Problem · Chicago area

Why does my basement smell like sewage?

A sewer smell usually means sewer gas is getting past a seal that should stop it. Most causes are simple; a few need a professional.

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Common causes

Sources of sewer odor
Source	Clue	What helps
Dry floor-drain or laundry-tub trap	Smell near a drain that is rarely used	Pour water into the drain
Loose or missing cleanout cap	Smell near a cleanout plug	Replace or tighten the cap
Ejector pit lid not sealed	Smell near a basement bathroom pit	Professional re-seal
Cracked cast-iron drain or stack	Rust, stains, moisture on old pipe	Repair or section replacement
Blocked or failed vent	Gurgling drains, smell after flushing	Vent inspection
Failed wax seal at a toilet	Smell around the base of a toilet	Reset the toilet

Older buildings

Chicago bungalows and two-flats often still have cast-iron drains and stacks. Hairline cracks and rusted joints can release odor before they leak. In overhead-sewer conversions, the sealed ejector pit lid is another common source.

Diagram An overhead sewer keeps the building's sewer connection above the basement floor so a backed-up street sewer cannot flow in through floor-level openings.

Ejector pump with check valve lifts basement wastewater up; it needs power, so outages matter.
Overhead sewer line runs near the basement ceiling.
Building sewer exits the wall at a higher elevation.
Former floor-level openings are sealed under the design.

Simplified schematic, not to scale. Actual layouts vary by building.

Safety

Safety first A strong sewer smell with headaches or dizziness, or any smell of gas (rotten egg), is a reason to ventilate and leave. Natural gas has an added odorant — if you smell gas, leave and call the gas utility or 911 from outside.

Frequently asked questions

Is sewer gas dangerous?

Sewer gas can contain hydrogen sulfide and other gases. Persistent odor should be found and fixed, and strong odor means ventilate and step away.

Why does it smell only sometimes?

Traps dry out slowly, and pressure changes in the sewer (wind, heavy flows) can push gas through weak seals.

Can a plumber find the source?

Yes — a smoke or peppermint test of the drain system is a common way to find leaks in drains and vents.

Sources

Metropolitan Water Reclamation District — Understanding Your Sewer Official / primary
Chicago Metropolitan Agency for Planning — Local Housing Profiles Official / primary

Information can change; official sources control.