Step One: Determine the square footage of your roof.
Step Two: Calculate the runoff of your roof using the following equation:
(square footage of your roof × 623 ) ÷ 1000 = gallons of runoff per average storm Example: (960sqft × 623) ÷1000 = 598 gallons per storm
•That sure is a lot of water! But, rain barrels are not 100% effective. Most rain barrels are inbetween 70% and 90% effective. To factor this into your equation, follow step three.
Step Three: Determine minimum and maximum amount of run off that can be collected during an average storm using the following equation:
gallons of runoff per storm × 0.7(70%) = min gallons per storm that can be collected ex. 598 gallons × 0.7 = 419min gallons per storm that can be collected gallons of runoff per storm × 0.9(90%) = max gallons per storm that can be collected ex. 598gallons × 0.9 = 538 max gallons per storm that can be collected
•So, a rain barrel in South Florida could collect a minimum of 419 gallons and a maximum of 538 gallons of rooftop runoff from a 960 square foot roof from a 1 inch rain event.
Step Four: Determine how many rain barrels you need.
min gallons per storm that can be colleted ÷ 55 gallon rain barrel = min number of rain barrels ex. 419 gallons per storm ÷ 55 gallon rainbarrel = 7.6 --- > 8 rain barrels
max gallons per storm that can be collected ÷ 55 gallon rain barrel = max number of rain barrels ex. 538 gallons per storm ÷ 55 gallon rain barrel = 9.8 --- > 10 rain barrels
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