Where this starts
Drain air is held back by water, not by anything mechanical. Each sink, bath, shower, washing machine, floor gully and toilet has a trap holding a few centimetres of water against the pipe beyond it, and as long as that water is there, nothing comes back. The smell is always a trap that no longer has its water, or a pipe that is open somewhere it should not be.
EVAPORATION IS THE COMMONEST CAUSE and it points at the least-used fitting. A trap left alone loses its seal over weeks — a guest shower, a utility sink, a floor gully in a garage, a basin in a room nobody goes into. So the first move is not to investigate but to run every outlet in the building for half a minute each, including the ones that are obviously never used, and see whether the smell goes.
IF IT COMES BACK WITHIN DAYS, the seal is being removed rather than evaporating, and there are two ways that happens. SIPHONAGE: a discharge elsewhere in the system drags the seal out behind it, which is a venting problem and usually announces itself with a gurgle at another fitting. Or the trap is damaged, disconnected, or was never fitted — most often behind a washing machine or a dishwasher where a standpipe or an appliance hose has been pushed straight into a waste.
WHERE the smell is matters. Confined to one room points at that room's fittings. Present through the house, worst in still weather or when the heating is off, suggests a soil vent terminating badly or a drain fault outside. Worst when it rains points outside almost always.
One thing worth stating plainly: a persistent drain smell with no trap explanation is a reason to look at the drain itself, because a cracked or displaced pipe under a floor smells exactly like a dry trap and does a great deal more damage while you are working through the fittings.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
A trap dried out through disuse
Most commonA smell in one area, often near a rarely used shower, utility sink, floor gully or basin, and clearing after water is run and not returning for weeks.
How to check: Run every outlet in the building, including ones nobody uses. If the smell goes and stays gone, evaporation was the whole story.
An appliance waste with no trap, or a trap bypassed
Most commonSmell around a washing machine or dishwasher, often with the hose pushed directly into a waste pipe or a standpipe that is too short.
How to check: Follow the appliance hose to where it enters the waste. A hose into an untrapped pipe is an open connection to the drain.
Siphonage pulling seals out
CommonSmell returning within days of running the water, usually with gurgling at another fitting when something discharges, and often in a house where waste runs have been altered.
How to check: Whether anything gurgles when a bath or a toilet empties. A gurgle is air being pulled through a seal.
A failed or missing seal at a WC pan connector
CommonSmell localised to a bathroom, sometimes strongest at floor level behind the WC, occasionally with staining on the floor around the base.
How to check: Whether the smell is strongest low down behind the pan. A perished connector is an open joint into the soil pipe.
A soil vent pipe terminating badly or blocked
Less commonSmell through the house rather than in one room, worst in still or damp weather, sometimes with slow drainage everywhere at once.
How to check: Where the vent pipe ends outside, and whether it is near an opening window. A blocked or badly placed vent puts drain air back into the building.
A defective drain outside or under the floor
Less commonPersistent smell with no trap explanation, sometimes worse after rain, occasionally with a soft or damp patch of ground or floor nearby.
How to check: Whether the smell survives every trap being filled and every fitting checked. What is left is the drain itself.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
It refills every trap at once, costs nothing, and settles the commonest cause outright — including the fittings nobody thinks to mention because nobody uses them.
- 2
Weeks means evaporation and the fitting is simply unused. Days means something is removing the seal, which is a venting or a trap fault and needs finding.
- 3
A washing machine or dishwasher hose pushed into an untrapped pipe is an open connection to the drain, and it is behind an appliance where nobody looks.
- 4
A gurgle is air being drawn through a water seal, which identifies siphonage and tells you the problem is the venting rather than the trap.
- 5
One room is a fitting; the whole house points at the vent; worse after rain points outside, and those are three different jobs.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Where the smell persists after every trap has been filled and every appliance connection checked, since what remains is the pipework or the drain.
- Where drainage is slow at several fittings at once as well as smelling, which indicates a blockage or a fall problem rather than a seal.
- Where there is any sign of sewage outside — a wet patch, a surcharging chamber, a smell over a drain run — because that is a drain failure and it does not improve on its own.
- Where the smell is in a basement or a room below drain level, where a backflow device and its condition become part of the question.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.