What is defined as the lowest piping of a gravity drainage system in a building?

In a gravity drainage system, the term that describes the lowest piping in a building is often referred to as the house drain. This component is crucial in the plumbing system as it serves to collect waste and wastewater from various fixtures throughout the building and convey it to the building sewer. The house drain typically connects to the building sewer at a point where the waste can be transported away from the structure effectively through gravity.

Understanding the functionality of the house drain is essential for plumbers and designers, as it influences the overall efficiency of the drainage system. It is designed to ensure that wastewater is properly directed without causing blockages or backflow issues.

Other components mentioned, such as the building sewer, waste stack, and drainage pipe, play different roles in the plumbing system. The building sewer refers to the segment of piping that carries waste away from the property to the municipality's system or sewer treatment, whereas a waste stack may refer to vertical pipes that manage waste flow but are not necessarily the lowest point. Drainage pipe is a more general term and does not specify placement within the system. Thus, the house drain’s specific definition and role make it the correct choice for the lowest portion in a gravity drainage system.