A trap holds a plug of water that blocks drain air, and a smell with no leak means that plug has gone. There are only three ways it can go, and they are quick to tell apart.
1. It evaporated
An unused bathroom — a holiday flat, a guest shower — loses its seal to evaporation in weeks. Run the tap for thirty seconds and the smell stops. That is also the test: if it comes back after a week away, this was it.
2. The seal is too shallow
Space-saving and slimline traps buy their compactness partly out of the water seal depth. The European reference is 50 mm; a 30 mm seal in a rarely used room dries out much sooner. Makers publish the figure, and it is the one to read.
3. It was siphoned out
A long or badly vented waste run can pull the seal out when a large volume drains — a bath emptying, or a washing machine discharging. This one is a plumbing question rather than a product question.
What does not fix it
Drain cleaner, bleach, and anything poured in. The smell is not a deposit; it is a missing barrier.
Questions people actually ask
How deep should a water seal be?
50 mm is the usual European reference figure, and manufacturers publish the depth of their traps. Anything shallower is a trade against space, and it is worth knowing you are making it.
Do the non-return valves that replace a trap work?
Membrane and flap devices exist and are sold for exactly this. They are a mechanical seal rather than a water one, they have a wear part, and the manufacturer states the conditions they are approved for — which is what to read rather than a review.
Last reviewed 5 September 2026