Sewer gas and natural gas smell different. If the smell is like rotten eggs near a gas appliance, treat it as a gas leak: leave the house and call the gas utility from outside.
How plumbing keeps the smell out
Every drain has a U-shaped trap that holds a small amount of water. That water is a seal that stops gas in the sewer line from coming back into the room. Vent pipes through the roof let the rest escape outdoors.
Likely causes
- A floor drain, guest shower or basement sink that has not been used and whose trap has dried out
- A worn wax ring under a toilet
- A cracked drain pipe or loose fitting inside a wall or crawl space
- A blocked roof vent that lets drains pull water out of their traps
- A cleanout cap that is loose or missing
What you can safely check
Run water for a minute in every rarely used drain, including floor drains, to refill the traps. Check that cleanout caps are in place. If the smell is strongest near one toilet, the seal under it is the likely source.
When professional help makes sense
If the smell persists after the traps are refilled, the source is a failed seal, a broken pipe or a venting problem. A smoke test or camera inspection can find a leak that cannot be seen.