Believe it or not, for every inch of rain that falls on a catchment area of 1,000 square feet, you can expect to collect approximately 600 gallons of rainwater. Ten inches of rain falling on a 1,000 square foot catchment area will generate about 6,000 gallons of rainwater! That’s right, 6,000 gallons! More than you were expecting?
Your roof catchment area is equal to the total square feet of your house plus the extension of your eaves. You don’t need to consider the angle of your roof, like you would if you were buying roofing material, because rain falls evenly on every part of the roof.
To calculate the square feet of your house’s catchment area, measure the area of the outside walls and then include the overhang of any eaves. For example, let’s say you have an oblong house with outside dimensions of 36 feet by 46 feet. You’ve calculated the overhang of your eaves as 2 feet. So, add the 4 feet of the eaves to each wall length (2 eaves of 2 feet equals an additional 4 feet for each wall) to get the total length of the walls plus the eaves (40 by 50 feet).
Now multiply 40 times 50 (length times width) to get your total roof catchment area.
(36 + 4) x (46 + 4) = 2,000 sq ft
Your roof catchment area is thus 2,000 square feet.
Since one inch of rainfall provides approximately 600 gallons of water for a 1,000 square foot catchment area, and our theoretical house has a 2,000 square foot catchment area (twice the area), you will multiply 600 gallons by 2.
600 gal x 2 = 1,200 gallons
If you have an average rainfall of say 20 inches per year, you have the potential to collect 24,000 gallons of water in one year. (You can use the following website to get a good idea of the average rainfall in your area: http://countrystudies.us/united-states/weather/) 1,200 gal x 20 inches of rain = 24,000 gal
Depending on the needs of your household, that can be significant amount of water to augment your water supply.
You should consider that rainwater harvesting systems aren’t necessarily 100% efficient. Most sources estimate efficiency between 70% and 90%. All rainwater harvesting systems lose some of the rainwater. It may spill out of the gutters or the wind may blow it away. Evaporation will undoubtedly affect some of it. To maximize your collection of rainwater, you can use out buildings such as barns or sheds. If you’re creative, you can even use rainwater from a patio or other paved areas around your house.
Now that you’ve got an idea how much water you can collect, we can help you calculate: How Much Municipal Water Do I Already Use Without a Rain Barrel?