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The flow rate (gpm) shall be determined by the use of Chart 18-29-1110.1.1 A for flat roofs and Chart 18-29-1110.1.1 B for slope roofs. The size of the vertical pipe and horizontal pipe shall be determined by the use of Tables 18-29-1110.1.1 C for vertical pipe and Table 18-29-1110.1.1 D for horizontal pipe and in no case shall be less than 3 inches (75 mm). The capacity rating (gpm) of any controlled flow downspout, branch or storm drain shall be converted to an equivalent square foot area by multiplying the gallons per minute (gpm) by 24 (24 square feet of roof area equals 1 gpm) before connecting to any combined house drain or house sewer, and add this value to the total drained area of Section 18-29-1106 of this chapter. Downspouts that are offset at 90 degrees shall be full size to the storm drain or sewer or combined drain or sewer.
Diameter of Downspout | Max. Horizontal Projected Roof Area Square Feet |
Diameter of Downspout | Max. Horizontal Projected Roof Area Square Feet |
2" | 720 |
2 1/2" | 1,300 |
3" | 2,150 |
4" | 4,600 |
5" | 8,300 |
6" | 13,000 |
8" | 29,000 |
10" | 52,000 |
12" | 83,000 |
13" | 155,000 |
Diameter of Downspout | Max. Vertical Projected Roof Area Square Feet |
Diameter of Downspout | Max. Vertical Projected Roof Area Square Feet |
3" | 88 |
4" | 190 |
5" | 345 |
6" | 560 |
8" | 1,210 |
10" | 2,185 |
12" | 3,570 |
15" | 6,450 |
Maximum Controlled Flow in GPM | ||||
Pipe Diameter in Inches | Slope in Inches in gpm | |||
1/2 | 1/4 | 1/8 | 1/16 | |
Maximum Controlled Flow in GPM | ||||
Pipe Diameter in Inches | Slope in Inches in gpm | |||
1/2 | 1/4 | 1/8 | 1/16 | |
3" | 72 | 51 | N.P. | N.P. |
4" | 156 | 110 | 78 | N.P. |
5" | 282 | 200 | 140 | N.P. |
6" | 460 | 325 | 230 | N.P. |
8" | 990 | 700 | 495 | N.P. |
10" | 1,795 | 1,265 | 895 | 630 |
12" | 2,065 | 1,460 | 1,030 | |
15" | 2,650 | 1,870 | ||