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1103.0 Size of Leaders, Conductors, and Storm Drains

California Plumbing Code 2019 > 11 Storm Drainage > 1103.0 Size of Leaders, Conductors, and Storm Drains

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1103.1 Vertical Conductors and Leaders

Vertical conductors and leaders shall be sized by the maximum projected roof area and Table 1103.1.

TABLE 1103.1
SIZING ROOF DRAINS, LEADERS, AND VERTICAL RAINWATER PIPING^{2, 3}
SIZE OF DRAIN, LEADER, OR PIPE	FLOW	MAXIMUM ALLOWABLE HORIZONTAL PROJECTED ROOF AREAS AT VARIOUS RAINFALL RATES (square feet)
inches	gpm¹	1 (in/h)	2 (in/h)	3 (in/h)	4 (in/h)	5 (in/h)	6 (in/h)	7 (in/h)	8 (in/h)	9 (in/h)	10 (in/h)	11 (in/h)	12 (in/h)
2	30	2880	1440	960	720	575	480	410	360	320	290	260	240
3	92	8800	4400	2930	2200	1760	1470	1260	1100	980	880	800	730
4	192	18400	9200	6130	4600	3680	3070	2630	2300	2045	1840	1675	1530
5	360	34600	17300	11530	8650	6920	5765	4945	4325	3845	3460	3145	2880
6	563	54000	27000	17995	13500	10800	9000	7715	6750	6000	5400	4910	4500
8	1208	116000	58000	38660	29000	23200	19315	16570	14500	12890	11600	10545	9600

For SI units: 1 inch = 25 mm, 1 gallon per minute = 0.06 L/s, 1 inch per hour = 25.4 mm/h, 1 square foot = 0.0929 m²
Notes:

¹ Maximum discharge capacity, gpm (L/s) with approximately 1³/₄ inch (44 mm) head of water at the drain.
² For rainfall rates other than those listed, determine the allowable roof area by dividing the area given in the 1 inch per hour (25.4 mm/h) column by the desired rainfall rate.
³ Vertical piping shall be round, square, or rectangular. Square pipe shall be sized to enclose its equivalent round pipe. Rectangular pipe shall have not less than the same cross-sectional area as its equivalent round pipe, except that the ratio of its side dimensions shall not exceed 3 to 1.

1103.2 Size of Horizontal Storm Drains and Sewers

The size of building storm drains, or building storm sewers or their horizontal branches shall be based on the maximum projected roof or paved area to be handled and Table 1103.2.

1103.3 Size of Roof Gutters

The size of semi-circular gutters shall be based on the maximum projected roof area and Table 1103.3.

TABLE 1103.3
SIZE OF GUTTERS
DIAMETER OF GUTTER (¹/₁₆ inch per foot slope)	MAXIMUM RAINFALL RATES BASED ON ROOF AREA (square feet)
inches	2 (in/h)	3 (in/h)	4 (in/h)	5 (in/h)	6 (in/h)
3	340	226	170	136	113
4	720	480	360	288	240
5	1250	834	625	500	416
6	1920	1280	960	768	640
7	2760	1840	1380	1100	918
8	3980	2655	1990	1590	1325
10	7200	4800	3600	2880	2400

DIAMETER OF GUTTER (¹/₈ inch per foot slope)	MAXIMUM RAINFALL RATES BASED ON ROOF AREA (square feet)
inches	2 (in/h)	3 (in/h)	4 (in/h)	5 (in/h)	6 (in/h)
3	480	320	240	192	160
4	1020	681	510	408	340
5	1760	1172	880	704	587
6	2720	1815	1360	1085	905
7	3900	2600	1950	1560	1300
8	5600	3740	2800	2240	1870
10	10200	6800	5100	4080	3400

DIAMETER OF GUTTER (¹/₄ inch per foot slope)	MAXIMUM RAINFALL RATES BASED ON ROOF AREA (square feet)
inches	2 (in/h)	3 (in/h)	4 (in/h)	5 (in/h)	6 (in/h)
3	680	454	340	272	226
4	1440	960	720	576	480
5	2500	1668	1250	1000	834
6	3840	2560	1920	1536	1280
7	5520	3680	2760	2205	1840
8	7960	5310	3980	3180	2655
10	14400	9600	7200	5750	4800

DIAMETER OF GUTTER (¹/₂ inch per foot slope)	MAXIMUM RAINFALL RATES BASED ON ROOF AREA (square feet)
inches	2 (in/h)	3 (in/h)	4 (in/h)	5 (in/h)	6 (in/h)
3	960	640	480	384	320
4	2040	1360	1020	816	680
5	3540	2360	1770	1415	1180
6	5540	3695	2770	2220	1850
7	7800	5200	3900	3120	2600
8	11200	7460	5600	4480	3730
10	20000	13330	10000	8000	6660

For SI units: 1 inch = 25 mm, ¹/₁₆ inch per foot = 5.2 mm/m, 1 inch per hour = 25.4 mm/h, 1 square foot = 0.0929 m²

1103.4 Side Walls Draining Onto a Roof

Where vertical walls project above a roof to permit storm water to drain into the roof area below, the adjacent roof area shall be permitted to be computed from Table 1103.1 as follows:

For one wall - add 50 percent of the wall area to the roof area figures.
For two adjacent walls of equal height - add 35 percent of the total wall areas.
For two adjacent walls of unequal height - add 35 percent of the total common height and add 50 percent of the remaining height of the highest wall.
Two opposite walls of same height - add no additional area.
Two opposite walls of differing heights - add 50 percent of the wall area above the top of the lower wall.
Walls on three sides - add 50 percent of the area of the inner wall below the top of the lowest wall, plus an allowance for the area of the wall above the top of the lowest wall, in accordance with Section 1103.4(3) and Section 1103.4(5) above.
Walls on four sides - no allowance for wall areas below the top of the lowest wall - add for areas above the top of the lowest wall in accordance with Section 1103.4(1), Section 1103.4(3), Section 1103.4(5), and Section 1103.4(6) above.