Rain Runoff From Roof

To calculate the runoff from any given rainfall.

Rain runoff from roof.

Convert the percentage to a decimal by dividing the percentage by 100. If your downspouts are connected directly to a storm drain disconnecting them is the single most important step you can take to reduce runoff. The area we are calculating here is the same as if your roof were flat. Count the number of downspouts coming from your roof.

Determine the percentage of water collected from each downspout p. However there is a way that you can redirect the rain off the roof so it falls harmlessly out of the way. 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. The answer is about 623 gallons.

These techniques will minimize the amount of water that accumulates or that causes erosion in the problem area. 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. 1 place a piece of standard metal drip edge onto a table or between a pair of sawhorses. 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. 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.