Rainfall on Roofs and Gutter Slopes

Rainfall and recommended gutter slopes when draining roof areas:

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.

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