What It Is
Sewer gas is a mixture of gases produced by the decomposition of organic waste in sewer and drain pipes. It typically contains hydrogen sulfide - which smells like rotten eggs - along with methane, ammonia, and carbon dioxide. In normal operation, your home's plumbing keeps these gases sealed inside the drain system using two mechanisms: water-filled traps and vent pipes.
What It Does
Sewer gas exists continuously inside the drain system. The question is whether it stays inside the pipes or finds a path into the living space. A properly maintained plumbing system creates two barriers: the water seal in the trap beneath every fixture, and the vent stack that routes gases out through the roof rather than back into the house.
When either of those barriers fails - a trap runs dry, a vent pipe gets blocked, or a wax ring under a toilet cracks - sewer gas finds its way into the room. The rotten egg smell from hydrogen sulfide is detectable at very low concentrations, so even a small breach can produce a noticeable odor.
At typical household concentrations, sewer gas is unpleasant but not acutely dangerous. The greater concern is with methane, which is flammable. An unusual concentration in an enclosed basement, for example, could pose a fire or explosion risk - which is why persistent sewer gas odors should be addressed rather than ignored.
Where It Is Located
Sewer gas exists throughout the drain-waste-vent (DWV) pipe network inside and under the house. It is kept out of living spaces by traps under every sink, tub, shower, and floor drain, and by water in the toilet bowl. If sewer gas odor appears in a specific room, the source is usually a nearby fixture whose trap has run dry, or a wax ring or drain seal that has failed.
How It Works
The trap beneath every fixture holds a few inches of water that physically blocks gas from traveling up through the drain opening. As long as the trap has water in it, gas cannot pass. This water evaporates if a fixture goes unused for weeks - a common scenario in vacation homes, guest bathrooms, or floor drains in rooms that don't see regular use.
The vent stack provides a pressure-equalizing path for the gas to escape harmlessly through the roof. Without it, negative pressure in the drain pipes could siphon water out of traps - defeating the barrier.
Common Problems or Signs of Failure
- Rotten egg smell from a specific drain or room
- Smell coming from a bathroom that is rarely used
- Odor from a basement floor drain
- Smell near a toilet - may indicate a failed wax ring
- Persistent smell that doesn't correspond to a specific fixture
Can a Homeowner Check It?
- Run water into every drain in the house, including floor drains and guest bathroom sinks. If a trap was dry, the smell should diminish within a day or two as the water seal is restored.
- Check the wax ring under a toilet by looking for staining or soft flooring around the base. A failed wax ring often causes a toilet to rock slightly and allows gas to escape.
- Check the trap primer on floor drains if the home has one - these devices automatically add water to floor drain traps on a schedule. A failed primer leads to a dry trap.
- If you run water in all drains and the smell persists, the issue is likely a vent problem or a pipe breach rather than a dry trap.
When to Call a Plumber
If running water into all drains doesn't resolve the smell within a couple of days, call a plumber. The source may be a blocked vent stack, a cracked pipe under a slab, or a failed wax ring - all of which require diagnosis and repair. Persistent sewer gas odors are worth tracing and fixing promptly.
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