A building sewer runs 62 ft from the building drain to the connection at the main and is to be graded at 1/4 in per foot. The invert elevation at the upstream end is 100.00 ft. What is the invert elevation at the downstream end?

Explanation

Total fall is 62 ft x 1/4 in per ft = 15.5 in, and 15.5 in divided by 12 is 1.29 ft, so the downstream invert is 100.00 - 1.29 = 98.71 ft. The value 98.45 uses 3/8 in per ft or rounds the fall to 18.6 in. The value 99.35 halves the fall, as if the grade were 1/8 in per ft, and 101.29 applies the fall in the wrong direction, which would run the sewer uphill toward the main.