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

The extension of a soil or waste stack above the highest horizontal drain connected to it is referred to as a stack vent. The primary function of the stack vent is to allow for the equalization of pressure within the plumbing system, which is crucial for the proper drainage of wastewater. By extending above the highest horizontal drain, the stack vent facilitates the escape of sewer gases and prevents the formation of a vacuum that could impede the flow of wastewater.

Furthermore, the stack vent is an essential component for maintaining the integrity of the plumbing system and ensuring that it operates efficiently. It allows for the proper venting of gases while also preventing potential issues that could arise from trapped air or negative pressure within the stack.

This understanding is fundamental in plumbing design and facilitates compliance with codes that require effective venting to maintain sanitary conditions.