Vertical
conductors and leaders shall be sized on the basis of the
maximum projected roof area and Table 1101.12.
The size of building storm drains or building
storm sewers or their horizontal branches shall be based
upon the maximum projected roof or paved area to be
handled and Table 1101.8.
The size of semi-circular
gutters shall be based on the maximum projected roof area
and Table 1103.3.
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
| 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 |
Where vertical
walls project above a roof so as to permit storm water to
drain to the roof area below, the adjacent roof area shall
be permitted to be computed from Table 1101.12 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 lower wall.
- Walls on three sides - add 50 percent of the area of the inner wall below the top of the lowest wall, plus 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 11 03.4(3), Section 1103.4(5), and Section 1103.4(6) above.