Leaking Toilet? Here's Why It's Running and How to Fix It

A toilet that keeps running almost always has a flush valve or flapper that isn't sealing the bottom of the cistern properly, letting water constantly leak into the bowl below. The fill valve then detects the dropped water level and keeps topping the cistern back up, which is the trickling or refilling sound you hear every few minutes.

Less often, the fill valve itself is worn and lets water dribble into the overflow tube even when the cistern is full. Both faults waste water and push your bill up, but neither is usually urgent — this is a fix-it-this-week job, not a call-out-now job.

A cistern that keeps filling on its own, without anyone flushing, means water is escaping past the flush valve seal or the flapper and draining into the bowl faster than it should. As the water level in the cistern drops, the fill valve's float drops with it, triggering the fill valve to open and top the cistern back up — so the cycle repeats every few minutes, sometimes audibly.

This is different from a toilet leaking at the base, where water pools on the floor around the pan — that's a seal or supply pipe fault, not a cistern fault, and is covered in our toilet leaking at the base guide.

The fastest way to tell is to watch where the water is going: if the bowl water level rises or changes colour after a dye test even without flushing, the flush valve or flapper is letting water through; if water trickles into the overflow pipe at the back of the cistern, the fill valve is at fault.

• Fill valve — controls water coming into the cistern from the supply pipe. A worn washer or a float set too high causes it to keep dribbling water into the overflow.
• Flush valve — the larger valve mechanism at the base of the cistern that lifts to release water into the bowl on a flush. A warped or cracked seal here lets cistern water leak into the bowl constantly.
• Flapper — the rubber seal (found on some flush valve designs, particularly older or imported cisterns) that sits over the flush valve outlet. Flappers harden and crack with age, especially in hard-water areas of London, and stop forming a tight seal.

On most UK toilets it's the flush valve seal that fails first, simply because it sits in water permanently and is the part doing the most work on every single flush.

Fixing a running toilet is a 20-30 minute job for most people once you know which part has failed, and it doesn't require draining the whole system — just the cistern.

1. Find the isolation valve on the pipe behind or beside the toilet and turn it clockwise until the water supply stops.
2. Flush the toilet with the supply off so the cistern empties and you can see the fill valve, flush valve and flapper clearly.
3. Look for a warped, cracked or grit-covered seal where the flush valve or flapper meets the base of the cistern — this is the most common cause of a toilet that keeps running.
4. Inspect the fill valve for a worn washer or a float sitting too high, both of which cause water to keep trickling into the overflow.
5. Fit a like-for-like replacement flush valve seal, flapper or fill valve, matched to your cistern's make and model — mismatched parts are the most common reason a DIY fix doesn't hold.
6. Turn the water supply back on, let the cistern fill, then wait 20 minutes and check it has stopped and isn't quietly refilling itself.

If the leak keeps coming back after a like-for-like part swap, the cistern itself may be cracked or the overflow may be routed wrong — both need a plumber to inspect rather than another part swap.

Repairing a leaking cistern means isolating the water, draining the cistern, and replacing whichever of the fill valve, flush valve or flapper is worn — the lid usually just lifts off, so there's no need to disconnect the whole cistern from the pan for a straightforward seal or valve swap.

Where it gets more involved is older cisterns with a side-entry ballcock rather than a modern bottom-entry fill valve, or close-coupled cisterns where the overflow and flush mechanism are combined — these sometimes need the whole cistern lifted off the pan to access the base seal, which is where it's worth calling in a plumber rather than guessing.

Call a plumber if the running or leaking doesn't stop after replacing the obvious part, if you can see water pooling around the base of the toilet rather than just hearing it run, or if your cistern has an unusual or older fitting you can't source a matching part for. RenoPlumb carries common fill valves, flush valves and flappers on every van and covers Tower Hamlets, Hackney, Islington, Newham, Southwark and the rest of East and Central London.

For the full service, including cistern replacement and diagnosing leaks at the base of the pan, see our toilet repair service or call 07460 824073 for a free quote.

The cost depends on which part has failed and whether your cistern takes a standard or a less common fitting, so we always confirm the price with a free, no-obligation quote before any work starts. For a general sense of how plumbing jobs in London are priced, see our pricing page.

Need this fixed by someone local? See our page for a plumber in East London for borough-specific coverage and response times.