The system of large-diameter pipes that carries water and waste to a sewer line or septic tank is called the drain-waste-vent (DWV) system. As the name implies, it has three components.. Drain lines collect water from sinks, showers, and tubs; waste lines carry waste from toilets; and vent lines exhaust sewer gasses and provide the necessary air pressure to allow wastes to flow freely. All drain and waste lines slope slightly downward from the fixture toward the sewer or septic system. Water and wastes are carried by gravity.

The pipes are large in diameter—typically 1 1/4 inches to 4 inches—to minimize the possibility of blockages. The main soil stack for toilets is normally a 4-inch pipe; showers usually have 2-inch pipes.

Sinks, lavatories, bathtubs, and laundry tubs may be served by 1 1/4-inch to 2-inch pipes. Though older homes may have pipes made of lead, most drain piping is cast iron, plastic, or copper. Some vent pipes are galvanized iron.

To operate properly and safely, each drain must be served by a vent line that carries sewer gasses out through the roof. Several vents may be connected together and joined to the soil stack as long as there is no drain above the connection point. Or vents may pass through the roof on their own.

All waste lines should have cleanouts at easily accessible locations. A cleanout is simply a Y-shaped fitting in the line that is capped off. If a blockage occurs, this is the easiest place for a plumber to snake out the line.

To prevent sewer gasses and odors from entering the house, drains are protected by traps. A trap is a curved section of drain pipe that fills up with water, providing a seal.

Drains that penetrate a wall have a P trap, and those that go through the floor have an S trap. The water held by the trap is replaced each time the fixture is used.

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