Common causes
- Dry traps in rarely used floor drains, showers or basement sinks
- A failed toilet seal at the floor
- A blocked or damaged vent
- A cracked drain pipe behind a wall or under a floor
- A loose cleanout cap
Checks you can do
- Pour water into every drain that is rarely used, especially basement floor drains.
- Check the toilet: does it rock, or is the floor stained around it?
- Look at cleanouts in the basement to make sure caps are tight.
- Notice timing. Does the smell appear when other fixtures drain? That suggests venting.
Floor drain trick. A tablespoon of mineral oil poured on top of the water in a rarely used floor drain slows evaporation.
When to call
If refilling traps does not solve it, the source may be a hidden crack or a vent issue. A smoke test or leak investigation can locate it. Buying a home? A smell during a viewing is worth noting; see plumbing checks before buying.
Safety
Strong, persistent sewer odours should not be ignored. If you smell natural gas, which has a rotten egg odour added, leave and call the gas utility from outside.
Worth knowing
Answers our Mississauga crew gives most
Why does my basement smell only sometimes?
Floor drain traps dry out, especially in winter when heating dries the air.
Is sewer gas dangerous?
In high concentrations it can be; any persistent odour should be investigated.
Can a vent be blocked?
Yes, by debris, nests or ice. A plumber can check it.