Drain smells from plugholes, showers and gullies

A smell of drains at a plughole, a shower or a yard gully is usually sewer air getting past a trap. The trap is the bend, or the gully, that is meant to stay full of water. This page is about which trap has failed, and about telling a private drain from a water company one. It does not recommend chemical drain cleaner.

The seal

Every basin, sink, bath, shower and washing-machine waste should discharge through a trap. A yard gully works the same way. After you use the fitting, a pool of water stays in the bend. Foul air from the drain should stop there.

If only one fitting smells, start there. If every waste smells, or a toilet gurgles when a sink empties, look further along the pipe, at the ventilation, or at a shared drain. A sink that will not empty is covered in blocked sink. A shower tray that fills is covered in blocked shower drain.

Dry, siphoned, or just dirty

Dried out. Water in a trap evaporates. A guest bathroom, a floor gully or an outside gully that nothing has run into for weeks can lose its seal. The waste is then an open pipe, and the smell arrives even though nothing is blocked. Pour a litre or two of clean water in, or a bucket into the gully, and wait. If the smell fades, use the fitting now and again, or pour water in after a holiday. A gully that only smells in hot weather is often this, especially if the grid was dry. Bleach does not restore a seal, and it should not be mixed with other cleaners already in the waste.

Siphoned. Sometimes the water is pulled out as another fitting drains. You may hear a gulp from a basin or shower as the washing machine empties, or as an upstairs toilet flushes, and the smell follows. A partial blockage can do that. So can a vent that cannot let air in. Many soil stacks are open above the roof. Some houses use an air admittance valve, often in the loft or a cupboard on the stack. If that vent is blocked, or the valve has stuck, several traps can be sucked dry. We do not give a method for opening a soil stack, working on a roof, or replacing an air admittance valve. If the smell returns as soon as another fitting is used, tell a plumber which rooms are affected.

Biofilm. A trap can be full of water and still smell. Soap, grease and hair form a film, and the smell comes from the plughole or from the overflow, which shares the waste on many basins and baths. Clean it as you would a slow waste: gloves on, hair and slime out, and the trap washed if it comes apart. Basin wastes and bottle traps explains overflows, which is where a bathroom smell often sits. Let cooking fat cool and bin it. If the trap is clean and full, and the smell is still strong, the cause is downstream: a blockage, a broken pipe or a dry gully.

How the soil stack, its roof vent and an air admittance valve keep traps full is explained in soil stacks and air admittance valves.

Outside gullies

A yard gully holds a seal just as a sink trap does. Leaves, silt and fat fill it, the water disappears, and the smell comes through the grid. Clearing the grid and pouring water back is the householder check. The fittings around a house, and when to stop, are in outside drains.

A gully in the public road can be a different owner. Thames Water's manhole covers page says roadside gullies in its area are owned by local authorities or Transport for London, and to contact the council about a blocked one. That is one company's area, not a rule for every street.

Drainage firm or water company?

A smell does not, by itself, prove who owns the pipe.

CCW's page who is responsible for my drains or sewers? describes England and Wales. A drain serves one property. The homeowner is responsible for repairs to drains that serve their property and lie under the property, garden or driveway. CCW lists a bad smell as a sign of a blocked pipe, with a slow toilet and waste coming back up. Inside the boundary it says to call a plumber or an independent drain-clearing company, and it links to the Drainsafe website for firms. If you are unsure, contact the sewerage provider and keep a record. Wastewater companies are responsible for the large sewers under the road or pavement, and for shared pipework that connects to a public sewer. CCW's page gives its contact numbers for England and Wales.

Thames Water's blockages page says the same kind of thing for its customers: a bad smell is one sign of a blockage, a blockage in the home is the owner's, and it advises a plumber. It may be the company's if it has public sewers there and the problem is outside the boundary or more than one property is affected. Its sewer pipe page says you are responsible for gullies and drains within the boundary when they serve only your property, and that the company owns the joint part where your drain joins a neighbour's. Other companies publish their own versions. The CCW page we read does not cover Scotland or Northern Ireland, so ask the provider where you live.

Call a plumber or a drainage firm when only your home is affected. Call the sewerage company when several homes are affected, sewage is in the street, or you have been told the pipe is shared. If sewage is coming indoors, stop using the fittings and keep people away. If the smell is gas rather than drains, follow when to call a Gas Safe engineer.

Why does one plughole smell when the others do not?

That trap has probably dried out, been siphoned, or collected soap and grease. Run water in, then clean the waste if the smell remains. If every room smells, look beyond that one trap.

Can a shower smell after I have cleaned the grid?

Yes. Film sits in the trap under the grid, and an overflow or a dry gully can smell as well. If the tray is also slow, treat it as a blockage. If a full, clean trap does not hold the smell off, ask a plumber to check the waste and the vent.

When should I call the water company about a smell?

When it looks shared or public: more than one property, sewage at a manhole in the street, or a pipe you have been told is the company's. For a single house, CCW and Thames Water both point first to a plumber or a drain firm. If you are unsure, contact the sewerage company and keep a note of what they say.