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  3. Leaking toilet: working out where the water is coming from

Leaking toilet: working out where the water is coming from

By Declan Barry, plumber, Stockport. Updated 2026-09-10.

A toilet leaks from one of four places: the supply pipe or fill valve, the cistern-to-pan joint, the pan connector to the soil pipe, or a crack in the pan or cistern. Wipe everything dry, flush, and watch where the water first appears. A leak at the pan connector is foul water and needs fixing quickly; the others are less urgent.

What to do

  1. Dry everything and wait

    Wipe the whole toilet dry, put a piece of kitchen roll around the base, and leave it ten minutes without flushing. If the base gets wet, it is the supply or the cistern leaking constantly.

  2. Now flush

    Watch the joint between cistern and pan, and the back of the pan where it meets the soil pipe. Water appearing during or just after the flush points to the cistern joint or the pan connector.

  3. Check for a running toilet

    If the cistern never stops filling, or water trickles into the pan, the fill valve or the flush valve seal has failed. That is not a leak onto the floor but it wastes water and it means the cistern may overflow.

  4. Look for a crack

    Hairline cracks in the cistern or pan show as a wet line that reappears after drying. A cracked pan or cistern needs replacing; it cannot be sealed.

When to ring a plumber

  • Water at the back of the pan after a flush. That is the pan connector, and it is foul water.
  • A cracked pan or cistern. It needs replacing, usually the same day.
  • A constantly running toilet. A cheap valve fix that saves a lot of water.

Questions

Is water around the base always a leak?

Not always. In a cold bathroom it can be condensation running off the cistern. If the kitchen roll test shows nothing and it only appears on cold mornings, that is what it is.

Can a leaking toilet damage the floor?

Yes, particularly a timber floor. A slow leak at the base rots the boards and eventually the joists under them. It is worth fixing early.

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