No downspout shall be of smaller size than shown in Table 18-29-1106.2, based on the maximum horizontal projected roof area drained. Downspouts that are offset at 90 degrees shall be sized as a horizontal storm drain and shall be full size to the storm drain or sewer or combined drain or sewer. The above sizes of downspouts are based on the diameter of circular downspouts and other shapes shall have equivalent cross-sectional area.
Diameter of Downspout (inches) | Max. Horizontal Projected Roof Area (square feet) |
Diameter of Downspout (inches) | 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 |