How much rainwater can I collect from my roof?
You can collect 0.62 gallons of water per square foot of roof space, per inch of rainfall. With factors such as evaporation, splash out, leaks, overflow from gutters and first flush we use a collection efficiency percentage of 75-85%. Example: (0.62 x .85 x 1500 x 36) Seattle WA. Annual Rainfall 36 Inches Sqaure Feet of Roof Area 1500 Rainwater Collection Efficiency 85% Gallons of Water Per Square Foot/ Per Inch 0.62 Total Annual Rainwater Collected 28,458 Gallons.