Quick Answer
The amount of runoff per square foot of roof area can vary depending on the area's rainfall, but a general rule of thumb is to assume 0.6-0.7 gallons per square foot per inch of rainfall. This translates to around 0.4-0.5 gallons per square foot per 0.25 inches of rainfall. Roof area is a critical factor in determining water collection.
Calculating Roof Runoff Potential
When designing a water collection system, understanding the potential runoff from your roof is essential. This calculation helps you determine the size of your cistern and the number of gutters and downspouts required. To calculate the roof’s runoff potential, first, determine your roof’s total square footage. Then, multiply this number by the assumed gallons per square foot of rainfall (0.6-0.7 gallons per square foot per inch of rainfall).
Roof Runoff Example
For example, let’s say your roof measures 1,500 square feet and you expect 0.5 inches of rainfall. To calculate the total gallons of runoff, you would multiply the roof’s square footage (1,500) by the gallons per square foot of rainfall (0.5 gallons per square foot per inch). This gives you 750 gallons of potential runoff from your roof. This number will help you determine the required cistern size and overall water collection system.
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