Tracing a Sewer Odour Indoors

In a nutshell

Most indoor sewer odours come from a trap that has dried out, a toilet seal that has let go, a cracked pipe, or a vent that is blocked. Start with the quick checks and work toward the harder ones.

Sewer gas is mostly methane and hydrogen sulphide, the rotten-egg smell. A properly built drain system keeps it out with a water seal in every trap and a vent pipe that carries the gas up through the roof. When the smell appears indoors, something has broken that chain. In older Toronto houses the usual suspects are a dry floor drain, a basement shower nobody uses, or a cast iron stack with a crack near the floor. Newer homes more often have a loose toilet seal or a failed air admittance valve under an island sink.

Working through it

1. Refill floor drains

Pour a jug of water in each basement floor drain.

2. Run unused fixtures

Guest bathrooms, laundry tubs and basement showers dry out.

3. Check toilets

A toilet that rocks may have a broken wax ring.

4. Look at the roof vents

Nests, leaves or snow can cap them.

5. Check cleanout plugs

A missing or loose cap is an open door.

Clues worth noticing

• Odour near a floor drain.
• Odour in a bathroom nobody uses.
• Smell during or after heavy rain.
• Smell around the base of a toilet.

Signals that it needs a licensed plumber

If the smell persists, a smoke test or camera can find a cracked pipe. Start with drain cleaning or call.