Toilet Cistern Leaking? Find Where It's Coming From

A leaking toilet cistern is almost always a seal that has hardened, shifted or been disturbed, not a toilet that needs replacing. Water on the floor usually comes from one of four places: the supply connection where the pipe meets the fill valve, the large rubber seal between the cistern and the pan on a close-coupled toilet, the washers on the bolts that hold the cistern down, or a hairline crack. If water is running into the bowl rather than onto the floor, the flush valve seal inside the cistern is letting by. Condensation can look like a leak as well, especially in winter. Finding the exact source takes a few minutes and tells you whether it's a quick fix or a job for a plumber.

What to do first

If water is reaching the floor, stop it before you start looking.

1. Turn off the isolating valve on the pipe that feeds the cistern. It's usually a small valve with a screwdriver slot, just below or beside the cistern. Turn it a quarter turn so the slot is across the pipe. Our guide to the toilet isolation valve shows what it looks like on different toilets.
2. Flush to empty the cistern, then hold the handle down or press the button until it stops.
3. Mop up and dry the floor round the base of the toilet. A slow leak that sits under vinyl or lino can rot the floorboards and mark the ceiling below, which matters in flats and conversions.

If there is no isolating valve, or it won't turn, use the main stopcock. Water that is coming from the pan itself rather than the cistern is a different problem, covered in our guide to a toilet leaking at the base.

Finding where the water is coming from

Dry every joint under and behind the cistern with a towel. Tuck a piece of dry toilet tissue under each one: the supply pipe connection, both cistern bolts and the joint where the cistern sits on the pan. Turn the water back on, let the cistern fill, wait a few minutes, then flush. The first piece of tissue that gets wet tells you where the leak is.

What you see	When	Most likely cause
Drip from the pipe or valve under the cistern	All the time, flush or not	Supply connection, fibre washer or isolating valve
Water from the joint between cistern and pan	Mainly when you flush	Cistern-to-pan seal (the close-coupling seal)
Drip from one or both bolt heads under the cistern	Constant, or worse after flushing	Hardened or split bolt washers, or a loose bolt
Trickle down the back of the bowl	With nobody using the toilet	Flush valve or siphon seal letting water into the pan
Wet line on the ceramic itself	Constant while full	Hairline crack in the cistern
Fine droplets all over the outside	Cold weather, after a shower	Condensation

Supply connection and isolating valve

Water enters the cistern through the fill valve, which has a threaded tail coming out of the bottom or side of the cistern. The supply pipe connects to that tail, usually with a flexible connector or a tap connector that seals on a fibre or rubber washer. Those washers harden and shrink with age, and a connector that has been knocked or overtightened can start to weep.

The isolating valve itself can also leak, either from its compression joints or from round the spindle. A small weep from a compression nut can sometimes be stopped with a gentle nip, holding the valve body still with a second spanner. If it carries on, the joint needs remaking. Our guide to a leaking compression fitting explains why these joints fail and how they're remade properly.

Replacing a perished washer or a tired flexible connector is a small, cheap job, but it does need the water off and a new washer of the right size. Braided flexible connectors do wear out, and one that's bulging, rusty or kinked should be replaced before it fails.

Cistern-to-pan seal and bolts

On a close-coupled toilet, where the cistern sits directly on the back of the pan, the flush water passes through a large rubber seal between the two. The cistern is held down by two bolts that pass through the base of the cistern and the pan, each with rubber washers. These are the most common source of a cistern that leaks only when flushed.

• A loose bolt. Bolts can work loose over time, especially if the cistern gets leaned on. Tighten gently and evenly, a little at a time on each side. Ceramic cracks if you overtighten, and that turns a small repair into a replacement cistern.
• Hardened washers or a shrunken pan seal. If a gentle tighten doesn't stop it, the rubber has lost its shape. The fix is to isolate the water, empty the cistern, disconnect the supply, lift the cistern off and fit a new seal and bolt kit. Kits are made for most common toilets, but some need the manufacturer's own part.

A close-coupled cistern is heavy and awkward to handle once it's off the pan, and the flush mechanism often has to come out to reach the seal. It's a straightforward job for a plumber and usually takes around an hour, but it's easy to crack a cistern or strip a plastic nut if you haven't done one before.

Concealed cisterns behind a wall panel or inside furniture work the same way but are harder to reach. If water is appearing at the bottom of boxing or a vanity unit, it's worth getting the panel off before it soaks the chipboard.

Cistern leaking into the bowl

If you see a thin, constant trickle down the back of the pan, or hear the cistern topping itself up every so often with nobody using it, water is passing the seal at the bottom of the cistern. On modern toilets that's the seal on the flush valve. On older lever toilets with a siphon, it's the flap valve washer inside the siphon.

This doesn't damage the floor, but it wastes water all day and adds to a metered bill. A few drops of food colouring in the cistern that show up in the pan without flushing confirm it. Cleaning scale and grit off the seal sometimes cures it, and replacing the seal or the whole flush valve is the lasting fix. Our guide to a toilet running constantly covers how to tell this apart from a fill valve that won't shut off, and our guide to toilet noises after flushing explains the hissing that often goes with it.

Every WC cistern also has an overflow. On most modern toilets it runs internally into the pan, so a fill valve that is set too high sends water into the bowl rather than out of a pipe on the outside wall. If the water level sits right at the top of the overflow tube, lower the float before blaming the flush valve.

Cracks and condensation

A cracked cistern. A hairline crack shows up as a wet line on the ceramic that stays wet while the cistern is full. Cracks usually follow a knock, an overtightened bolt or frost in an unheated room. Sealant might slow it down, but it won't hold reliably under the weight of water, and a crack can open without warning. The fix is a new cistern, or a new toilet if the model is no longer made. Our guide to how long a toilet lasts covers that choice.

Condensation. In cold weather the mains water in the cistern keeps the ceramic cold. In a warm, steamy bathroom, moisture condenses on the outside, runs down and drips onto the floor. If the whole outside of the cistern is covered in fine droplets and the joints underneath stay dry during the tissue test, it's condensation. Opening a window, running the extractor fan for longer after a shower and keeping the room evenly heated usually stop it.

When to call a plumber

Call a plumber if the leak is from the cistern-to-pan seal or the bolts and a gentle tighten hasn't stopped it, if the cistern is concealed, if the isolating valve won't turn off, if the ceramic is cracked, or if water has been getting under the floor or through to the ceiling below. A leak that appears only when you flush is still worth sorting quickly, because every flush puts more water under the toilet.

We charge £95 an hour for labour, plus a £60 call-out that's waived when the job runs over 2 hours. Parts are extra and we confirm the price before any work starts. Most cistern leaks are fixed in one visit. Our toilet repair service covers close-coupled, concealed and older high-level toilets across London, and the pricing page explains how jobs are charged. For other flush and fill faults, see our overview of toilet cistern problems.

Frequently Asked Questions