Dealing with Sewer Gas Smell in Your Home

What Causes Sewer Gas Odors

Sewer gas is a mixture of gases including hydrogen sulfide (the rotten-egg smell), methane, ammonia, and carbon dioxide. In normal operation, your plumbing system prevents these gases from entering your home through water-filled traps in every drain and vent pipes that route gases up and out through the roof. When you smell sewer gas indoors, one of these protective mechanisms has failed. While occasional faint odors may not be dangerous, persistent sewer gas exposure can cause headaches, nausea, and in extreme cases with methane accumulation, explosion risk.

Dried-Out Traps

The most common cause of sewer gas smell is a dried-out drain trap. Every drain has a P-trap or S-trap — a curved section of pipe that holds water to create a seal against sewer gas. Drains that are not used regularly (guest bathrooms, basement floor drains, unused utility sinks) can dry out through evaporation, breaking the seal. The fix is simple: pour a gallon of water into each unused drain. For drains that are rarely used, add a tablespoon of mineral oil on top of the trap water to slow evaporation.

Checking Other Sources

If refilling traps does not eliminate the smell, check for other sources. A loose or missing toilet wax ring can allow sewer gas to escape around the toilet base — check by gently rocking the toilet (if it moves, the wax ring may have failed). Cracked or deteriorated vent pipes in the attic can leak gas into the living space. A missing or damaged cleanout plug allows gas to escape from the drain system. Check washing machine drain connections — the drain hose should loop up and connect with a proper air gap, not be sealed into the standpipe.

Vent System Issues

If sewer gas smell coincides with gurgling drains, the vent system may be blocked or insufficient. A blocked roof vent causes negative pressure in the drain system, pulling water out of traps and allowing gas to enter. Check the vent stack opening on your roof for obstructions — bird nests, leaves, or ice. If the vent is clear but problems persist, a vent pipe may be disconnected or damaged inside the wall or attic. A professional plumber can perform a smoke test — pumping non-toxic smoke into the drain system — to pinpoint where gas is escaping into the living space.