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Rainfall on Roofs and Gutter Slopes

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Rainfall and recommended gutter slopes when draining roof areas:

Recommended gutter slopes for draining roof areas 
Gutter DiameterGutter Slope
mminches0.5% 1%2%
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.


  • 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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