A 3 in horizontal branch must gain 4-1/2 in of fall between two fixed points. At the minimum slope for 3 in pipe, what is the longest run that produces exactly that fall?

Explanation

At 1/8 in per ft, a run of 36 ft yields 36 x 1/8 = 4-1/2 in of fall. The 18 ft answer uses 1/4 in per ft, which is the slope for 2-1/2 in and smaller pipe, and 54 ft and 24 ft do not produce 4-1/2 in at any code slope for 3 in pipe.