Which valve type is expressly forbidden in plumbing for drainage lines?

Saddle valves are expressly forbidden in plumbing for drainage lines primarily due to their design and functionality. These valves are typically used for connecting small-diameter water lines to larger pipes, but they create a significant risk when used in drainage applications. The reason for this prohibition is that saddle valves can become easily clogged due to their small openings and design, leading to potential drainage problems. Additionally, they are prone to leaks and failures, which can contribute to water damage and other plumbing issues.

Furthermore, in drainage systems, the flow of waste and the need for reliable operation are critical. Using a valve that does not provide a full flow and can easily obstruct the passage of waste is undesirable. Other valve types, like gate valves, globe valves, and check valves, generally have designs that better accommodate the necessary flow characteristics and reliability required in drainage systems.