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Where there is a continuous or semi-continuous discharge into the building storm drain or building storm sewer, such as from a pump, ejector, air conditioning plant or similar device, each gallon per minute (L/m) of such discharge shall be computed as being equivalent to 24 square feet (2.25 m
2
) of roof area. (See Table 18-29-1108.1)
The roof of a structure shall be designed for the storage of water where the storm drainage system is engineered for controlled flow. The controlled flow roof drain system shall be an engineered system in accordance with this section and the design, submittal, approval, inspection and testing requirements of Section 18-29-1105.4. In lieu of the conventional downspout requirements of Section 18-29-1106, controlled flow roof drainage may be used. The water of a 25 year frequency shall not be stored on the roof for more than 24 hours. The water depth on flat roofs shall not exceed 3 inches (75 mm) during the above storm and 3 inches (75 mm) average depth on sloped roofs not exceeding 6 inches (32 mm) rise or depth of slope.
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 | ||
The control devices shall be installed so that the rate of discharge of water per minute shall not exceed the values for continuous flow as indicated in Section 18-29-1110.1.1. A scupper drain or drains shall be placed in the parapet wall at an invert elevation 1/2 inch (12 mm) above the maximum designed head. The scupper shall have a cross sectional area equal to that as required for conventional roof drainage. Control of runoff from roofs shall be by pre-calibrated tamper proof weirs; no valves or mechanical devices shall be permitted.
(Amend Coun. J. 4-10-19, p. 100029, Art. XVI, § 13)
Subsoil drains shall be open-jointed, horizontally spill or perforated pipe conforming to one of the standards listed in Table 18-29-1102.5. Such drains shall not be less than 4 inches (100 mm) in diameter. Where the building is subject to backwater, the subsoil drain shall be protected by an accessibly located backwater valve. Subsoil drains shall discharge to a trapped sump or approved location above ground. The subsoil sump shall not be required to have either a gastight cover or a vent. Access for the purpose of cleaning and removing obstructions in subsoil drains shall be provided at every change of direction.
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