Bathroom Smells Like Sewer: Causes and How to Fix It

Cause 1: dry traps

A shower, bidet, bath or floor drain that is rarely used loses its water by evaporation — especially in summer or after a holiday. Once the trap is dry, there is nothing between you and the sewer.

Pour a jug of water into every drain in the bathroom, including the bidet and any floor drain. If the smell fades within an hour, you have found it. Do it every week or two for drains you rarely use.

Cause 2: failed toilet seal

The toilet sits on a seal that connects it to the waste pipe. If it dries out, cracks or shifts, sewer air escapes around the base. A smell strongest near the base of the toilet, a toilet that rocks slightly, or discoloured grout around it all point here.

The fix is to lift the toilet and fit a new seal. With a wall-hung toilet the connection is behind the wall, but it is reached from the front.

Cause 3: shower tray and basin connections

The outlet of a shower tray or bath has its own seal. If it fails, you get a smell and often a small leak — sometimes noticed first as a stain on the ceiling below. See leaking shower tray.

Basin waste pipes can also come loose inside the cabinet. Check that every joint is tight and that there is a trap under every basin.

Cause 4: siphoning and poor venting

Waste pipes need air to flow properly. In buildings, the shared column has a vent that normally goes up to the roof. If it is blocked or missing, water rushing down the column sucks the water out of your traps — you hear gurgling, and then you smell the sewer.

Gurgling when you flush, or when a neighbour drains a bath, is the tell-tale sign. The fix is at building level. See gurgling drains.

Cause 5: build-up and hidden cracks

A partly blocked pipe holds decaying waste and smells even though water still drains. A cracked waste pipe inside a wall or under the floor smells too, often with damp nearby.

When traps and seals are fine, a camera survey finds the cause without opening walls at random.

When it is the building's problem

Smells in several flats, in the stairwell or in the entrance, or smells that get worse after heavy rain, usually come from the shared column or building drain. That is for the building administrator — tell them in writing, with dates and times.

What not to do

Air fresheners and scented blocks only mask the smell; the cause stays and usually gets worse. Pouring bleach down drains does not fix a dry trap or a failed seal, and mixing products can release toxic fumes.

A step-by-step way to find the source

Day 1: fill every trap with water (sinks, shower, bath, bidet, floor drain). If the smell goes, it was a dry trap.

Day 2: if not, check the toilet base — any movement, stained grout or smell strongest there points to the seal.

Day 3: look under the basin and bath panel for loose joints or missing traps.

Then notice timing: after your own shower (shower waste), after neighbours use water (venting or column), after rain (building drainage).

Anti-siphon and air admittance valves

Where a building vent is poor, a plumber can sometimes fit an air admittance valve on your branch pipe. It lets air in when water flows, so your traps are not sucked dry. It is not a fix for a blocked building vent, but it can solve the problem inside the flat.

Smells from the washing machine area

A washing machine standpipe without a proper trap, or with the hose pushed too far in, lets sewer air into the room. A sealed connection with a trap solves it. Also clean the machine's filter and door seal — they smell too, but not of sewer.

When to get a camera survey

If traps, seals and joints are all fine and the smell persists, the cause is inside the pipes — a crack, a displaced joint, or build-up. A camera survey shows it without opening walls, and gives you something concrete for the landlord or building.

Frequently asked questions