How do traps and roof vents keep sewer gas out of a house?

Two parts work together. A trap is a curved section of pipe that stays full of water, and that water forms a seal against sewer gas. A vent is a pipe open to outside air that keeps the pressure in the drain pipes close to neutral, so the water seal stays in place. Vent pipes also give sewer gas a route up and out through the roof rather than into rooms. [1] [2]

According to a home inspectors' association article, sewer gas is more than a bad smell. It contains methane, which can explode in sufficient quantities, and it may carry pathogens that can cause illness. Keeping it out of living space is the main reason plumbing codes require traps and vents. [1]

Each fixture needs a trap, usually a U-shaped or P-shaped bend downstream of the drain opening. The bend holds water, and that water blocks gas from rising through the fixture. Fresh water refills the trap each time the fixture is used. A toilet has its trap built into the porcelain. King County, Washington, describes a trap as a device that provides, when properly vented, a liquid seal that stops air passing back while letting wastewater through, and its rules call for a seal of between 2 and 4 inches. [3] [4]

Flowing wastewater pushes air ahead of it and leaves a vacuum behind it. Fine Homebuilding explains that this moving slug of water can suck the water out of a nearby trap, or push air through and make nearby traps gurgle. The home inspectors' article adds that negative pressure can siphon the seal away, while positive pressure can force sewer gas past it, making a trap burp. A vent is simply a connection to outside air that neutralizes these pressure swings. A trap whose seal has been lost lets gas flow directly into the house. [2] [1] [5]

Vent lines connect to a main stack, a large vertical pipe running from above the roof down to the sewer line. Gas travels up the stack and out through the roof. The sources give some code details: King County requires vents to end at least 6 inches above the roof, and the inspectors' article says vents must stay clear of windows, doors and mechanical air intakes, and that a home needs at least one vent ending outdoors rather than in an attic or crawl space. Requirements vary by locality, and the article notes the six-inch rule may not be enforced everywhere. [4] [1]