It seemed to rain in Atlanta every day last week. Though some of us thought the rain was an inconvenience, those people collecting rain water made money.
The National Weather Service Forecast Office in Peachtree City, Georgia (the NWS office closest to Atlanta) has recorded ten inches of rainfall so far this year. You can search their NOWData page if you’d like to figure out the value for your area.
A ranch-style house typically has a roof pitch of about 4-in-12. That means it rises four inches per foot from the outside wall of your house toward the high point of the roof. Steeper roofs collect more rainfall because the wind drives more rain onto those roofs. The multiplier for a 4-in-12 roof is 1.05.
Also, not all of the rainfall is collected into the gutter system, some of it bounces off the roof, some is deflected by trees, etc. Most of the rainwater harvesting companies with which we’re familiar use a 0.50 multiplier to account for this. So 1 inch of rain on 1 square foot of roof area yields about 0.5 gallons of water. Don’t worry, this may sound like a lot of math but it’s the last multiplier.
On a ranch-style house with a simple roof [a ridge (the high point) running from one side to the other above the middle of the house], it would be easy to collect rainwater on the back side of the house – the back half of the roof. We’d have the same gutter and down spout system except the downspouts would direct the water to a collection tank or tanks. Collecting on the back side of the house means the collection points and storage wouldn’t be seen from the front of the house.
The area of that half of the roof would be many hundreds of square feet – slightly more than half the square footage of the house because the roof is pitched. To use a round number, let’s say that we’re collecting water from five hundred square feet of roof on our ranch-style house.
Here’s our calculation:
500 square feet of roof X 1.05 multiplier = 525 square feet
525 square feet X 10 inches of rain = 5,250 gallons of water
5,250 gallons of water X 0.50 multiplier = 2,625 gallons collected
So, in the first three weeks of 2013, we could have collected 2,625 gallons of water to use for irrigation, flushing toilets or some other use (other than drinking water). To put that in perspective, we want to offer two observations:
1) If we flush our toilet four times each day, we’re using about five gallons of water per day (assuming 1.28 gallons per flush). If we’re using collected rainwater to flush the toilet, we have enough water from the first three weeks of 2013 to last 525 days!
2) The City of Atlanta water rate is graduated into three tiers – the more you use, the more you pay. The rate for the middle tier, where we think most people would fall, works out to about 2 cents per gallon. So, that 2,625 gallons of rain we’ve collected is worth approximately $52.50.
But wait, there’s more! Many municipalities charge a sewer rate based on water consumption under the assumption that the water that flows through the meter into the house will eventually go down into the sewer system. In effect, we’re charged twice for the water that flows through the meter. The sewer rate for the middle tier is also about 2 cents per gallon. If we’re flushing our toilets with the water we collected, we’re really saving a total of about $105 – $52.50 multiplied by two.
Atlanta’s average rainfall over the last thirty years is about fifty inches per year. If you plug fifty inches into the calculation above, you get 13,125 gallons of water. That’s worth $262.50 of water per year or $525.00 when you multiply by two to figure the total savings. We only used five hundred square feet of roof – catching water off 1,500 square feet of roof would be worth $1,575 per year: $787.50 multiplied by two.
Using these simple calculations, we can figure out how quickly a rain water harvesting system would pay for itself. Even if you use the lower rainfall numbers from recent years, we think the system will pay for itself long before it’s at the end of its useful life.
We hope you’ll share this information with your friends and family – especially children. As a society, Americans have a lot to learn about the value of water. The picture above is from a town outside Nairobi, Kenya. You can see the gutters are connected to pipes that feed the storage tank. The tank could go under your deck, patio or driveway so you (and your neighbors) don’t see it.
The countries we consider third-world nations think rain water harvesting is common sense. A simple system collects such a precious local resource!