What governs the size of the common waste and vent pipes?

The size of the common waste and vent pipes is primarily governed by air movement above the flow line. Adequate venting is crucial for maintaining proper air pressure within the plumbing system, which ensures that waste can flow freely without being hindered by siphoning or backpressure. The vent pipes allow for the equalization of pressure and the release of sewer gases, which is essential for both the efficient movement of waste and the health and safety of the building's occupants.

While factors like building design, flow rate of fixtures, and the diameter of the sewer line all play roles in overall plumbing system design, they do not directly dictate the size of waste and vent pipes as effectively as the considerations related to air flow and pressure dynamics. Proper venting is integral to the functionality of the waste system, emphasizing the critical nature of air movement above the flow line in maintaining system efficacy.