Have you ever watched how much rain runs off your roof during a heavy rainstorm? The water coming out of your downspouts can look like a water cannon! During a 1" rainstorm, a house with a 2000 square foot roof will have over 1000 gallons of water running off of it!
Let's calculate how much water runs off your roof get a measuring tape and measure how big your house is. This is a rough estimate of how big your roof is (we will ignore the fact that roofs might have peaks or be different shapes and simply assume your roof is the size of a square the length and width of your house). You can use the example below to calculate how much rain will run off your roof in a 1" rainstorm.
The model house in the picture above is 40 feet long by 25 feet wide
40 feet x 25 feet = 1000 feet^{2 }= 1000 square feet
We will use this as our estimate for the area of the roof.
If a 2000 square foot roof has approximately 1000 gallons of water running off of it in a 1" rainstorm, how much rain runs off our model roof in a 1" rainstorm?
x gallons = 1000 gallons
_____________ ______________
1000 square feet 2000 square feet
Now we can simplify the term on the right
x gallons = 1000 gallons
________________ _______________
1000 square feet 2000 square feet
The next step is to multiply both sides by the denominator of the term on the left, and then simplify both sides of the equation
(1000 square feet) x gallons = (1000 square feet) 1 gallon
______________ _____________
1000 square feet 2 square feet
This gives us
x gallons = 1000 gallons which is 500 gallons, so that means that there will be 500 gallons of rain running off of a
__________ 1000 square foot roof.
2
Now use your measurements from your own house and calculate how much rain will run off your roof in a 1" rainstorm.

Updating...
ĉ Dr. Cynthia Annett, Jan 3, 2012, 6:38 PM
