(The Maximum Annual Gallons of Rain Capture on a 2,000 square foot roof with 14 inches of rain are calculated as follows: 14 inches x 2,000 square feet x .623 gallons = 17,444 gallons)http://www.harvesth2o.com/Capture_System.shtmlYou can go all the way......Or do just a little bit. Yes, you can go green too !" A water garden is not just a change in your landscape, it's a change in your lifestyle.