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When planning your home and landscaping drainage needs, it is necessary to accurately calculate the amount of rain runoff from your roof. This is especially important if you are installing a rain catchment system for use in irrigating your garden. Before you know what size rain barrels or cisterns to install, you will need to calculate the average amount of rain runoff from your roof.

Calculate the total area of your roof. For a rectangular house, this is simply done by multiplying the length by the width of the roof. If your roof plan is more complicated, break it up into smaller more easily calculable areas and then add those areas together for the total area. Even though most roofs are pitched at an angle, the only part that will affect the rain runoff is the actual area underneath your roof. The area we are calculating here is the same as if your roof were flat.

Multiply the area of your roof by the amount of rainfall you wish to calculate for. For example, if you have a roof that measures 50 square metres, that equals 500,000 square cm (1 square metre = 10,000 square cm). Multiply this by the rainfall to get the volume of rain. Assuming 5 cm of rain per hour: 500,000 square cm multiplied by 5 cm is 2,500,000 cubic cm of rain.

In imperial units: a 1,500 square feet roof equals 216,000 square inches. (The number of square feet multiplied by 144 equals square inches; 1,500 X 144=216,000.) Multiply those 216,000 square inches by the rainfall. In this example, we'll assume 2 inches of rain per hour: 216,000 square inches multiplied by 2 inches of rain yields 432,000 cubic inches of rain.

Convert the value from cubic cm into litres, using the fact that 1 cubic cm = 1 ml. This means that in the example, 2,500,000 cubic cm of rain is 2,500 litres.

Convert the amount of rain from cubic inches into gallons, if necessary. There are 231 cubic inches of water in a gallon, so, using our example above, divide cubic inches by 231, and you have a total runoff of approximately 1870 gallons of water (or 432,000/231=1870.129).

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- The basic calculation in metric units is (a X r)/1000 = v, where "a" is the roof's area in square cm, r is the amount of rain in cm and v is the runoff litres. For imperial units, divide by 231 instead of 1000 to obtain the runoff in gallons.

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