I wanted to provide a resource on how to actually calculate the amount of rainwater you could actually collect off of a rooftop. It’s a pretty simple calculation.
First, you need to figure out big your roof will be. Now don’t worry, you don’t have to be exact! And don’t just look at your roof and try to figure out the angles, etc either. None of it is relevant!
The only thing matters is the square footage that the roof covers. And I mean literally that, covers. So if you have a 24×30 cabin, that roof will cover about 720 square feet.
Rudy’s Note: I’m serious. Forget about the angles and gables and all that. Rain doesn’t care. it’s basically a two dimensional problem here.
The next thing you need to find out is how much rainwater falls in your area. Now sometimes this can be tough since most places report overall precipitation and not just rain. You’ll need to figure this part out on your own.
I like using the site ‘HomeFacts‘ since their climate data has monthly precipitation numbers. You’ll probably know which months are rain vs snow, so this makes life easier!
Using Seattle as a guide, we find that our average annual precipitation is 38.25 inches. Since it doesn’t snow much in Seattle, we’ll pretend it’s all rain.
Rudy’s Note: If you DO want to include snowfall, it may be useful to know that on average 10 inches of snow is equivalent to one inch of rainfall. Slushy snow is less, light and fluffy powder is of course more than ten to one.
ANYHOW. Back to our rainwater collection system calculations!
Now we have all of the key information, so we pull out our handy calculator, and do the following:
Square Footage * Avg Rainfall (inches) / 12 = Rainfall in Cubic Feet
Now, we don’t really want just Cubic Feet, so we multiply the result by 7.48 to get rainfall in gallons.
For our example, it looks like this
720 sq ft * 38.25 inches / 12 * 7.48 = 17,166.6 gallons
So now we know that our little cabin in Seattle could potentially collect over seventeen thousand gallons of rainfall for us each year.
Rudy’s Note: Since this is all averages, and I’m lazy, instead of dividing by twelve and multiplying by 7.48, I just multiply by 0.623 (which is the result of 7.48 / 12) … it’s not going to give you the same result, but close enough:
720 * 38.25 * 0.623 = 17,157 … Definitely close enough for my purposes!
Anyhow, for what it’s worth, you now know how to calculate runoff for your new rainwater collection system! Or at least the rainwater harvesting you PLAN to do…