Loft Tank Overflowing? How to Stop the Overflow Pipe Dripping

If a small pipe high on your outside wall or under the eaves is dripping or running, your loft tank is overflowing. In most homes the cause is the float valve (also called a ball valve) that tops the cold water tank up: it isn’t closing fully, often because of a worn washer or limescale. You can usually stop it in a few minutes by shutting off the water feeding the tank. If you’re in Clare, Sudbury, Long Melford, Haverhill or a village nearby, here’s how to do it safely, and when to leave it to us.

Is it the loft tank, the heating tank or a toilet?

Older homes with a hot water cylinder often have two tanks in the loft, and each one has its own overflow pipe to the outside. The overflow (sometimes called the warning pipe) is there on purpose, so a fault shows up outside instead of coming through your bedroom ceiling.

• The large cold water storage tank feeds your hot water cylinder and sometimes the bathroom cold taps. Its overflow usually comes out high up, near the roof line. This is the most common culprit.
• The small feed and expansion tank tops up an older “open vented” heating system. If this one overflows, or the water from the pipe feels warm, it can be a sign of a heating fault, so turn the heating off at the programmer and call us.
• A toilet cistern also has an overflow, usually lower down on the wall outside the bathroom, or it may run into the toilet pan. If lifting the loft float arm makes no difference, check your toilets.

Before you go into the loft

Only look at the tank if there’s safe access, as Which? Trusted Traders advises. That means a proper loft ladder, a light, and boards to stand on. Step only on boards or joists, never on the plasterboard between them, and don’t lean right over an open tank. Leave any cables and light fittings alone, and if old insulation looks loose and fibrous or the tank lid looks like old cement sheeting, don’t disturb it. If any of that isn’t possible, skip straight to turning off your stopcock (step 5) and call us.

Step by step: how to stop a loft tank overflowing

1. Take the lid off gently and look at the water level. If it’s at or above the overflow outlet and you can hear or see water still coming in through the float valve, the valve isn’t shutting off.
2. Lift the float arm gently by hand. If the flow stops, the valve is at fault. Sometimes giving the float valve a gentle wiggle frees limescale and it starts shutting off again. Don’t bend, force or unscrew the arm: plastic arms snap, and it won’t fix a worn washer.
3. Check the float itself. If it’s cracked, half full of water or rubbing on the side of the tank, it can’t rise far enough to close the valve. Note it down for the plumber rather than trying to repair it.
4. Look for a service valve on the pipe feeding the tank. It’s a small valve near the float valve, with a screwdriver slot or a little lever. Turn it a quarter turn so the slot or lever sits across the pipe. That stops the tank filling while the rest of the house keeps its mains water.
5. No service valve, or it won’t turn? Turn off your inside stopcock instead (see how to find and turn off your stopcock). The tank still holds water, so you’ll have hot water for a while, but be sparing with it until it’s fixed. If the stopcock won’t move, don’t force it; our guide to a seized or leaking stopcock explains what to do.
6. Protect the ceiling and the path outside. If water is dripping onto the ceiling below the tank, put a towel or tray under the drip in the loft. Outside, a constant drip can make a path slippery and turns to ice in a frost, so put a bucket under it or warn the family.
7. Keep water away from electrics. If water has reached a light fitting, cables or the ceiling below a light, don’t touch it or use that light. Switch off at the consumer unit (fuse box) only if you can stand somewhere completely dry.
8. Put the lid back on and take a photo of the float valve and the tank label if there is one. It helps us bring the right valve first time.

Avoid these mistakes

• Never block, cap or cut back the overflow pipe to stop the drip. Under the Water Supply (Water Fittings) Regulations 1999, a cold water storage tank should have a warning pipe that shows a fault somewhere you can see it, and a lid that keeps out dirt and insects. Blocking it just moves the flood into your house.
• Don’t take the float valve apart with the water still on, or swap parts you’re not sure about. The valve and its fittings need to be the right type for the tank and the water pressure.
• Don’t touch the hot water cylinder, the immersion heater or any wiring in the loft. Electrical work needs a registered (Part P) electrician, and you should only reset a tripped RCD or breaker once. If it trips again, leave it off.
• Don’t take the boiler casing off or adjust anything inside it. If you have a sealed system, your boiler actions are limited to what the user manual allows, such as the reset button and topping up the pressure through the filling loop. Repeated lockouts or pressure loss need a Gas Safe registered engineer.

When to stop and call us

• The overflow keeps running even after you’ve lifted the float arm, or you can’t find a way to shut off the feed.
• The float valve, float or arm is broken, or the tank is cracked, bowed, leaking from a joint or badly rusted (older galvanised steel tanks corrode from inside). Our plumbing team can replace the valve or the whole tank.
• It’s the small heating tank that’s overflowing, or the water from the pipe is warm. That points to a heating fault that needs an engineer; our boiler repair team can look at the whole system.
• There’s a damp patch on the ceiling under the tank and you can’t tell where it’s coming from. Our leak detection service can find it.
• Water is coming through the ceiling or near electrics. Turn off your stopcock and call our emergency plumbing team. Our guide to a flood or burst pipe covers what to do while you wait.
• You’re not happy going into the loft. That’s a perfectly good reason to call.

The outside or boundary stop tap is usually owned by your water company (Anglian Water or Essex & Suffolk Water) and may need a special key, so don’t dig it up or force it. If you think there’s a leak on their side, check whether neighbours are affected and contact the water company. You can also use your water meter to check for a hidden leak.

Smell gas or suspect carbon monoxide? Leave the property, open doors and windows if it’s safe, don’t use light switches or naked flames, turn the gas off at the meter only if it’s safe to do so, and call the National Gas Emergency Service on 0800 111 999. Get medical advice if anyone has CO symptoms such as headaches, dizziness or nausea. Only a Gas Safe registered engineer may work on gas appliances; you can check an engineer on the Gas Safe Register.

Sources and further reading

• Which? Trusted Traders: how to stop a water leak (tank overflows)
• Anglian Water: leakage leaflet (check your loft tank’s float valve isn’t running constantly)
• The Water Supply (Water Fittings) Regulations 1999

This is general safety guidance. If you’re unsure, stop and call a qualified engineer.