Who said rain barrels can’t be pretty?
Most people don’t want a giant water tower in their side yard. I get that. My wife definitely gets that. So what do you do? Well, I suggest you put in the biggest and prettiest rainwater harvesting system you’re comfortable having in your yard.
Yes, I said ‘prettiest.’
So let’s start with the how much water you can get, and then we’ll get into what these things look like and the basics of how they’re set up.
How much rain hits your roof?
Once you figure out how to calculate the amount of rain hitting your roof, you can better determine an appropriate size of your rain system.
If you don’t want to manually do this calculation, never fear! I have compiled a rainwater harvesting calculation worksheet at the bottom of the page that will do the work for you! This worksheet also contains a metric version.
But for those of you who want to know how to do this:
First, you need the total square footage of your roof footprint.
This is the horizontal space covered by your roof. You don’t need any fancy calculation to factor in slopes or angles here. All you do is take the width and length of your roof and multiply them to get your square footage. If you have a fancy house that isn’t a rectangle like ours, you can do this in multiple steps by calculating each rectangle, then adding them together.
Here’s an example. You measure the width of your roof, from end to end of your gutter and come up with 40’. Then you measure the depth of your roof from front to back and get 30’. You also have a garage that measures 20’ by 20’.
For a block footprint house, your square footage is:
Length x Depth = Square feet
For a house that is a fancy shape, you just do the same calculation for each section, and then add each answer up.
So our calculation is:
(40’ x 30’) + (20’ x 20’) = 1200sf + 400sf = 1600sf
Once you get the square footage, you then get your total rainfall in your area. There are many online resources for obtaining your annual precipitation. Here is one such resource I just found with a quick search: https://www.usclimatedata.com/
To get the total number of gallons that hit your roof, you need to do another calculation:
Square feet x Annual Rainfall ÷ 12 x 7.48 = Total gallons
(For you nerdy folks like me, the ‘÷ 12’ converts annual rainfall to feet. The ‘x 7.48’ converts cubic feet to gallons)
So for our pretend roof, let’s say we live in Tucson. We get only 11.3 inches of rain per year. Let’s see how it stacks up.
1600sf x 11.3” ÷ 12 x 7.48 = 11,269 gallons!
And that’s with only 11.3 inches of rain per year!
Now that we know our total rainfall catchment, how much do we get in a one-inch storm at our home? Well, all we do is substitute our 11.3” in the previous calculation with 1”.
1600sf x 1” ÷ 12 x 7.48 = 997 gallons
Even with one good thunderstorm, we get almost 1,000 gallons of water!
With these calculations, you can now make an informed decision on the type of rainwater harvesting system to install.