Which floor levels are used to determine the building drain elevation?

• A. The lowest floor outside the building.
• B. The lowest floor levels on which fixtures may be installed.
• C. The highest floor levels on which fixtures may be installed.
• D. The grade level at the street.

Answer: (C)

Gravity drainage inside a building must have a continuous downward slope toward the building drain, so the drain’s elevation is set with respect to the highest point from which wastewater could originate. That highest floor level where fixtures may be installed determines how high the interior drain must be able to receive and carry waste by gravity. If the building drain were placed too high, fixtures on lower floors could still drain, but fixtures on the top floor would have insufficient slope to reach the drain without pumping, which is not allowed in gravity systems. By aligning the building drain elevation with the topmost potential fixture level, all lower fixtures can discharge by gravity along proper slopes. The exterior grade at the street or the lowest exterior floor don’t govern the interior drain elevation.