The basement is clean. The laundry is done. There is still an unpleasant smell near the floor drain.
Before buying a stronger cleaner, think about the water that is supposed to stay inside the plumbing trap.
Why a quiet drain can start smelling
A trap holds water to separate indoor air from the drainage system. If the drain is rarely used, that water can evaporate. Toronto Water's plumbing explanation shows the water barrier and advises adding water to drains before sewer smoke testing.
For an accessible floor drain that is not backing up, replenishing the trap with water is a reasonable first check. Toronto's smoke-testing guidance uses one to two litres per drain. Pour slowly and stop if it does not drain normally. There is no need to begin by mixing cleaning chemicals.
Notice what happens afterwards
| Observation | Useful next step |
|---|---|
| Odour clears and the drain remains normal | Keep the seldom-used trap from drying out and watch for recurrence. |
| Odour returns quickly | Have the trap, any primer, venting, and connections assessed. |
| Several drains gurgle or drain slowly | Ask a plumber to investigate the drainage system. |
| Sewage rises through the drain | Treat it as a backup, not a dry-trap problem. |
A returning smell is information. Photograph the drain and note whether it follows laundry, heavy rain, or long periods without use. Those observations are more helpful than repeatedly masking the odour.
Know when to stop investigating
If you suspect a fuel-gas leak or anyone feels unwell, leave the building and call emergency services or the gas utility from outside. Do not assume an unidentified odour is harmless sewer gas.
For an actual backup, avoid contact with sewage and follow the local utility's reporting guidance. Toronto Water's backed-up-drain page explains its response process for Toronto properties.
Before finishing the basement
Keep required drains and service points accessible. A floor covering, cabinet, or decorative cap should not become the solution to an unresolved smell.
Sometimes the fix really is water in a forgotten trap. The catch is knowing when that explanation has stopped fitting the evidence.