Rule out condensation first
Before assuming there is a leak, check whether the water might simply be condensation. A cold cistern filled with cold mains water can sweat in a warm, humid bathroom, especially after a shower, and droplets running down the outside of the cistern and pan can pool at the base and look exactly like a leak. Dry the outside of the cistern and pan thoroughly, run the shower for ten minutes with the bathroom door shut, and see if moisture reappears on the outside surfaces rather than seeping from underneath. If so, it is a ventilation issue rather than a plumbing one.
Leaking pan connector or soil pipe joint
If water appears specifically when you flush, and it is coming from behind or underneath where the pan outlet meets the soil pipe, the most likely cause is a failed pan connector: the flexible rubber or plastic sleeve that joins the toilet outlet to the waste pipe in the wall or floor. These can perish, shift out of alignment if the toilet has ever rocked, or simply not have been fitted tightly enough in the first place. This is a common issue after a toilet has been refitted or a floor has been relaid without care taken over realigning the outlet.
Cracked pan or failed cistern-to-pan seal
A crack in the ceramic pan itself, sometimes hairline and hard to see, will let water escape continuously rather than only on flushing, and tends to get worse over time. Similarly, on close-coupled toilets the cistern sits on the pan with a rubber doughnut washer and fixing bolts sealing the join; if that washer perishes or the bolts loosen, water leaks from between the cistern and pan every time it is used. Run a dry paper towel around the join and around the pan itself to see exactly where dampness first appears.
Why a rocking pan makes everything worse
If the toilet rocks even slightly when you sit on it or press against it, this is not just uncomfortable, it actively causes and worsens leaks. Movement stresses the pan connector, can crack the ceramic at the fixing points, and disturbs any sealant around the base. A rocking pan is usually due to an uneven floor, missing or loose fixing bolts, or packing that has come away underneath. This should be fixed as soon as it is noticed, since a leak from a rocking toilet will typically get worse rather than better.
| When it happens | Likely cause | Action needed |
|---|---|---|
| Only after a shower, on the outside of the pan/cistern | Condensation | Improve ventilation, not a leak |
| Only on flushing, from behind/below the pan | Failed pan connector | Toilet needs to be pulled and connector replaced |
| Constant, small and steady | Cracked pan | Toilet likely needs replacing |
| Between cistern and pan on close-coupled toilet | Worn doughnut washer or loose bolts | Reseal or tighten fixings |
| Around the base sealant line, following floor joints | Failed sealant or grout, water tracking | Reseal base and check floor tiling |
Sealant, floor joints and tracked water
Sometimes water is not coming from the toilet at all but tracking across the floor from elsewhere, such as a shower tray or bath, and pooling against the toilet base because that is the lowest point or where a grout or sealant line has failed. If the toilet itself is dry when isolated and flushed with the water supply off, but dampness still appears at the base after someone showers, look at the sealant around the shower tray or bath and the grout lines nearby rather than at the toilet.
A silicone bead around the base of a toilet is common practice to stop water pooling underneath, but it can also hide a genuine leak by trapping water where you cannot see it evaporate. If you find damp sealant, it is worth cutting it away, drying the area fully, and re-checking before resealing.
When the toilet needs to come off the floor
Diagnosing exactly which of these is happening usually means removing the toilet, which is a bigger job than most homeowners want to take on themselves, particularly disconnecting the waste connection cleanly without damaging the floor or the soil pipe. If dye testing and drying tests point to the pan connector, cistern seal, or a cracked pan, this is a sensible point to call in a plumber. Left unaddressed, a leaking base can rot chipboard flooring, damage a ceiling below, and in the worst case lead to a much larger repair than the original fix would have cost.
Where to read next
Questions we get asked most
How can I tell if it is condensation or a real leak?
Dry the outside of the cistern and pan completely, then run the shower with the door shut for ten minutes. If moisture reappears on the outer surfaces of the toilet rather than from underneath it, it is condensation caused by poor ventilation rather than a plumbing leak.
Why does my toilet only leak when I flush it?
Leaking specifically at the moment of flushing points to the pan connector, the joint between the toilet outlet and the waste pipe, since that is when the largest volume of water passes through the system. A steady, constant leak is more likely to be a cracked pan.
Can I use silicone sealant to stop a leak at the base?
Sealant around the base is often used to stop water pooling underneath, but it will not fix an underlying leak and can actually hide one by trapping moisture out of sight. If there is an active leak, sealant is not the fix; the connector, seal or pan itself needs attention first.
Is a rocking toilet dangerous?
A rocking toilet will not usually be dangerous in the sense of collapsing, but the movement puts repeated stress on the pan connector and fixing points, which tends to cause or worsen leaks and can eventually crack the ceramic. It is worth fixing as soon as you notice it.
Will a leaking toilet damage my floor?
Yes, given time. Water pooling repeatedly around the base can soften chipboard subfloors, lift floor tiles, and in upstairs bathrooms eventually show up as damage on the ceiling below. It is worth tracing and fixing the source promptly rather than just mopping up.