01 Sept 2026
Your bathroom smells of drains, and it is worse first thing in the morning, or after a few days away, or every time somebody showers. A drain smell is not one fault. It is four or five different faults with the same symptom, and people bleach a floor trap for months because nobody told them how to tell the causes apart.
It all comes back to the water seal
Every waste fitting has a trap: a bend holding a permanent plug of water. That water is the only thing between your bathroom and the inside of the sewer. It has no moving parts, and it is what fails. So the first question is always the same. Is the plug still there, or has something taken it away?
Cause one: the trap has dried out
Water evaporates. A bathroom nobody uses, a spare toilet, a floor trap in a dry corner, a laundry standpipe used once a fortnight, all lose their seal to plain evaporation, faster than people expect.
The test is free. Pour a bucket of water slowly into the floor trap and down every basin and shower waste, then leave it a few hours. If the smell goes and stays gone, that was it, and topping up unused fittings every week or two keeps it away. If it is back within a day, something is removing the seal, which is cause three.
Cause two: the trap is clean but the pipe is not
This one catches everybody, because the water seal is intact and the smell is still there.
Hair, soap, skin, shampoo and conditioner build a soft coating inside the pipe above the water line. That layer is organic, it rots, and it sits in warm air just below the grating. No amount of water rinses it off, because the water runs straight past.
The fix is physical. The coating has to be scrubbed off the pipe wall, and lifting the grating to wipe what you can reach is not enough, because the worst is further down. It is also why bleach disappoints: it deodorises for a day and leaves the coating where it was.
Cause three: the seal is being pulled out
If a trap keeps losing its water while the room is in normal use, something is siphoning it.
The giveaway is sound. A gurgle from the floor trap or the basin when a different fitting drains, or when a washing machine empties, means air is being dragged through a water seal, and every gurgle takes water with it. The second giveaway is timing: a smell arriving shortly after a large discharge elsewhere, coming and going rather than sitting there all day.
This is a system fault rather than a cleaning fault, and nothing you do at the grating will touch it. It comes down to how the waste run and its venting are arranged.
Cause four: the fitting no longer holds water
Some floor traps are cracked, some are worn, and some were swapped for a shallow pattern that never held a proper seal. A basin waste can also work loose where it meets the trap, letting air escape while the water below is fine. The clue is a smell strongest at one fitting that does not change however much water you pour in.
A washing machine hose pushed straight into a floor trap is a common version. The hose fills the opening, air comes back up around it, and the room smells whenever the machine runs.
Cause five: it is not the drain at all
Two smells get confused constantly. Sewer smell is sharp and eggy, and comes and goes with drainage. Musty smell is flat and mouldy, sits there all day, and belongs to a bathroom that never dries out. That is a ventilation problem, not a drainage one.
Two more worth checking. The overflow slot near the top of a basin collects the same rotting film and is almost never cleaned. And a failed seal where a toilet pan meets its waste smells only after a flush.
Working through it in order
- Pour water into every waste in the room, then wait a day.
- If the smell went, keep unused fittings topped up and stop there.
- If it came back, listen for gurgling when another fitting drains.
- If nothing gurgles, clean the pipe below the grating physically.
- If it gurgles, or more than one room is affected, the waste run needs looking at.
Our drain choke clearing work covers clearing a trap and the pipe below it properly rather than just at the grating, and there are short answers on smells and floor traps on our FAQ page. If the same trap keeps blocking as well as smelling, our post on why a floor trap keeps choking goes into that side of it.
When to call a plumber
Call when the smell survives a bucket of water and a proper clean, when one fitting gurgles as another drains, when more than one room is affected, or when it started after building work nearby. A dried-out trap in a spare bathroom is yours to fix in two minutes, and it is worth ruling out first.