A site plan shows the invert elevation of a building sewer as 102.50 ft at the upstream end and 101.90 ft at the downstream end, with 48 ft of run between the two points. What is the slope of the sewer in inches per foot?

Explanation

The fall is 102.50 - 101.90 = 0.60 ft, which is 0.60 x 12 = 7.2 in, and 7.2 in divided by the 48 ft run gives 0.15 in per ft. The value 0.0125 is the slope expressed in feet per foot, not inches per foot, so it is off by a factor of 12. One tenth of an inch per foot would produce only 4.8 in of fall over 48 ft, and 0.125 in per ft, the familiar 1/8 in per ft grade, would produce 6 in of fall. Both are plausible-looking but leave the pipe higher than the plan calls for at the connection.