Drain smells in a London flat: causes and fixes

A drain smell in a flat can be hard to pin down. It may come and go, be worse in the morning or only appear in warm weather. In blocks and converted buildings, the source can be in your own bathroom, in a shared soil stack, or in a neighbour’s flat entirely.

How a drain stops smells normally

Every sink, basin, bath, shower and toilet has a trap, a bend in the pipe that holds a small amount of water. That water forms a seal between your room and the sewer. Soil stacks also have vents to the open air, usually through the roof, so pressure in the pipes does not suck traps dry.

Most drain smells happen when one of these protections fails.

The common causes

Cause	Typical sign	What to try
Dry trap	Smell from a basin, shower or gully nobody uses	Run water for a minute to refill it
Gurgling trap	Smell after another fitting drains	Points to a venting problem in the stack
Failed toilet pan seal	Smell behind the toilet, sometimes a damp floor	Needs resealing or a new connector
Cracked waste or stack joint	Smell inside boxing or a cupboard	Needs the joint repairing
Slow blockage	Smell and slow draining	Clear the blockage
Blocked vent	Traps empty, smells after flushes	Needs the vent clearing, often a roof job
Dirty overflow or waste	Smell from the basin or bath overflow	Clean the overflow and waste

Empty flats and dry traps

A trap that is not used slowly loses its water to evaporation. In a flat that has been empty for a few weeks, between tenants or during a long holiday, every trap can dry out. When you come back, the whole flat smells of drains. Running each tap and flushing each toilet usually fixes it within minutes.

Floor gullies in wet rooms and utility cupboards are the worst for this, because nobody pours water into them. If you have one, tip a jug of water in now and then.

Blocks and warehouse conversions in Southwark

Southwark has one of the highest shares of purpose built flats in London. In the 2021 Census, 64.4% of households lived in purpose built flats or tenements (Nomis, TS044), from the estates in Peckham and Walworth to the converted warehouses along Shad Thames and in Bermondsey.

In these buildings, your bathroom joins a soil stack shared by every flat above and below. If the stack’s vent is blocked, or a joint on the stack has cracked inside your boxing, you may smell drains even though your own fittings are perfect. In converted warehouses, wastes often run a long way sideways before reaching the stack, and long flat runs can hold waste and smell.

If you suspect the stack, the Leasehold Advisory Service says the freeholder is usually responsible for communal pipes serving more than one flat. Report it to your managing agent or housing provider.

What you can check yourself

1. Run water into every fitting, including any you rarely use and any floor gully.
2. Clean the overflows on basins and baths. They collect soap and grime that smells.
3. Look behind the toilet for damp or a gap where the pan meets the pipe.
4. Sniff inside cupboards and boxing where pipes run. A strong smell in one spot points to a joint.
5. Notice when it happens. After a neighbour flushes, after rain, or all the time.

When it is more urgent

A drain smell becomes urgent when it comes with:

• water or sewage backing up into a shower, bath or toilet
• damp patches spreading from a pipe
• fittings draining slowly across the flat
• a smell that is getting stronger day by day

Those point to a blockage or a leak, not just a dry trap. Our blocked drains page explains what the visit involves.

If the smell is of rotten eggs or gas rather than drains, leave the plumbing. Go outside and call the National Gas Emergency Service on 0800 111 999.

Outside the building

Sometimes the smell is from the drains outside, not inside. If the inspection chambers in the yard or garden are full or slow, the drain may be partly blocked. Since 2011, shared drains and lateral drains in London have mostly been Thames Water’s responsibility. Our guide on blocked drains and who is responsible explains how to tell.

If you rent

Under section 11 of the Landlord and Tenant Act 1985, your landlord must keep sanitation installations in repair. A persistent drain smell from a faulty seal, pipe or stack is their problem to fix. Report it in writing and say when it happens. If you are a council tenant, use the council’s repairs line.