Dry everything completely, put paper under each joint, and use the basin normally. The paper tells you in one use what an hour of looking will not.
The four candidates
The waste flange where it meets the ceramic. Its seal hardens and water tracks down the outside of the waste rather than through it.
The trap joints. Hand-tight nuts that have loosened, or a washer sitting crooked after a previous clean.
A supply hose, at the crimp or at the nut. This one is under pressure and gets worse quickly rather than slowly.
A slotted waste in a basin with no overflow. Water finds the slot on its way past the seal, and it leaks only when the basin is filled rather than when the tap runs.
Telling them apart
Only when the basin is filled and released — the waste or the trap. Whenever the tap runs — the tap tails or the waste flange. All the time, wet and increasing — a supply hose, and that one is urgent.
What to do first
Close both isolating valves. That eliminates the supply side immediately and tells you whether you have time to think.
Questions people actually ask
Can I just tighten the trap nuts?
It is the right first move and it often works. Hand-tight plus a small nudge is the intent — plastic trap nuts strip easily, and a cracked nut leaks worse than a loose one.
Why does it only leak sometimes?
Because it depends on how the basin is used. A leak that appears only when the basin is filled to the overflow points at the overflow channel and the waste, and it is the clue that separates that case from all the others.
Last reviewed 5 September 2026