Where must all building drains be installed in relation to the lowest floor levels, assuming sewer elevation permits?

• A. Below the lowest floor levels on which fixtures may be installed if the public sewer, POWTs or private interceptor main sewer elevation permits.
• B. Above the highest floor levels.
• C. At the grade level of the lowest floor.
• D. Adjacent to the building exterior wall.

Answer: (A)

The main idea here is gravity drainage and proper elevation relative to where fixtures can be located. The invert of the building drain—the bottom inside surface of the pipe—must be lower than the lowest floor that may contain a fixture. Why this matters: with the drain running below that level, every fixture on any floor above has a clear downward path to the sewer, so waste and water can flow by gravity toward the public or private sewer without needing pumps. This also helps ensure adequate slope and allows cleanouts and venting to function properly, while reducing the chance of backflow creating problems on upper floors, assuming the sewer can accept the flow.

If the building drain were not below the lowest possible fixture level, some fixtures could end up with insufficient fall or would require pumping, which complicates design and maintenance. The exact placement also depends on sewer elevation; if the sewer is high enough to accept the flow, gravity drainage can be maintained by keeping the building drain below that lowest fixture level. The other options would not guarantee that consistent gravity flow from all fixtures or proper fall to the sewer, and they don’t address the vertical relationship between the drain and the fixture levels.