What is the extension of a soil or waste stack above the highest horizontal drain called?

The extension of a soil or waste stack above the highest horizontal drain is known as a stack vent. This component is crucial for maintaining proper airflow within the plumbing system, allowing sewer gases to escape and ensuring that the pressure within the system remains balanced. By extending above the highest horizontal drain, the stack vent prevents the potential for siphoning of water from traps, which could lead to sewer gases entering the living spaces of a building. This function emphasizes the stack vent's role in health and safety by ensuring proper ventilation in the drainage system.

Other terms, such as "vent pipe," typically refer to similar but specific pipes that may not necessarily be extensions of a soil or waste stack; rather, they could serve individual fixtures or parts of the system. A "drain stack" is generally used to describe the vertical conduit that carries waste from the fixtures to the sewer, while a "soil stack" typically refers to the vertical pipes that handle human waste. Thus, stack vent is the precise term used to describe the extension that provides necessary ventilation for the entire plumbing system.