How Much Rain Runs Off A Roof

How much rain water runs off your roof.

How much rain runs off a roof.

The basic calculation is a x r 231 g where a is the area of your roof in square feet r is the amount of rain in inches and g is the amount of runoff in gallons. Divide 100 by the number of downspouts in order to obtain the percentage of water collected from each downspout. Measure along the length of the house or building and along the width of the house or building in feet. Count the number of downspouts coming from your roof.

Assuming 5 cm of rain per hour. The answer is about 623 gallons. Take a guess what is the volume of water that runs off a 1 000 square foot roof during a 1 rain storm. To find how much rain you can collect in an average rain year multiply this number by the average inches of rain.

Although this calculation is simple in principle the units can make it a headache. Show comments you may like. Step 1 determine the footprint of your structure f in square feet. It isn t uncommon for an area to collect up to 350mm of rain in just one month even in rural areas.

For example on a 2 000 square foot roof you can collect 2 000 x 0 56 1 120 gallons inch of rain. While this is a normal aspect of roofing and gutters water that pours off of a roof and drenches an area like a door stoop or porch not only makes these spots slippery but water falling. If your average rainfall was 25 inches year your annual collection potential is 1 120 x 25 28 000 gallons year. However calctool s unit menus remove this issue doing all the unit conversion for you.

Multiply this by the rainfall to get the volume of rain. Just multiple the square footage of roof space you have available x 0 6 gallons per square foot per inch of rain and you can see how much water you can collect from each inch of rain that falls. An average suburban roof of 200m 2 with such a rain fall can potentially harvest 100 000l of rainwater 200m 2 500mm 100 000l. 500 000 square cm multiplied by 5 cm is 2 500 000 cubic cm of rain.

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. You may be as astonished as we were to learn how much water runs off a roof in a rain storm. Multiply the two figures together to obtain the square footage of your roof step 2 determine the percentage of water collected from each downspout p.