Rainfall on Roofs and Gutter Slopes
Draining of roof areas and recommended gutter slopes based on rainfall intensity of 100 mm per hour (4 inches per hour)
Rainfall on Roofs and Gutter Slopes
Draining of roof areas and recommended gutter slopes when draining roof areas.
Recommended Gutter Slopes for Draining Roof Areas
The following table provides recommended gutter capacities based on gutter diameter and slope at a rainfall intensity of 100 mm per hour (4 inches per hour).
| Gutter Diameter | Gutter Slope | 0.5% | 1% | 2% |
|---|---|---|---|---|
| mm (inches) | m² (ft²) | m² (ft²) | m² (ft²) | |
| 80 (3") | - | 16 (170) | 22 (240) | 32 (350) |
| 100 (4") | - | 33 (360) | 47 (510) | 67 (720) |
| 125 (5") | - | 58 (625) | 82 (880) | 116 (1250) |
| 150 (6") | - | 89 (960) | 126 (1360) | 178 (1920) |
Slope Definitions
- 0.5% = 0.5 cm per 100 cm run = 0.0625 inch per 1 ft run
- 1% = 1 cm per 100 cm run = 0.125 inch per 1 ft run
- 2% = 2 cm per 100 cm run = 0.250 inch per 1 ft run
Gutter Size vs. Downspout Size
- 100 mm (4") gutter → 75 mm (3") downspout
- 125 mm (5") gutter → 100 mm (4") downspout
- 150 mm (6") gutter → 100 mm (4") downspout
Note: Anticipated rainfall intensity should always be checked with local authorities. As a reference, the maximum rainfall intensity used in the UK is normally 75 mm per hour.