Toilet Leaking From the Back? Causes and Fixes

Updated July 2026 · Emergency Plumber Chelsea

First, limit the damage. Most toilets have a small isolation valve on the supply pipe, usually a slotted screw you turn a quarter turn with a flat screwdriver so the slot sits across the pipe. That stops water refilling the cistern. If there is no isolation valve, or it will not turn, use the main stopcock; our guide on how to turn off your stopcock shows where to find it. Flush once to empty the cistern, put down towels, and if you live above someone, check whether water has reached their ceiling. Our guide to a leak in the ceiling below a bathroom covers what to do next.

Next, find the source. Dry the back of the toilet, the pipes and the floor completely with kitchen roll. Then turn the water back on and leave it without flushing for ten minutes. If water appears, the leak is on the supply side: the pipe, the isolation valve or the fill valve connection. If everything stays dry, flush and watch. Water that appears only during or straight after a flush is coming from the cistern-to-pan seal, the flush pipe or the pan connector. Running a dry finger along each joint and checking for drips is more reliable than looking at where the puddle ends up, because water runs along pipes and drips from the lowest point.

A leaking supply pipe or isolation valve is the easiest to spot. Look for drips on the underside of the valve or at the compression nuts either side of it. A nut that is weeping can often be stopped with a gentle quarter turn using two spanners, one to hold the valve and one to turn the nut. Do not overtighten, and do not use one spanner on its own, as you can twist the pipe. If the valve body itself is leaking or the slotted screw drips when turned, the valve needs replacing. Flexible hoses with braided covers can also fail with age; a bulging or rusty hose should be replaced before it bursts.

A leaking fill valve connection shows up at the bottom of the cistern where the supply pipe enters. On most modern cisterns the fill valve is held by a plastic back nut with a washer under it. If the washer has hardened or the nut has worked loose, water seeps out around the thread whenever the cistern is full. Sometimes tightening the nut by hand is enough. If not, the washer or the whole valve needs replacing. If water is dripping from the overflow pipe outside instead, that is a different fault, covered in our guide to an overflowing cistern.

The cistern-to-pan seal is the usual cause on a close coupled toilet, where the cistern sits directly on the back of the pan. A large rubber washer, often called a doughnut washer, seals the flush opening, and two or three bolts hold the cistern down through rubber washers. When the doughnut washer flattens or the bolt washers perish, water escapes every time you flush and runs down the back of the pan. Tightening the bolts slightly can help, but ceramic cracks easily under uneven pressure. A proper repair means emptying the cistern, lifting it off and fitting new seals, which is a straightforward job for a plumber.

On older high-level or low-level toilets with a separate cistern, the flush pipe runs from the cistern down to the back of the pan, and it seals into the pan with a rubber cone or flush pipe connector. These cones split and harden over time, so water sprays or dribbles out at the back of the pan on each flush. They are common in Chelsea's Victorian and Edwardian houses and mansion flats where original or reproduction toilets are still in use. Replacing the cone is usually quick, though period pans sometimes need a specific size.

The pan connector is the joint between the toilet outlet and the soil pipe, and a leak here matters most because it carries waste water, not clean water. Signs include water pooling behind the pan after flushing, a smell, or staining on the floor and ceiling below. The connector may have been pushed off when the toilet was knocked or the floor moved, the seal may have perished, or the connection may never have been made properly after a bathroom refit. Do not keep using a toilet that leaks here. The toilet usually needs to come off to fit a new connector and reseat it, which our toilet repairs team does as a routine job.

Make sure it is really a leak. Condensation forms on the outside of a cistern and on cold supply pipes when warm, moist air meets cold water, especially after showers and in winter. It drips down the back of the pan and can look exactly like a leak. If the water appears on the outside of the cistern in beads rather than at a joint, read our guide to toilet cistern condensation. Also check the toilet is not rocking, because a loose pan can break its seals repeatedly.

Concealed cisterns behind a wall or boxing complicate things. In many refurbished Chelsea and Knightsbridge bathrooms the cistern is hidden, so water can escape inside the frame for weeks before it shows as damp on a wall, a loose tile or a stain downstairs. Our guide to concealed cistern problems explains how to get access through the flush plate. If you suspect water is getting into the floor or wall, call us rather than waiting, because hidden leaks in flats can quickly become a problem for neighbours as well as you.

When to call a plumber: any leak from the pan connector, a cracked cistern or pan, a cistern that needs lifting to replace seals, a concealed cistern you cannot reach, or a leak that is reaching a ceiling below. Emergency Plumber Chelsea covers Chelsea, Knightsbridge, Sloane Square, South Kensington and Fulham. We find the leaking joint, explain the fix before any work starts, and there is no call-out fee on jobs over 3 hours. Call 020 3002 6667 or see our bathroom plumbing page.

Quick tips

• Turn the isolation valve on the toilet supply a quarter turn so the slot sits across the pipe to stop the cistern refilling.
• Dry every joint, wait ten minutes without flushing, then flush and watch. That tells you supply side or flush side.
• Use two spanners on compression fittings: one to hold, one to turn, and never more than a quarter turn at a time.
• Beads of water all over the cistern are condensation, not a leak.
• A leak from the pan connector carries waste water. Stop using that toilet until it is fixed.

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