For every inch of rainfall a typical roof captures a usable 60 to 70 gallons per 100 square feet of roof.**Example**: Thus, our 3,500 square-foot roof yields approximately 30,000 gallons of water per year according to the formula:

15 inches average annual rainfall on the Peninsula x 60 gallons per 100 square feet x 3.5 = 30,000 gallons per year.