Which section of a building's drainage system is located at the lowest point and receives discharge from soil, waste, and other interior drain pipes?

• A. Building Sewer
• B. Building Drain
• C. Soil Stack
• D. Drainage Manifold

Answer: (B)

The building drain is the section located at the lowest point inside the structure that collects discharge from soil, waste, and other interior drain pipes and carries it toward the exterior. Inside a building, waste from fixtures travels into vertical soil/waste stacks and then into the horizontal building drain, which serves as the final interior conduit before the wastewater leaves the building through the building sewer. The soil stack is the vertical line that carries waste downward from fixtures, not the lowest collecting point of all interior branches. A drainage manifold collects individual fixture drains but isn’t the ultimate low point that gathers all interior discharges. The building sewer runs outside the building to the public sewer, not within the interior.