By Bill Ivey

Principal Consultant

Proper venting is essential for a sanitary drainage system, as it ensures the system functions effectively. The venting system maintains airflow and relieves air pressure caused by flowing liquids.

Purpose of System

Proper airflow and pressure relief prevent corrosive gases from building up in pipes and keep fixture traps sealed, stopping odors from entering occupied spaces. A well-designed venting system promotes flow, prevents gurgling toilets, and stops foul odors from escaping through compromised traps.

Plumbing fixtures like sinks, showers, tubs, and toilets require a trap—a U-shaped bend in the piping, or in toilets, built directly into the fixture. This trap holds water to create a seal that blocks gases and odors. Without proper venting, pressure differences between building air and the plumbing system can displace this water seal. As water flows through the system, it draws air along with it; without adequate venting, negative pressure develops, compromising the traps.

High-Rise Buildings and Venting

Proper venting in high-rise buildings is especially critical because of the diversity in usage demand, which causes constant pressure fluctuations in the drainage system. A vent stack is required for drainage in buildings that have five floors or more. Pressure is high at the base of the drainage stack where the vertical stack transitions to horizontal. To alleviate the pressure at the base of the drainage stack, the vent stack is required to connect to the drainage stack at or below the lowest fixture branch.

Pressure is also balanced by relief vents located at every 10th floor. Where drainage stack offsets occur, the upper and lower sections of the offset are required to be vented with additional relief vents. The top of the drainage stack must vent through the roof or connect to other vents that go through the roof. This is called a stack vent.

The vent stack must run uninterrupted and at full size from the base of the drainage stack up through the roof to ensure proper air circulation. If the vent stacks, relief vents and stack vents are not properly provided, flow problems resulting in blockages and backups can occur.

The Sudsing Issue

In high-rise buildings, a related issue is “sudsing,” where soap suds back up through fixture traps. This happens when washing machine drainpipes connect too close to the base of the drainage stack. For example, the Uniform Plumbing Code requires that stacks for suds-producing fixtures not connect within eight feet of the bottom of a non-sudsing stack.

In summary, a properly designed waste and vent system is especially important for high-rise construction.  The venting system will prevent drainage problems, accumulation of corrosive gases and liquids, sewer odors and associated health hazards.