The short answer
An infrequently used floor drain can smell when water evaporates from its trap and opens a path for sewer gas. Slowly adding clean water is a useful first check: if the odor fades and the drain accepts water without backing up, a dry trap is plausible. A smell that remains, returns unusually quickly, or appears with gurgling or backup needs a broader plumbing inspection.
Confirm the drain is the source
Remove loose debris from the grate without reaching into the pipe. Note whether the odor is strongest directly above the drain, then check nearby sinks, laundry standpipes, cleanouts, sump covers, and toilet bases. Odors move with air currents, so proximity alone does not prove the source.
Pour water slowly enough to watch the drain. Stop if the level rises instead of draining. Do not mix cleaners, bleach, acids, or biological products in the drain; unknown residues can react and the treatment does not diagnose a missing water seal.
| What happens | What it suggests | Next step |
|---|---|---|
| Water drains normally and odor subsides | Evaporation from an unused trap is likely | Refill after long periods of non-use and monitor |
| Odor returns within a short interval | Trap leakage, air movement, or a failed or missing primer may be involved | Have the trap and any primer inspected |
| Water drains slowly or backs up | A restriction or downstream drainage problem is possible | Stop adding water and arrange drain service |
| Odor persists with a full trap | Another fixture, cleanout, vent, sump cover, or drain-body seal may be the source | Use a plumber to locate the gas path |
Long-term options
Some floor drains are served by a trap primer that periodically adds water. Confirm whether one exists and follow its manufacturer instructions; do not assume every drain has one. Where evaporation is persistent, a code-compliant waterless trap-seal device may be an option, but it must fit the drain and local requirements.
A small amount of water can support the evaporation explanation. It cannot rule out a cracked trap, failed seal, blocked vent, or drain-line problem.
Stop and call a plumber
Stop if sewage backs up, the drain overflows, the odor causes symptoms, or the drain appears connected to an unknown or abandoned line. Keep children and pets away from wastewater. A plumber can inspect the trap, primer, cleanout, and venting without masking the symptom.