The International Plumbing Code (IPC) provides sizing tables for roof drainage systems including roof drains, leaders (downspouts), gutters, and horizontal storm drain piping. These tables are based on the design rainfall intensity for the building location.
Values shown are derived from the 2021 IPC. Always verify with the code edition adopted in your jurisdiction.
Roof Drain and Leader Pipe Sizing (IPC Table 1106.2)
Maximum allowable roof area (sq ft) served by vertical leaders/conductors based on rainfall intensity. Flow capacity assumes the pipe is flowing full.
| Leader Size (in) | 1 in/hr | 2 in/hr | 3 in/hr | 4 in/hr | 5 in/hr | 6 in/hr |
|---|---|---|---|---|---|---|
| 2 | 2,880 | 1,440 | 960 | 720 | 575 | 480 |
| 3 | 8,800 | 4,400 | 2,935 | 2,200 | 1,760 | 1,470 |
| 4 | 18,400 | 9,200 | 6,135 | 4,600 | 3,680 | 3,065 |
| 5 | 34,600 | 17,300 | 11,530 | 8,650 | 6,920 | 5,765 |
| 6 | 54,000 | 27,000 | 18,000 | 13,500 | 10,800 | 9,000 |
| 8 | 116,000 | 58,000 | 38,660 | 29,000 | 23,200 | 19,315 |
Gutter Sizing (IPC Table 1106.6)
Maximum allowable roof area (sq ft) served by semicircular gutters at various slopes. Values shown are for 1 in/hr rainfall — divide by your design intensity to adjust.
| Gutter Diameter (in) | 1/16 in/ft slope | 1/8 in/ft slope | 1/4 in/ft slope | 1/2 in/ft slope |
|---|---|---|---|---|
| 3 | 340 | 480 | 680 | 960 |
| 4 | 720 | 1,020 | 1,440 | 2,040 |
| 5 | 1,250 | 1,770 | 2,500 | 3,540 |
| 6 | 1,920 | 2,770 | 3,900 | 5,520 |
| 8 | 4,640 | 6,560 | 9,280 | 13,120 |
| 10 | 8,800 | 12,400 | 17,600 | 24,800 |
Horizontal Storm Drain Sizing (IPC Table 1106.3)
Maximum roof area (sq ft) drained by horizontal pipes at 1 in/hr rainfall. Divide by your design intensity to adjust for other rainfall rates.
| Pipe Size (in) | 1/8 in/ft slope | 1/4 in/ft slope | 1/2 in/ft slope |
|---|---|---|---|
| 3 | 1,644 | 2,320 | 3,288 |
| 4 | 3,760 | 5,320 | 7,520 |
| 5 | 6,680 | 9,440 | 13,360 |
| 6 | 10,700 | 15,100 | 21,400 |
| 8 | 23,000 | 32,600 | 46,000 |
| 10 | 41,400 | 58,400 | 82,800 |
Design Notes
- • Design rainfall: The IPC uses the 100-year, 1-hour rainfall intensity from the local weather authority (typically NOAA Atlas 14).
- • Secondary drainage: The IPC requires an independent overflow drainage system sized for the full design storm, assuming the primary system is completely blocked.
- • Controlled-flow drains: Siphonic roof drain systems have different sizing — consult the manufacturer and IPC Section 1106.7.
- • Adjusting for rainfall: Tables at 1 in/hr can be adjusted for any intensity by dividing the tabulated area by the design intensity.
Primary Source
- International Code Council (2021). International Plumbing Code, Chapter 11: Storm Drainage. ICC.
- ASPE (2017). Plumbing Engineering Design Handbook, Volume 2: Plumbing Systems. American Society of Plumbing Engineers.