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Run Off from Roof Areas

Rainfall runoff from shed and house roofs can be a reliable, efficient water source that is easily overlooked. Roofs are high yielding and can turn even minor rainfall events into a useful supply of good quality water. Such supplies are ideal for stock and domestic consumption or for use in spraying equipment.

The yield from a roof is dependent upon the area of the roof and the rainfall received, however not all of this is always captured as at times the tank may overflow.

To calculate the volume of rainfall that can be collected from roof area the following formula is used: Volume of water (litres) = Annual average rainfall (mm) x Roof area (m²) x 0.95
*Note: for a rectangular roof the area is the length (m) x width (m)
For example: A shed with the dimensions 15m x 9m has a roof area of 135m² and an annual rainfall of 400 mm will yield 51,300 litres/year (400mm x 135m² x 0.95).

Roof catchment area (m2)

Annual Rainfall

50m2

100m2

150m2

200m2

300m2

500m2

1000m2

2000m2

300 mm

14,250L

28,500L

42,750L

57,000L

85,500L

142,500L

285,000L

570,000L

400 mm

19,000L

38,000L

57,000L

76,000L

114,000L

190,000L

380,000L

760,000L

500 mm

23,750L

47,500L

71,250L

95,000L

142,500L

237,500L

475,000L

950,000L

600 mm

28,500L

57,000L

85,500L

114,000L

171,000L

285,000L

570,000L

1,140,000L

700 mm

33,250L

66,500L

99,750L

133,000L

199,500L

332,500L

665,000L

1,330,000L

800 mm

38,000L

76,000L

114,000L

152,000L

228,000L

380,000L

760,000L

1,520,000L

Source: Managing Farm Water Supplies (2011) Victoria Government Department of Primary Industries