How Much Rainwater Runs Off Your Roof During a Storm?

During the spare time you have on a rainy day like today, you may have asked yourself, “Just how much water do people collect with those rain barrels they have attached to their downspouts?”  That’s a good question.  Let’s do a little math to figure it out.

First, we need to determine the area of the roof in square feet.  There’s no need to climb up there and measure angles, etc.  After all, we’re not ordering shingles—we’re trying to figure out the surface area upon which rain will fall vertically and be collected.  For our estimate, knowing the footprint of the building under roof will suffice.  We’ll use a very common footprint as an example—1,200 square feet.

40′ x 30′ = 1,200 sq. ft.

By dividing the area of the roof by 12, we can calculate the volume of water in cubic feet that is drained by the spouting for each inch of rainfall…

1,200 ÷ 12 = 100 cu. ft. per inch of rainfall


Next, we multiply the volume of water in cubic feet by 7.48 to convert it to gallons per inch of rainfall…

100 x 7.48 = 748 gallons per inch of rainfall


That’s a lot of water.  Just one inch of rain could easily fill more than a single rain barrel on a downspout.  Many homemade rain barrels are fabricated using recycled 55-gallon drums.  Commercially manufactured ones are usually smaller.  Therefore, we can safely say that in the case of a building with a footprint of 1,200 square feet, an array of at least 14 rain barrels is required to collect and save just one inch of rainfall.  Wow!

Why send that roof water down the street, down the drain, down the creek, or into the neighbors property?  Wouldn’t it be better to catch it for use around the garden?  At the very least, shouldn’t we be infiltrating all the water we can into the ground to recharge the aquifer?  Why contribute to flooding when you and I are gonna need that water some day?   Remember, the ocean doesn’t need the excess runoff—it’s already full.

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