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# Rainfall on Roofs and Gutter Slopes

## Draining of roof areas and recommended gutter slopes

Rainfall and recommended gutter slopes when draining roof areas:

Recommended gutter slopes for draining roof areas
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

• ### Sanitary Drainage Systems

The purpose of the sanitary drainage system is to remove effluent discharged from plumbing fixtures and other equipment.

## Related Documents

• ### Horizontal Storm Drain Piping

Capacities of horizontal storm drain piping systems.
• ### Manning's Formula and Gravity Flow

Calculate cross-sectional average velocity flow in open channels.
• ### Rainfall Draining of Roof Areas and Vertical Downspouts

Capacities of vertical downspouts when draining roof areas in rainfall.
• ### Rainfall Units Converter

Convert between different rainfall or storm-water units
• ### Roof Framing

Run, roof slope, gable height and gable area.
• ### Storm-Water Conductors

Roof drain capacities.
• ### Storm-Water Runoff Coefficients vs. Surface

Storm-water runoff coefficients for various surfaces

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## Citation

• The Engineering ToolBox (2008). Rainfall on Roofs and Gutter Slopes. [online] Available at: https://www.engineeringtoolbox.com/sloopes-roof-drainage-d_1107.html [Accessed Day Month Year].

Modify the access date according your visit.

6.3.17

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