SECTION P-1109 COMBINED SANITARY AND STORM PUBLIC SEWER
P-1109.1 General. Where the public sewer is a combined system for both sanitary and storm water, the storm sewer shall be connected independently to the lateral public sewer.
Exception: The repair or replacement of an existing combined building sewer as of the initial adoption date of this code.
P-1109.2 Size of the existing combined building drains and building sewers. The size of the existing combined building drains and building sewers when being replaced in kind shall not be less than 4" and not having a slope less than 1/8" per foot (1 percent).
P-1109.3 Size of existing combined building drains and building sewers when adding additional load. The size of a combination sanitary and storm drain or sewer shall be computed in accordance with the method in Table 1109.4. The fixture units shall be converted into an equivalent projected roof or paved area. Allowance in square feet of pitched roofs or paved areas for fixture units shall be as follows: 7 square feet for each of the first 1,500 Fixture Units; 5 square feet for each of the next 1,500 Fixture Units, 4 square feet for each of the next 2,000 Fixture Units, and 3 square feet for each Fixture Unit thereafter. These values are based on a rainfall rate of 6 inches (127 mm) per hour.  411
Note: Bring in Table 1106.3 from the 2012 IPC for 6" rainfall rate only
* * *
TABLE P-1109.4
MAXIMUM HORIZONTAL PROJECTED ROOF AREA IN SQUARE FEET FOR BUILDING STORM DRAINS AT VARIOUS SLOPES  412
Diameter (Inches)
Fall Per Foot
Vertical Leaders
1/8"
1/4"
1/2"
Diameter (Inches)
Fall Per Foot
Vertical Leaders
1/8"
1/4"
1/2"
3
930
1,300
1,750
4
1,585
2,100
3,300
3,650
5
2,875
3,800
5,300
6,000
6
4,300
6,000
9,000
10,800
8
9,200
13,000
18,000
23,000
10
16,500
25,000
35,000
40,000
12
26,600
40,000
60,000
65,000
15
47,500
75,000
100,000
115,000
16
57,250
92,500
131,000
18
67,000
110,000
162,000
20
85,500
135,000
196,000
24
155,000
225,000
30
295,000
416,000
 
* * *

 

Notes

411
   Amended, Bill No. 230653 (approved December 20, 2023), effective April 1, 2024.
412
   Amended, Bill No. 230653 (approved December 20, 2023), effective April 1, 2024.