Rainfall Harvesting Math
...signing a rainwater harvesting system is to figure out how much water your rooftop will provide. To do this measure the outside perimeter of your roof–you need not take into account the pitch or slant of the roof, since this does not affect the amount of water collected. Next, use the following formula: collection area (square feet) x 0.6 x collection efficiency factor x rainfall (inches) = gallons per year The collection efficiency factor is basi...