Rainfall on Roofs and Gutter Slopes
Draining of roof areas and recommended gutter slopes
Rainfall and recommended gutter slopes when draining roof areas:
| Gutter Diameter | Gutter Slope | ||||||
|---|---|---|---|---|---|---|---|
| mm | inches | 0.5% | 1% | 2% | |||
| 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 |
The table is based on rainfall intensity 100 mm per hour (4 inches per hour). Anticipated rainfall intensity should always be checked with local authorities. As a reference - maximum rainfall intensity used in the UK is normally 75 mm per hour.
Slope
- 0.5 % grade = 0.5 cm per 100 cm run = 0.0625 inch per 1 ft run
- 1 % grade = 1 cm per 100 cm run = 0.125 inch per 1 ft run
- 2 % grade = 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
Example - Draining Roof
A roof with area 45 m2 can be drained with a gutter with slope 1%. From the table above a gutter with diameter 100 mm (4 inches) can be used.
The downspout can be 75 mm.
