What is the term for the lowest horizontal piping of a drainage system that extends from the main stack?

The term for the lowest horizontal piping of a drainage system that extends from the main stack is known as the building drain. This pipe serves a crucial role in a plumbing system as it collects wastewater from the drain lines of various fixtures such as sinks, toilets, and showers, and transports it to the sewer line or septic system.

The building drain is typically located below the level of the fixtures it serves, allowing gravity to assist in the flow of waste away from the building. This terminology is important in plumbing codes and standards, as it defines the specific function within the drainage system.

In contrast to the building drain, terms like "building intercept," "drainage outlet," and "sewer connection" refer to different parts or functions within a plumbing system. A building intercept generally refers to a point where drainage from the building connects to a public sewer or larger drainage system. A drainage outlet is more of a general term that can refer to any exit point of drainage from a system. Lastly, a sewer connection represents the point of linkage between the building's drainage system and the municipal or private sewer system. Understanding these distinctions is vital for proper plumbing design and compliance with relevant codes and regulations.