How Much Rain Runs Off A Roof

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.

How much rain runs off a roof. Count the number of downspouts coming from your roof. Although this calculation is simple in principle the units can make it a headache. Step 1 determine the footprint of your structure f in square feet. 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.

For example on a 2 000 square foot roof you can collect 2 000 x 0 56 1 120 gallons inch of rain. How much rain water runs off your roof. So if you have 2 500 square feet of roof available for water catchment and a single inch of rain falls one day we see that. 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.

500 000 square cm multiplied by 5 cm is 2 500 000 cubic cm of rain. Show comments you may like. 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. You may be as astonished as we were to learn how much water runs off a roof in a rain storm.

It isn t uncommon for an area to collect up to 350mm of rain in just one month even in rural areas. Keep in mind most rain falls over 2 3 months of a year. Take a guess what is the volume of water that runs off a 1 000 square foot roof during a 1 rain storm. However calctool s unit menus remove this issue doing all the unit conversion for you.

If your average rainfall was 25 inches year your annual collection potential is 1 120 x 25 28 000 gallons year. Multiply this by the rainfall to get the volume of rain. Divide 100 by the number of downspouts in order to obtain the percentage of water collected from each downspout. Here you can easily find how much rainwater you can collect from you roof how much you need to remove from a courtyard or how much runoff you can expect from an area of land.

An average suburban roof of 200m 2 with such a rain fall can potentially harvest 100 000l of rainwater 200m 2 500mm 100 000l. Assuming 5 cm of rain per hour. 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.