Determine the percentage of water collected from each downspout p.
Rain runoff from roof.
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.
Divide 100 by the number of downspouts in order to obtain the percentage of water collected from each downspout.
Step 2 multiply the area of your roof by the amount of rainfall you wish to calculate for.
The answer is about 623 gallons.
Use the water that drains off your 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.
Count the number of downspouts coming from your roof.
These techniques will minimize the amount of water that accumulates or that causes erosion in the problem area.
However there is a way that you can redirect the rain off the roof so it falls harmlessly out of the way.
1 place a piece of standard metal drip edge onto a table or between a pair of sawhorses.
If you have heavy runoff from roof downspouts and neighboring properties redirecting or capturing runoff can provide better control.
So a 50 x 20 roof is 600 x 240 multiply the roof dimensions by the number of inches of rainfall.
The area we are calculating here is the same as if your roof were flat.
Take the dimensions of the footprint of your roof and convert them to inches.
If your downspouts are connected directly to a storm drain disconnecting them is the single most important step you can take to reduce runoff.