When figuring
out how much storm water runs off of your roof, a
general rule to use is that 1
inch of rainfall on a 1000 square foot roof will
yield 623 gallons of storm water runoff.
Calculate
the storm water runoff of your roof with the following equation:
Square
footage of your roof X 623 ÷ 1000
For
example, if you have a roof that is 20 feet long and 25 feet
wide (don't worry about the slope), the square footage of your
roof is 20 X 25, or 500 square feet.
So...
500
X 623 = 311,500, and then,
311,500 ÷ 1000
= 311.5 gallons of water
(or half of 623)
That
may seem like a lot of water, but rain barrel systems aren't
100% efficient. Most sources estimate an efficiency between
70% and 90%. Out
buildings like garages
or sheds
can also be used to increase how much water you collect.
311.5
gallons x .7 (70%) = 218 gallons
311.5
gallons x .9 (90%) = 280 gallons
So
a rain barrel system or cistern could collect between 218
and 280 gallons of water from a 500 square foot rooftop in
a 1-inch rain storm.
In
Washington, D.C., we get an average of 39 inches of
rain every year...
280
x 39 = 10,920 maximum gallons of saved rainwater per year!
Calculating
how many rainbarrels you need
The
results of your runoff calculation will help you to size
your rain barrel system (or rain garden). Simply divide the number
of gallons of water from you roof by the number of gallons the rain barrel
can hold.
For
example, if your roof yields a maximum of 280 gallons of
storm water and you are buying (or building) 55-gallon rain
barrels,
you
would need 6 of
them
connected
together
to
hold
all the storm
water
that flows off your roof during a one-inch rainstorm:
280 ÷ 55
= 5 rain barrels
Most
storms in D.C. produce about a half-inch of rain. So if you
drain your rain barrels after every storm, you will only need
about 3 rain barrels for a 500 square foot rooftop.
Up
to 40% of your summer water bill is spent on watering plants,
trees and lawns. Those 11,000 (or more) gallons of free water
will come in handy
when D.C.'s hot summer begins!
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