
If this all sounds beyond the realm of your DIY skills, get a plumber to help you out.
Boiler leaking water from overflow pipe?
Many types of boiler, including combi boilers, have an overflow pipe that leads to the outside of the house and consists of a pressure relief valve. This valve will release water when the system is under too much pressure.
The boiler pressure relief valve only flows when there is a problem in the system. If you see this valve leaking, you should immediately take heed and call a plumber.
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Is a leaking overflow pipe dangerous?
The leaking pipe itself isn't usually dangerous - it's what it leaks onto. For example, leaking water that's left can cause damage to walls and foundations.
A leaking overflow pipe isn't necessarily an urgent issue. But as with all plumbing problems, it's still best to get it dealt with as soon as you're able to.
On the other hand, if the leak is coming from an overflow pipe from a gas boiler, this could be dangerous. You will need to call a Gas Safe boiler engineer immediately to carry out the repairs safely.
Call a tradesperson urgently if:
The leak is coming from a gas boiler
The leak suddenly gets worse
Hot water is coming out of your pipe
Water is pouring out of the pipe
You notice damp, staining, structural damage, or other signs of water damage
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FAQs
The pipe dripping water outside your house is an overflow pipe that's most likely connected to an overflowing toilet cistern, cold water or central heating feed and expansion tank. It could also be your boiler overflow pipe dripping due to a faulty combi boiler. If you have any concerns, call in a plumber as soon as you can.
This is likely to be coming from your boiler or water heater. A water heater only has a life span of 8-15 years and can be blighted by rust, sediment build-up and corrosion that can eat away the bottom of the tank and cause a leak into the overflow.
To stop the leak, shut off the water supply, turn off the heating, and stay well away from any hot water that may scald. Then call a reliable plumber immediately for help.
No, water should not be coming out of an overflow pipe. This usually indicates that the water level has risen past its safe limit.
It costs around £330 on average to fix a leaking pipe. For more information, take a look at our guide to pipe repair costs.
Water leaking from your boiler's overflow pipe (aka PRV discharge pipe) usually means the system is under too much pressure. Either the pressure is too high, or you're facing a faulty pressure relief valve.
Some of the most common problems faced with overflow pipes include:
High system pressure
Faulty valves
Waterlogged float valves
Debris buildup
Worn-out rubber washers
Limescale
An overflow pipe should route excess water directly into a wastewater drain, or to an exterior location where it won't cause damage. The best location for a particular fixture's overflow pipe will depend on whether it's a boiler, sink, toilet, or other appliance.
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