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Drain-Waste-Vent Plumbing Systems

Understand how your home’s  drain-waste-vent plumbing systems work with this expert illustrated explanation.

The system of pipes that carries water and waste to a sewer line or septic tank is call 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 gases and allow wastes to flow freely.

Drain-Waste-Vent (DWV) System Diagram

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 a 2-inch pipe. Sinks, lavatories, bathtubs, and laundry tubs may be served by 1 1/4- to 2-inch pipes. Though some old homes may have pipes made of lead, most drain piping is ABS plastic, cast iron, 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 gases out through the roof. Several vents may be connected together and joined to one larger soil stack as long as there is no drain above the connection point. Or vents may pass through the roof on their own. Wherever vent pipes penetrate the roof, special flashing protects against roof leaks. (For a closer view of vent flashing, see How Roof Flashing Works.)

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 in the drainpipe, a cleanout offers a convenient place for a plumber to snake out the line.

To prevent sewer gases and odors from entering the house, drains are protected by traps. A trap is a curved section of drainpipe that fills up with water, providing a seal. Drains that penetrate a wall have a P trap; those that go through the floor have an S trap. The water held by the trap is replaced each time the fixture is used. (For more about drain traps, see Kitchen Sink Drain Plumbing.)

About Don Vandervort
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Don Vandervort developed his expertise more than 30 years ago as Building Editor for both Sunset Books and Home Magazine. He has written more than 30 home improvement books and countless magazine articles. He appeared regularly on HGTV’s “The Fix,” and served as MSN’s home expert. Don founded HomeTips in 1996.

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  1. This info was really helpful. We just had a clog and it was at the cleanout spot. I was wondering how this plumbing worked and this info helped me tremendously how to understand how this house stuff works. It helps to know what your dealing with if this happens(and it will) again. Thanks a lot!

    • Zabo, I’m glad we could help. Please come back to HomeTips often!

  2. Why are we seeing poop in the drain lines that are in the floor of the toilet room? Anyone?

  3. I am changing a toilet to a water saver, elongated style. The old toilet was never centered in the space provided; now the new one is larger and fitting very close to the wall. I seem to remember hearing about an off-set sewer drain. I am sure this would require replacing subfloor and flooring, but would this eliminate costly changing of a whole sewer line?


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