Gutter Slopes and Roof Drainage

Roof area drained with gutter slopes

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Recommended size of gutter slopes draining roofs can be found in the table below.

Roof area
Guttering diameter Slope of
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 a rainfall intensity of 100 mm per hour (4 inches per hour). The anticipated rainfall intensity should always be checked with the local authorities.

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