Whether you would describe it as a toilet leaking from the base, water appearing at the base after a flush, or just a wet patch round the foot of the pan, the shortlist is the same three things: the pan connector into the soil pipe, the seal under a close-coupled cistern, or a pan that has worked loose on its fixings. Which one it is decides whether this is a twenty-minute job or a floor that has to come up, and the paper test below tells them apart in about ten minutes.
Do this first
- Work out which water it is before you do anything else. Dry the floor completely, lay white kitchen roll all the way round the base, and wait. Clear, odourless water is the supply or the pan connector. Discoloured water with a smell is the waste seal, and that one is a health matter rather than a plumbing inconvenience.
- Isolate the supply to the cistern. Most have a small isolation valve on the feed pipe: turn the slot a quarter turn with a flat screwdriver so it sits across the pipe. If there is no valve, close the internal stopcock.
- Flush once to empty the cistern, then mop out what is left with a sponge.
- Stop using it if the water is dirty. Every flush pushes more waste under the floor covering.
- Check whether the pan rocks. Press down on the rim and see if it moves. Movement is the cause of most base leaks, not a consequence of them.
- Look at the ceiling below if you are above another room or flat. A base leak on an upper floor is already going somewhere.
The three things it is, and how the paper tells you
The kitchen roll test does nearly all the diagnostic work here, because the three causes wet different parts of the base at different times.
| What the paper shows | Cause | Urgency |
|---|---|---|
| Clear water, wettest at the back near the feed pipe, present with nobody using it | Supply connection: the isolation valve, the flexible tail or the cistern inlet | Continuous loss. Isolate now. |
| Clear water appearing a few seconds after each flush, wettest behind the pan | Pan connector seal between the pan outlet and the soil pipe, on the clean side of the flush | Only when used. Can wait a day. |
| Grey or discoloured water with an odour, after flushing, right at the floor line | Failed waste seal or a cracked pan outlet | Stop using it. Contamination risk. |
| Water only in cold weather, running down the outside of the cistern | Condensation, not a leak | None |
The one that catches people out is a rocking pan. A WC that moves a few millimetres under load works the pan connector loose over months until the seal no longer makes contact through its full travel. Re-bedding the seal without fixing the movement buys a few weeks. Fixing the fixings first is what makes the repair last. If the water is running inside the pan rather than onto the floor, that is a different fault entirely: see a leaking toilet, which costs you on the meter rather than damaging the floor.
What the repair involves
A supply-side leak is a new isolation valve, a new flexible tail or a remade compression joint, all done with the water off and tested under pressure afterwards. A pan connector is a pull-back, clean-off and new connector, and where the pan sits at an awkward offset to the soil branch it may need a different pattern of connector rather than the one that was forced in last time. The pan is then bedded and fixed properly to the floor so it cannot rock again.
The floor is usually the real damage. Water at a WC base runs under vinyl or between tiles and sits on the substrate, and on a timber floor it rots the boards around the fixings quietly for months. We lift what has to come up, dry it, replace failed boarding or screed, re-tile or re-lay, and reseal, which is the making-good half of bathroom and shower leak repair. If it has already reached the ceiling below, the stain downstairs tells you how long it has been going.
When the soil pipe is the problem, not the toilet
A toilet leaking at the base from the soil pipe side behaves differently from a supply leak, and it is worth separating because the repair is a different trade's worth of work. The pan connector is a flexible or straight rubber sleeve joining the pan outlet to the soil pipe, and it fails in three ways: it perishes with age, it was stretched or kinked at installation because the pan did not quite line up, or the pan has shifted on its fixings and pulled the joint open. In all three the water only appears when the toilet is used, and it is usually wettest behind the pan rather than at the front.
The case that needs more than a new connector is a soil pipe that has dropped or cracked below floor level, which happens in older properties where the pipe runs in a timber floor and something has moved. That shows as staining on the ceiling below before it ever shows upstairs, and it is the one version of this fault where the floor genuinely has to come up. If the ceiling under your bathroom has a mark on it, say so when you call — it changes what we bring.
When it is not us
- Water running down the outside of a cold cistern in winter. Condensation. Ventilation and a warmer room, not a plumber.
- The pan is loose and the seal is fine. Two new fixings into sound floor and it stops. Many people do this themselves in ten minutes.
- The flexible tail is weeping at its nut. A quarter turn often stops it. If the braiding is rusting, replace the tail rather than tightening a failing part.
- Waste water backing up out of the base of the pan. That is a blocked soil branch or stack, which is a drainage job, and in a block of flats the stack is usually communal and the agent's to clear.
- A cracked pan. A sanitaryware replacement. Worth pricing as a new WC rather than a repair.
- You rent. Report it in writing straight away. Sanitation installations are the landlord's to keep in repair, and a contaminated floor is not something to negotiate over.