A pool of water around the bottom of a toilet looks as though it has a single obvious cause. It rarely does. Water that collects at the base may have come from the joint between the pan and the soil pipe, from the cistern above, from the supply pipe, from condensation, or from a leak somewhere else that has crept under the flooring and surfaced at the lowest gap. Replacing the wrong seal wastes time and leaves the real leak running.
This guide covers the possible sources, how to test for each one without taking anything apart, and what a stain on the ceiling below can tell you.
Why is my toilet leaking at the base?
The usual sources are:
- The pan connector. The flexible or rigid fitting that joins the pan outlet to the soil pipe. It only carries water during a flush, so a leak here appears after flushing.
- The cistern-to-pan joint. On close-coupled toilets, a seal and two or three bolts join the cistern to the back of the pan. Water that escapes here runs down the back of the pan to the floor.
- The cistern itself. The fill valve backnut, the flush valve seal or a hairline crack in the ceramic.
- The supply connection. The isolation valve, the flexible hose or the compression fitting that feeds the cistern.
- Condensation. Cold water in the cistern and pan chills the ceramic, and moisture in a warm bathroom settles on it and drips.
- A leak elsewhere. A basin supply, a bath waste or a heating pipe under the floor, with the water appearing around the toilet because that is where it finds a way out.
If the cistern is running into the pan or out of an overflow, that is a different problem, covered in our guide to a toilet overflow that keeps running.
When does the water appear, and what does that tell you?
| When the water appears | Most likely source |
|---|---|
| Straight after a flush, at the back of the pan | Pan connector or pan outlet |
| While the cistern refills, or all the time | Supply connection, fill valve backnut or cistern-to-pan joint |
| Constantly, even with the toilet isolated | Not the toilet: another pipe under the floor or in the wall |
| Mostly after showers or in cold weather, with beads over the whole cistern | Condensation |
| After someone uses the bath or basin | A waste pipe from that fitting, running under the floor |
How do you do a dry test on a toilet?
A dry test shows where water forms first, rather than where it ends up.
- Dry everything. Mop the floor, then dry the pan, the cistern, the supply pipe, the isolation valve and the pan connector with a towel.
- Add markers. Wrap a strip of kitchen towel around the supply pipe and valve, tuck one under the cistern-to-pan joint at the back, and lay one on the floor at the pan connector. Put a sheet of dry paper on the floor around the base.
- Test at rest. Leave the toilet unused for a while and check which strip is damp.
- Test while refilling. Close the isolation valve, flush, then reopen the valve and watch the supply connection and cistern base as it fills.
- Test on a flush. Flush a few times and check the strip at the pan connector and the back of the pan.
- Isolate the toilet. If water still appears with the isolation valve closed and the toilet unused, the source is probably not the toilet.
How does a dye test help find a leak at the base?
Food colouring makes it clear which water is which.
- Dye in the cistern. Add a few drops to the cistern water and do not flush. Coloured water on the floor or down the back of the pan points to the cistern, its seals or the close-coupling joint.
- Dye in the pan. Add colouring to the water in the pan, then flush. Coloured water at the base afterwards points to the pan connector or the pan outlet.
- Clear water at the base when both tests are coloured suggests the supply, condensation or a source outside the toilet.
Use a colour that will not stain, and flush afterwards to clear it.
Is it condensation?
Condensation catches many people out. It forms on the whole outside of the cistern and sometimes the pan, not at one joint, and it is worst when the bathroom is warm and humid and the incoming water is cold. The drips gather at the lowest point and can leave a puddle that looks exactly like a leak.
Wipe the cistern dry and watch it. If fine beads return evenly over the surface, rather than a trickle from one place, condensation is the likely cause. Better ventilation, an extractor fan left running after showers, or an insulated cistern liner usually deals with it.
Could the seal between the pan and the floor be the problem?
Most toilets in UK homes connect to the soil pipe through a pan connector behind the pan, rather than through a seal underneath it, so the joint between the pan and the floor does not usually carry water. The bead of sealant around the base is there to finish the joint and stop the pan moving.
That bead can hide a leak, though. Water from the pan connector can run under the pan and sit trapped behind the sealant, so a stain or soft floor spreads out of sight. A pan that rocks, sealant that has cracked, or a smell at the base are worth mentioning to the plumber, because movement can also loosen the connector.
What if the water has travelled under the flooring?
Vinyl, tiles, laminate and floorboards all let water spread beneath them. Water from a basin pipe, a bath waste or a heating pipe can run along the subfloor and surface at the toilet, because the gap around the pan is often the easiest way out. In that case the toilet is simply where the leak shows.
Signs that point to a leak under the floor include damp or lifting flooring away from the toilet, a floor that stays wet with the toilet isolated, and moisture readings that rise as you move away from the pan. A moisture survey maps the floor to show where the wettest area really is, and our guide to finding a leak under a tiled floor explains how that is done without lifting tiles at random.
What does a stain on the ceiling below tell you?
If the bathroom is upstairs, the ceiling below is a useful clue.
- A stain directly below the pan that grows after flushing fits a pan connector leak.
- A stain that grows steadily whether or not the toilet is used fits a supply leak, either at the toilet or on a pipe running under the floor.
- A stain offset from the toilet suggests water travelling along a joist or pipe, and the source may be another fitting.
- A stain that grows after baths or showers points away from the toilet altogether.
Mark the edge of the stain in pencil with the date, so you can see whether it spreads. If the ceiling is sagging or water is near a light fitting, turn off the electrics for that area at the consumer unit, if it is dry and safe to reach, and keep clear. Our guide to a damp ceiling patch with no obvious leak covers other causes.
When should you call a leak detection specialist?
A pan connector or cistern seal that you can see leaking is a plumbing repair. A specialist is useful when:
- the dry and dye tests do not show a source at the toilet;
- the floor stays wet with the toilet isolated;
- a concealed cistern or boxed-in pipework may be leaking;
- a ceiling below is stained and you need to know which fitting is responsible;
- you need readings and a report for an insurer or landlord.
Our water leak detection uses moisture mapping, thermal imaging and pressure tests. A combined detection package is £250 to £500, and under-floor leak detection is £300 to £700. Call 020 3912 0200.
Frequently asked questions
Why is water leaking from the bottom of my toilet after I flush?
Water that appears straight after a flush most often comes from the pan connector, the joint between the pan outlet and the soil pipe. A dye test using coloured water in the pan helps confirm it.
Can condensation look like a toilet leak?
Yes. Cold water in the cistern can make moisture form over the whole outside of it, and the drips collect at the base. Even beads across the surface, rather than a trickle from one spot, point to condensation.
How do I know if my toilet leak is under the floor?
If the floor stays wet with the toilet's isolation valve closed and nobody using it, or the damp is worse away from the pan, the water is probably coming from another pipe under the floor.
Should I stop using a toilet that is leaking at the base?
If the leak appears when flushing, avoid using that toilet until it is checked, as waste water can soak into the floor. If the leak is on the supply, close the isolation valve.
Will resealing around the base stop the leak?
Usually not. The sealant around the base does not normally hold back water, and fresh sealant can trap a leak underneath and hide it. Find the source first.
