Does Plumbing Require a Vent?

Plumbers attach vent and drainpipes to the exterior of old buildings.

The technical term for a residential drainpipe system is drain, waste and vent system, or DWV. Vent pipes are essential components of DWV systems. Vents allow air into the piping system. Outside air encourages drainage by gravity, and provides pressure that seals interior fixtures from noxious sewer gases. Plumbing codes require drain vents. An overview of vent characteristics helps you construct drain systems or work with plumbing contractors.

Function of Vent Pipes

Vent pipes work with traps to prevent sewer gases from entering a home. Traps are the P-shaped pipe fittings below plumbing fixtures, such as below kitchen and bathroom sinks. The curved part of a trap holds a small portion of water and the trap's water creates a seal between the fixture's opening and the sewer. Without vents to regulate it, the flow of liquid waste can empty the trap and allow dangerous sewer gases to enter a home's interior. Additionally, as traps prevent gases from entering a home, gases exit the home through vent pipes.

Frequency and Location of Vent Pipes

Every plumbing trap must connect to a vent. However, several traps may share the same vent, and this vent pipe is called a "main" vent. A main vent may have several branches that extend to plumbing fixtures. Building codes specify the size of main and branch vent pipes according to the size of the drain system and the length and location of the pipes. Plumbers conceal vent pipes in wall cavities or run vent pipes along an exterior wall.

Vent Pipe Materials

The most common vent pipe materials are plastic and cast-iron. Although acceptable materials vary according to local building codes, most plastic vent pipes are made of acrylonitrile-butadiene-styrene, or ABS. Available in conventional home improvement stores, ABS pipes connect via a chemical welding process called "gluing." Heavy and expensive, cast-iron drain pipes are exceptionally durable. Cast-iron pipes usually connect via mechanical fittings.

Sealing Roof Penetrations

Although plumbers occasionally mount vents to exterior walls, the vents in newer homes usually route through the cavities of interior walls and protrude through roof coverings. Penetrations through roof surfaces present significant risk of leakage and damage to underlying roof materials. Plumbers and roofers protect vent pipe penetrations with special metal flanges, called "roof jacks" or "vent flashing." Vent flashing consists of a roughly cylindrical tube that protrudes from a flat, square base. The cylinder slips over vent pipes and acts as a protective sheath. The flat base slides beneath upper courses of shingles and rests on top of lower courses of shingles. Roofers or plumbers seal gaps and openings with roof sealant or caulk.

References

• Community College of Allegheny County: Plumbing-Traps and Vents
• Texas Tech University: Plumbing Vents
• University of Nebraska: Sizing DWV