Drain Smelling of Sewage or Rotten Eggs

Dry trap

The U-bend trap under every sink, basin, and floor drain holds water to block sewage gases. If a drain isn't used for weeks, the water evaporates. Running the tap for 30 seconds refills it.

Blocked soil stack vent

The soil stack has a vent at roof level. If it's partially blocked, negative pressure in the system can siphon the water out of traps, allowing gas to rise.

Cracked or failed drain seal

A crack in the underground drain or a failed seal allows sewage gas to enter the building. Needs a CCTV drain survey to locate.

Practical checks and next steps

Run water into the affected drain for 30 seconds to refill the trap. If the smell persists after the trap is wet, there's a deeper issue — a blocked vent, cracked drain, or failed drain seal.

• Run water down the smelly drain for 30 seconds — if the smell goes, it was a dry trap.
• Check all drains in the property — is it just one or multiple?
• Pour a small amount of water down floor drain traps that aren't regularly used.
• If smell persists after running water, or if it's coming from multiple drains, call a plumber.