How to Calculate Roof Runoff

Step-by-Step Guide

1. 1

   Step 1: Determine the footprint of your structure (F) in square feet: Measure along the length of the house or building and along the width of the house or building (in feet).

   Multiply the two figures together to obtain the square footage of your roof.
2. 2

   Step 2: Determine the percentage of water collected from each downspout (P).

   Count the number of downspouts coming from your roof.
   
   Divide 100 by the number of downspouts in order to obtain the percentage of water collected from each downspout.
   
   Convert the percentage to a decimal by dividing the percentage by
   100. , Multiply the area of your roof by the total obtained in step 2, as outlined in the following equation:
   F x P = A , For example, if it rained
   35.4 inches in 2010, with 129 calculated days of rainfall, you would divide
   35.4 by 129 to get the average of .274 inches per event.,,, Depending on the amount of rainfall which will be collected annually from your rooftop, you may opt to build and attach multiple rain barrels.
3. 3

   Step 3: Calculate the area of square footage that each downspout will be collecting from in a rainfall (A).

4. 4

   Step 4: Estimate the rainfall per event in your area (R) by dividing the annual precipitation rate by the number of rain days per year.

5. 5

   Step 5: Calculate the volume of water each downspout will collect in a rainfall by using the following equation: A x R x 0.62 = V (Volume in gallons)

6. 6

   Step 6: Calculate the average annual runoff coming out of each downspout (R1) by using the following equation: A x R1 x .062 = V

7. 7

   Step 7: Determine the gallon size of your rain barrel based on the calculations obtained from steps 6 and 7.

Detailed Guide

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