Design.
Municipal Code of Chicago § 18-29-1110.1.1
in effect18-29-1110.1.1 Design.
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.
Chart 18-29-1110.1.1 A and
Chart 18-29-1110.1.1 B
[image omitted; see the official code]
Table 18-29-1110.1.1
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
Table 18-29-1110.1.1C
Vertical
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
Table 18-29-1110.1.1D
Horizontal
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
Source: City of Chicago, Municipal Code of Chicago (American Legal Publishing, codelibrary.amlegal.com)URL: https://codelibrary.amlegal.com/codes/chicago/latest/chicago_il/0-0-0-2692819Fetched: 10/24/2026