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Roof drains shall have strainers extending not less than 4 inches (100 mm) above the surface of the roof immediately adjacent to the roof drain. Strainers shall have an available inlet area, above roof level, of not less than one and one-half times the area of the conduct or leader to which the drain is connected. All downspouts from gravel roofs shall be fitted with serviceable devices to screen out loose gravel.
Roof drain strainers for use on recreational decks, sun decks, parking decks and similar areas that are normally serviced and maintained shall comply with Section 18-29-1105.1 or shall be of the flat-surface type, installed level with the deck, with an available inlet area not less than two times the area of the conductor or leader to which the drain is connected.
The connection between roofs and roof drains which pass through the roof and into the interior of the building shall be made water tight by the use of approved flashing material. Refer to Section 18-29-305.7.
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.
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