What is defined as the lowest pipe in or under a home that carries sewage to the building sewer?

The lowest pipe in or under a home that carries sewage to the building sewer is referred to as a building drain. This is fundamental in plumbing systems because it plays a crucial role in transporting wastewater from the various parts of the home to the sewer system. The building drain collects the sewage from fixtures and directs it toward the building sewer, facilitating the proper functioning of the plumbing system as a whole.

In contrast, a vertical stack that runs up through the roof is primarily responsible for venting gases and allowing air to enter the plumbing system, which helps maintain proper pressure and drainage flow but does not function as the main transport line for sewage. The underground pipe that carries sewage from the building out to the public sewer connects the building drain to the public sewer system but does not specify that it is the lowest point or the initial transport conduit in the home. Similarly, the underground pipe carrying sewage from the building to an on-site sewage disposal system serves a similar purpose but is specific to scenarios without public sewer access. Thus, the building drain is specifically defined as the pipe that serves as the lowest point accumulating and directing waste materials from within the home, making it the correct answer.