Toilet Cistern Leaking Underneath

What's happening?

A cistern holds water at atmospheric pressure and meets the pipework in three places that can leak: the fixing bolts that pass through its base, the inlet connector where the supply pipe screws on, and the flush pipe or short connection to the pan on a low-level or close-coupled cistern. Each leaks differently. A bolt leak drips from under the cistern while it is full. An inlet leak drips only while it is under supply pressure. A flush pipe leak shows when you flush. This is not the toilet rocking or leaking at the base, which is the pan seal at floor level, and it is not the cistern running into the pan, which is on the toilet keeps running page.

What usually causes it?

• Perished rubber washers on the cistern fixing bolts
• Corroded or overtightened bolts that crack the ceramic or leave a gap
• A worn fibre or rubber washer at the inlet connector, or a loose back nut
• A split or loose flexible connector on the supply to the cistern
• A damaged cone or seal on the flush pipe or the cistern-to-pan connection
• A cracked cistern, often from an earlier overtightened bolt or a knock
• Condensation on a cold cistern that drips and looks like a leak

Is it dangerous?

Not dangerous, but it wets the floor and what is under it. A cistern that drips onto a laminate or timber floor rots it quietly over weeks. If the drip is near a socket or shaver point, keep clear and call an electrician as well as a plumber.

Can I fix it myself?

Dry everything, put a sheet of toilet paper under each joint, and run the cistern. Paper that wets under the bolts means the bolt washers; paper wet at the pipe nut means the inlet; flush to test the flush pipe. Turn the isolating valve off for any work. A weeping back nut often stops with a quarter turn and a new washer, never overtightened. Replacing bolt washers means emptying the cistern and holding the bolt heads inside while you undo the nuts below. A cracked cistern cannot be mended: replace it. Wipe the cistern dry first to be sure the drip is a leak, not condensation.

When should you call a plumber?

Call a plumber when the leak is at the bolts of a close-coupled cistern that has to be lifted off the pan, when the cistern or its connector is cracked, when the supply connector will not seal, when the floor is already damaged, or when the cistern is behind a unit.

What will the plumber actually do?

• Trace the leak joint by joint with the cistern full, the supply on, and during a flush
• Replace the bolt washers and seals, or the whole fixing set, and refit without overtightening
• Replace the inlet connector washer or the flexible connector
• Renew the flush pipe cone or the cistern-to-pan seal on a close-coupled toilet
• Replace a cracked cistern and dry out and check the floor beneath

Who fixes this

• Toilet repair and replacement
• Leak repair

What it usually costs

Leaking Pipe Repair Cost

£100–£500 for an emergency burst pipe

Questions people ask

Sources & References

1. WaterSafe: how can I tell if I have a leaky loo
2. The Water Supply (Water Fittings) Regulations 1999, Schedule 2 (watertight fittings, cisterns)
3. WaterSafe: how to stop a running toilet