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The size of the vertical conductors and leaders, building storm drains, building storm sewers, and any horizontal branches of such drains or sewers shall be in compliance with Sections 18-29-1106.2 through 18-29-1106.6.
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 |
The size of the building storm drain, building storm sewer and their horizontal branches shall be based on the maximum projected roof area in accordance with Table 18-29-1106.3. The minimum slope of horizontal branches shall be one-eighth unit vertical in 12 units horizontal (1-percent slope) unless otherwise approved.
In sizing roof drains and storm drainage piping, 25 percent of the area of any vertical wall that diverts rainwater to the roof shall be added to the projected roof area for inclusion in calculating the required size of vertical conductors, leaders and horizontal storm drainage piping.
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