Choose the roof-drainage component below, then compare its capacity in gallons per minute (gpm) with the design flow. These 2021 IPC tables distinguish storm piping, vertical leaders and gutters; use the edition adopted for your project.
Storm piping capacities — Table 1106.2
Capacities are gpm. Horizontal columns use pipe slope in inches per foot.
| Pipe (in) | Vertical | 1/16 in/ft | 1/8 in/ft | 1/4 in/ft | 1/2 in/ft |
|---|---|---|---|---|---|
| 2 | 34 | 15 | 22 | 31 | 44 |
| 3 | 87 | 39 | 55 | 79 | 111 |
| 4 | 180 | 81 | 115 | 163 | 231 |
| 5 | 311 | 117 | 165 | 234 | 331 |
| 6 | 538 | 243 | 344 | 487 | 689 |
| 8 | 1,117 | 505 | 714 | 1,010 | 1,429 |
| 10 | 2,050 | 927 | 1,311 | 1,855 | 2,623 |
| 12 | 3,272 | 1,480 | 2,093 | 2,960 | 4,187 |
| 15 | 5,543 | 2,508 | 3,546 | 5,016 | 7,093 |
Round leader capacities — Table 1106.3
Round sizes only; the source also lists rectangular leaders.
| Diameter (in) | Capacity (gpm) |
|---|---|
| 2 | 30 |
| 2.5 | 54 |
| 3 | 92 |
| 4 | 192 |
| 5 | 360 |
| 6 | 563 |
| 8 | 1,208 |
Semicircular gutter capacities — Table 1106.6
Only the listed semicircular size/slope combinations are shown. See the source for rectangular gutters.
| Diameter (in) | Slope (in/ft) | Capacity (gpm) |
|---|---|---|
| 4 | 1/8 | 39 |
| 5 | 1/8 | 74 |
| 6 | 1/8 | 110 |
| 8 | 1/16 | 172 |
| 8 | 1/8 | 247 |
| 10 | 1/16 | 331 |
| 10 | 1/8 | 472 |
Example: roof rainfall to flow
A 2,000 ft² roof receiving 3 in/hr produces approximately 2,000 × 3 × 0.623 ÷ 60 = 62.3 gpm before collection losses. The conversion uses approximately 0.623 gallons per inch-square-foot; see the TWDB collection-volume explanation.
At 1/8 in/ft, the 3-inch storm-pipe row gives 55 gpm and the 4-inch row 115 gpm. This compares table capacities with a rainfall-only load; it does not select a roof drain.
Complete the roof-drainage checks
Check the manufacturer's drain flow at the applicable ponding head (1105.2–1106.2), contributing wall area (1106.4) and secondary drainage (1108). Siphonic systems have a separate method in 1107; controlled-flow systems are covered in 1110.
Table source
- ICC: 2021 IPC, Chapter 11 — Tables 1106.2, 1106.3 and 1106.6. Selected shapes are identified above.