Rainfall on Roofs and Gutter Slopes

Draining of roof areas and recommended gutter slopes

Gutter - rainfall draining

Rainfall and recommended gutter slopes when draining roof areas:

Roof area
Gutter DiameterGutter Slope
mminches0.5% 1%2%
m2ft2m2ft2m2ft2
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.

Related Topics

  • Water Systems - Hot and cold water service systems - design properties, capacities, sizing and more

Related Documents

Matched Content

Tag Search

  • en: slopes roof drainage
  • es: drenaje laderas techo
  • de: Pisten Dachentw√§sserung

Search the Engineering ToolBox

- "Search is the most efficient way to navigate the Engineering ToolBox!"

Engineering ToolBox - SketchUp Extension - Online 3D modeling!

3D Engineering ToolBox Extension to SketchUp - add parametric components to your SketchUp model

Add standard and customized parametric components - like flange beams, lumbers, piping, stairs and more - to your SketchUp model with the Engineering ToolBox - SketchUp Extension/Plugin - enabled for use with the amazing, fun and free SketchUp Make and SketchUp Pro . Add the Engineering ToolBox extension to your SketchUp from the Sketchup Extension Warehouse!

Translate the Engineering ToolBox!
About the Engineering ToolBox!

close