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1103.0 Size of Leaders, Conductors, and Storm Drains
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Vertical conductors and leaders shall be sized by the maximum projected roof area and Table 1103.1.
| 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 | 18 400 | 9200 | 6130 | 4600 | 3680 | 3070 | 2630 | 2300 | 2045 | 1840 | 1675 | 1530 |
| 5 | 360 | 34 600 | 17 300 | 11 530 | 8650 | 6920 | 5765 | 4945 | 4325 | 3845 | 3460 | 3145 | 2880 |
| 6 | 563 | 54 000 | 27 000 | 17 995 | 13 500 | 10 800 | 9000 | 7715 | 6750 | 6000 | 5400 | 4910 | 4500 |
| 8 | 1208 | 116 000 | 58 000 | 38 660 | 29 000 | 23 200 | 19 315 | 16 570 | 14 500 | 12 890 | 11 600 | 10 545 | 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.
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.
| SIZE OF PIPE | FLOW (1/8 inch per foot slope) |
MAXIMUM ALLOWABLE HORIZONTAL PROJECTED ROOF AREAS AT VARIOUS RAINFALL RATES (square feet) |
|||||
|---|---|---|---|---|---|---|---|
| inches | gpm | 1 (in/h) | 2 (in/h) | 3 (in/h) | 4 (in/h) | 5 (in/h) | 6 (in/h) |
| 3 | 34 | 3288 | 1644 | 1096 | 822 | 657 | 548 |
| 4 | 78 | 7520 | 3760 | 2506 | 1880 | 1504 | 1253 |
| 5 | 139 | 13 360 | 6680 | 4453 | 3340 | 2672 | 2227 |
| 6 | 222 | 21 400 | 10 700 | 7133 | 5350 | 4280 | 3566 |
| 8 | 478 | 46 000 | 23 000 | 15 330 | 11 500 | 9200 | 7670 |
| 10 | 860 | 82 800 | 41 400 | 27 600 | 20 700 | 16 580 | 13 800 |
| 12 | 1384 | 133 200 | 66 600 | 44 400 | 33 300 | 26 650 | 22 200 |
| 15 | 2473 | 238 000 | 119 000 | 79 333 | 59 500 | 47 600 | 39 650 |
| SIZE OF PIPE | FLOW (1/4 inch per foot slope) |
MAXIMUM ALLOWABLE HORIZONTAL PROJECTED ROOF AREAS AT VARIOUS RAINFALL RATES (square feet) |
|||||
|---|---|---|---|---|---|---|---|
| inches | gpm | 1 (in/h) | 2 (in/h) | 3 (in/h) | 4 (in/h) | 5 (in/h) | 6 (in/h) |
| 3 | 48 | 4640 | 2320 | 1546 | 1160 | 928 | 773 |
| 4 | 110 | 10 600 | 5300 | 3533 | 2650 | 2120 | 1766 |
| 5 | 196 | 18 880 | 9440 | 6293 | 4720 | 3776 | 3146 |
| 6 | 314 | 30 200 | 15 100 | 10 066 | 7550 | 6040 | 5033 |
| 8 | 677 | 65 200 | 32 600 | 21 733 | 16 300 | 13 040 | 10 866 |
| 10 | 1214 | 116 800 | 58 400 | 38 950 | 29 200 | 23 350 | 19 450 |
| 12 | 1953 | 188 000 | 94 000 | 62 600 | 47 000 | 37 600 | 31 350 |
| 15 | 3491 | 336 000 | 168 000 | 112 000 | 84 000 | 67 250 | 56 000 |
| SIZE OF PIPE | FLOW (1/2 inch per foot slope) |
MAXIMUM ALLOWABLE HORIZONTAL PROJECTED ROOF AREAS AT VARIOUS RAINFALL RATES (square feet) |
|||||
|---|---|---|---|---|---|---|---|
| inches | gpm | 1 (in/h) | 2 (in/h) | 3 (in/h) | 4 (in/h) | 5 (in/h) | 6 (in/h) |
| 3 | 68 | 6576 | 3288 | 2192 | 1644 | 1310 | 1096 |
| 4 | 156 | 15 040 | 7520 | 5010 | 3760 | 3010 | 2500 |
| 5 | 278 | 26 720 | 13 360 | 8900 | 6680 | 5320 | 4450 |
| 6 | 445 | 42 800 | 21 400 | 14 267 | 10 700 | 8580 | 7140 |
| 8 | 956 | 92 000 | 46 000 | 30 650 | 23 000 | 18 400 | 15 320 |
| 10 | 1721 | 165 600 | 82 800 | 55 200 | 41 400 | 33 150 | 27 600 |
| 12 | 2768 | 266 400 | 133 200 | 88 800 | 66 600 | 53 200 | 44 400 |
| 15 | 4946 | 476 000 | 238 000 | 158 700 | 119 000 | 95 200 | 79 300 |
For SI units: 1 inch = 25 mm, 1 gallon per minute = 0.06 L/s, 1/8 inch per foot = 10.4 mm/m, 1 inch per hour = 25.4 mm/h, 1 square foot = 0.0929 m2
Notes:
1 The sizing data for horizontal piping are based on the pipes flowing full.
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.
The size of semi-circular gutters shall be based on the maximum projected roof area and Table 1103.3.
| 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 | 10 200 | 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 | 14 400 | 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 | 11 200 | 7460 | 5600 | 4480 | 3730 |
| 10 | 20 000 | 13 330 | 10 000 | 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
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.
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