How should plumbing vent pipes be sized according to the IRC?

The sizing of plumbing vent pipes according to the International Residential Code (IRC) is primarily determined by the total developed length of the vent. This approach considers not only the number of fixtures being served but also the complexity of the venting system, which can include the distance the vent pipes must travel and any changes in elevation or directional changes in the piping.

Sizing by total developed length helps ensure that the vent is adequate for maintaining the necessary pressure balance within the drainage system. An appropriate vent size prevents trap siphonage, ensures that drainage can occur freely, and allows sewer gases to vent effectively without causing negative pressure in the system.

While other factors, such as the number of fixtures, are important in determining overall design and function, they are usually integrated into calculations involving the length and configuration of the vent system. Thus, using total developed length as the basis for sizing ensures that all critical aspects of venting are adequately addressed.