Alternatively, to approximate how much water you can capture from your roof, use the following steps;
- The average annual rainfall in your area.
Example the annual rainfall of Florida Keys is 41.7";
the annual rainfall of northwest Arizona is 8.9"
- The area of your roof in square feet
Multiply the length x width of the building; if the building shape is irregular, work out each section of the building and add the totals together.
Example 80' long x 25' wide = 2000 sq ft.
- Calculate the catchment for each inch of rainfall
1" of rain falling on 1 square foot of roof catches 0.623 gallons of rainwater.
Example 2000 sq ft of roof x 0.623 gallons = 1246 gallons collected for each inch of rainfall.
- Calculate the annual catchment of your roof
Multiply the number of gallons your roof catches by the annual rainfall.
Example If our example house is in the Florida Keys, the amount of rainfall flowing from the roof is
1246 gallons x 41.7" annual rainfall = 51,958.2 gallons per year.
If our example house is in northwest Arizona, the amount of rainfall flowing from the roof is
1246 gallons x 8.9" annual rainfall = 11089.4 gallons.
The amount of water flowing into any one downspout will depend on how much of the roof is connected to it - you can work out the catchment of each downspout using the same steps above.