All storm water drains (house leads) shall be a minimum of four inches inner diameter. If it is determined to be in the interest of public safety to connect roof drains or downspouts directly to the storm sewer system in a given case, the diameter of the storm water drain shall be determined on both its slope and the projected roof area in accordance with the following table which is that prescribed in Section 102 of the State Plumbing Code, as amended, for storm systems only.
Projected Maximum Drained
Roof Area (Square Feet) for
Slope (Indicated)
SLOPE
Diameter of 1/8 inch fall 1/4 inch fall 1/2 inch fall to
Pipe to 1 foot or to 1 foot or 1 foot or 3.84
(in.) 0.96 percent 1.92 percent percent
3 865 1,230 1,825
4 1,860 2,610 4,170
5 3,325 4,715 7,465
6 5,315 7,515 11,875
8 11,115 15,745 24,890
10 19,530 27,575 43,625
12 31,200 44,115 69,720
14 42,600 60,000 95,000
The calculations in this table are based on a rate of rainfall of four inches per hour.