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 lateralpublic 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. Where the total fixture load on the combined drain is less than or equal to 256 fixture units, the equivalent drainage area in horizontal projection shall be taken as 4,000 square feet (372 m 2). Where the total fixture load exceeds 256 fixture units, each additional fixture unit shall be considered the equivalent of 15.6 square feet (1.5 m 2) of drainage area. These values are based on a rainfall rate of 6" (127 mm) per hour.
Note: Bring in Table 1106.3 from the 2012 IPC for 6" rainfall rate only
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TABLE P-1109.4
SIZE OF HORIZONTAL STORM DRAINAGE PIPING
 
SIZE OF HORIZONTAL PIPE
HORIZONTALLY PROJECTED ROOF AREA (square feet)
1/8 unit vertical in 12 units horizontal (1% slope)
3
548
4
1,253
5
2,227
6
3,566
8
7,600
10
13,800
12
22,200
15
39,650
1/4 unit vertical in 12 units horizontal (2% slope)
3
773
4
1,766
5
3,146
6
5,033
8
10,866
10
19,450
12
31,350
15
56,000
1/2 unit vertical in 12 units horizontal (4% slope)
3
1,096
4
2,500
5
4,450
6
7,140
8
15,320
10
27,600
12
44,400
15
79,250
 
For SI: 1 inch = 25.4 mm, 1 square foot = 0.0929 m2
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