A 1 000 square foot roof can produce more than 600 gallons 2 271 2 l of runoff for every 1 of rain that falls on it.
Rain runoff from roof.
In this example 600 x 240 x 1 144 000 cubic inches of water divide by 231 to get the number of gallons because 1 gallon 231 cubic inches.
The answer is about 623 gallons.
Divide 100 by the number of downspouts in order to obtain the percentage of water collected from each downspout.
Take the dimensions of the footprint of your roof and convert them to inches.
The area we are calculating here is the same as if your roof were flat.
However there is a way that you can redirect the rain off the roof so it falls harmlessly out of the way.
To calculate the runoff from any given rainfall.
Use the water that drains off your roof.
Determine the percentage of water collected from each downspout p.
If you have heavy runoff from roof downspouts and neighboring properties redirecting or capturing runoff can provide better control.
Even though most roofs are pitched at an angle the only part that will affect the rain runoff is the actual area underneath your roof.
Step 2 multiply the area of your roof by the amount of rainfall you wish to calculate for.
1 place a piece of standard metal drip edge onto a table or between a pair of sawhorses.
Convert the percentage to a decimal by dividing the percentage by 100.
Count the number of downspouts coming from your roof.