Quick Answer
To calculate the volume of rainwater from your roof area, multiply the roof area by the annual rainfall intensity in inches or millimeters, and then divide by 12 to account for the number of inches in a foot.
Calculating Roof Area
To determine the volume of rainwater your roof can collect, you first need to calculate the roof area. Measure the length and width of your roof in feet, and multiply them together to get the square footage. For example, a roof that is 40 feet long and 30 feet wide has a total area of 1,200 square feet.
Calculating Annual Rainfall Intensity
Next, you need to determine the annual rainfall intensity in your area. This can be expressed in inches or millimeters. In the United States, for example, the average annual rainfall intensity can range from 20 to 80 inches, depending on the region. To calculate the volume of rainwater your roof can collect, you’ll need to use a rainfall intensity figure specific to your location.
Calculating Rainwater Collection Volume
Once you have the roof area and annual rainfall intensity, you can calculate the volume of rainwater your roof can collect. Use the following formula: Rainwater Collection Volume (gallons) = Roof Area (square feet) x Rainfall Intensity (inches) / 12. For example, if your roof has a total area of 1,200 square feet and your annual rainfall intensity is 40 inches, the volume of rainwater your roof can collect would be: 1,200 x 40 / 12 = 4,000 gallons.
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