What causes a sewer smell in the bathroom?

A sewer smell in the bathroom is mostly caused by sewer air getting in when the water seal of a drain has dried out or was never there. The toilet seal, the washing machine connection and the building's ventilation line are other common causes.

Why does the drain in an unused bathroom smell?

The water in the trap of an unused drain evaporates over time and sewer air comes straight out into the bathroom. Pouring a jug of water into the drain regularly prevents this smell.

Does pouring bleach on a bathroom drain smell work?

Bleach hides the smell for a short time, but it does not close the route the smell comes in by. If it mixes with another cleaning product it can also give off harmful gas, so the source needs to be found first.

Can a smell in the bathroom be a sign of a blockage?

If the smell comes with water draining slowly or a rumbling noise, yes, the smell can be an early sign of a blockage. Standing water and sediment make the smell stronger, and in this case unclogging the drain also removes the smell.

How to get rid of a drain smell in the bathroom

The grate of a bathroom floor drain lifted off while the odour trap chamber inside is checked. — Clearing a bathroom toilet drain with a drain snake

Published: 17 October 2026

In short

If a rarely used drain smells, pouring a jug of water into it often stops the smell straight away.
Floor drains with no water seal, or with the odour trap removed, let sewer air straight into the bathroom.
A smell coming from the base of the toilet means the connection seal in the floor is leaking, and it is a job for a plumber.
If your drain rumbles when a neighbour runs water, the building's ventilation line is inadequate.
If several flats smell at the same time, the problem is in the shared stack and should be handled together with building management.

Under every bathroom drain there is a small amount of water sitting in the bend of the pipe. It works like a plug that stops air from the sewer rising into the home. Most smells come from this water disappearing, and some come from leaks at the pipe joints.

First, find where the smell is coming from

Close the bathroom door, wait a few hours, then go in and notice where the smell is strongest. Bending down and sniffing the washbasin drain, the shower or floor drain, the base of the toilet and the back of the washing machine in turn will often show the source in one go.

The timing of the smell is also a clue. A smell that is heavy when you first walk in during the morning and fades during the day usually comes from a drying trap, while a smell that appears only when water runs upstairs comes from the ventilation.

If the smell is concentrated at the mouth of a single drain: the water seal of that drain, or the biological film inside it.
If it is concentrated behind or at the base of the toilet: the toilet connection.
If it is concentrated behind the washing machine: the drain hose connection.
If it is concentrated around the ceiling or the pipe shaft: the building's shared line.

In a rarely used drain, the water seal may have dried out

In a guest bathroom, a second shower that is not used in summer, or a floor drain that never gets wet, the water evaporates within weeks. Once the water in the bend is gone, nothing stands between the sewer and the bathroom. This is the most common case and the easiest to fix.

The fix is to pour water into that drain regularly. A jug of warm water rebuilds the seal and the smell usually clears within a short time. If you will be away from home for a long period, covering the mouths of the drains with a damp cloth or a lid makes the water dry out later.

The same thing is common in floor drains next to the washing machine or behind the toilet. Because these drains get wet only when the floor is washed, they empty faster in winter, in bathrooms dried out by the radiators. Turning cleaning day into a habit that also reminds you to pour water into these drains stops the smell coming back.

What to do if the floor drain has no water seal at all

In some older bathrooms the floor drain opens straight into the pipe, with no chamber underneath to hold water. Sometimes the plastic odour trap cup inside the drain was taken out during cleaning and never put back. In both cases the drain acts like a window onto the sewer.

Lift the grate and look inside. If you cannot see a cup holding water, that is what is missing. The problem is solved by fitting an odour-trap insert of the right diameter, or a drain strainer with a membrane that closes even when the water has dried. If the body of the drain is cracked or there is a gap between it and the floor, it needs to be replaced.

The film inside the trap smells too

Even when the water seal is in place, a slippery film of soap, hair and skin residue builds up on the inner wall of the trap. This film is a home for bacteria and gives off a smell of rotten eggs or mould all by itself. In this case the smell is usually strongest close to the mouth of the drain.

Taking the washbasin trap off and washing it, or lifting out the shower chamber and scrubbing it, stops this smell. If it cannot be removed, flushing with warm water, washing-up liquid and a little baking soda thins the film, but it is not enough for a thick build-up.

If the smell comes from the base of the toilet

The toilet sits on the drain pipe in the floor with a gasket or a flexible connector. Over time this part hardens, and if the toilet rocks slightly it slips out of place and sewer air starts to leak out from under the toilet. Darkening in the joint around the toilet, damp at the base, or wetness on the floor when you flush are signs of this.

Lifting the toilet and renewing the gasket is a plumber's job. Trying it at home can crack the porcelain and make the connection worse. Until the plumber arrives, make sure the toilet does not move.

The washing machine connection and the building's ventilation

The washing machine's drain hose is sometimes just pushed into the wall pipe or left in the floor drain. With no water seal in between, sewer air comes out around the hose into the room even when the machine is not running. Connecting the hose with a sealed fitting to a washing machine trap that holds water stops this smell.

In apartment buildings the drain of every flat is connected to a shared stack, and this stack runs up to the roof to take in air. If the outlet on the roof is blocked, or the line has never been ventilated, suction builds up inside the pipe when water is discharged on the upper floors. This suction pulls the water out of your traps. A rumbling noise and a smell straight after it are the sign.

If your washbasin gurgles when a neighbour flushes the toilet, the ventilation is inadequate.
If the smell is strong around the cover of the pipe shaft in the bathroom, the stack pipe may be leaking.
If drains on the lower floors have water rising together with the smell, a blockage has started in the stack.

If the problem is in the building's stack

If the smell is noticed in several flats at the same time, or continues even after you have fixed everything inside your own flat, the shared line needs to be checked. Sediment built up in the stack, a cracked joint or a blocked vent are things a single flat cannot solve. The best step is to inform building management and ask for the stack to be inspected with a camera.

The camera footage shows on which floor the leak or narrowing is. After that the line is flushed with high-pressure water and, if needed, the faulty PVC drain pipe section is replaced.