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

The correct term for the extension of a soil or waste stack above the highest horizontal drain connected to the stack is known as a stack vent. This component is critical in plumbing systems as it serves several functions. Primarily, a stack vent allows air to enter the plumbing system, which helps to maintain proper pressure and prevents siphoning of trap seals, ensuring that wastewater can flow freely and efficiently.

The stack vent also aids in venting sewer gases safely outside of the building, contributing to a healthier indoor environment. Proper venting is fundamental in preventing issues such as odors from entering living spaces or plumbing backs-up situations.

Understanding the function of the stack vent is essential for effective plumbing design and compliance with codes, as it ensures the entire drainage system operates correctly and safely.