Sewer Smell in Bathroom? Finding a Sewage Smell in the House

Start with when and where it smells

Notice the pattern before you look for the fault. A smell that appears after the toilet is flushed points to the toilet connection or the soil stack. One that comes and goes with windy weather suggests a problem with the vent at the top of the stack. A smell in a spare room that fades once the shower has been run is almost always a dry trap, covered in our guide to smells coming from drains. A smell that hangs around all the time, worst near one wall or the floor, suggests a leaking joint or pipe.

The toilet seal and pan connector

Where the back of the toilet pan joins the soil pipe there is a rubber or plastic connector. If the pan has been knocked, the floor flexes, or the connector has perished with age, foul air can escape around it, and sometimes a little water too. Signs include a smell strongest at the base of the toilet, staining on the floor behind it, or a pan that rocks slightly when you sit on it. A new connector and a firmly fixed pan usually cure it.

Soil stack joints and boxing

In many houses the soil stack runs inside a boxed-in corner of the bathroom or through a cupboard. Joints on the stack, and the branches where the toilet, basin and bath connect, can work loose over time or be poorly made during a refit. Because the pipe is hidden, a slow leak of air or water can go unnoticed for months. If the smell is strongest near the boxing, it is worth taking off an access panel and checking the joints by eye and torch.

Vent pipes and air admittance valves

The top of the soil stack lets air into the system so that water flowing down does not suck the traps dry. On some houses the vent goes out through the roof; on others there is an air admittance valve, a plastic cap fitted to the stack in the loft or behind boxing. If a roof vent is blocked by a nest or leaves, or an air admittance valve sticks open, sewer air can escape into the house. A valve stuck open usually smells all the time; a blocked vent tends to cause gurgling traps and smells after flushing.

Other places to check

• Basin and bath overflows coated in slime
• A shower waste under the tray that has lost its seal
• A washing machine standpipe with no trap or a loose hose
• Unused waste pipes left open behind units after a refit
• Cracks in the toilet pan itself, often at the back

Smells in flats and shared buildings

In flats, the stack is usually shared with the homes above and below. A neighbour's blocked or leaking branch, or a vent problem at roof level, can bring a smell into a flat where nothing is wrong. If yours appeared after work in another flat, or affects several homes, mention it to your factor, because the shared parts of the stack are often common property.

When to call in help

If the smell persists after you have refilled traps, cleaned overflows and checked the toilet and boxing, the fault may be in a joint you cannot reach or in the pipework below the floor. A drainage engineer can test the stack, check the vent and, where needed, inspect the pipes with a camera to show exactly where the air is escaping. Sewer smells indoors should not simply be masked with air freshener: the cause needs finding.

Questions we get asked