Sunday, September 4, 2011

The Sustainability Review – Rainwater Geometry

By Mike Forbes, Co-op Volunteer Writer

I’ve written about rainwater systems for several years now, specifically about our experience with installing, filtering, and using it.  I’ve received many questions in that time regarding everything about our system.  There is one question that keeps rearing its head.  How much water can I collect off of my roof?

I remember doing the calculations years ago and quickly realized that in our circumstance that we would have more water falling on our roof than we could easily store.  Storage became our issue but for you it might not be since our roof is rather large.  My point in writing this is that it’s important to know what your water capture potential is and design your storage and useage accordingly.  I’m not going to go through any of the techniques or roof materials of rainwater collection as I’ve done that in the past, this article is purely theoretical.  Previous articles can be found on the Co-op website or from me directly.

Let’s walk through the math with an imaginary 20’ x 20’ small house.  Time to put on your high school geometry thinking cap. The most important thing here is to visualize the roof area that the rain is falling on, not the length of the roof line. The slope of the roof isn’t important here. The dimensions we are looking for are the lengths of the walls plus any overhang, ultimately the total roof area that the rain will see.  With our house we have a 20’ x 20’ roof giving us an area of 400 sq. ft.

Next we need to know how much rain comes out of the sky.  Where do you find this data?  There are various sites out there with specific data for a location but this one is a simple to use and easy to decipher.  Many other sources require you to dig a bit and don’t give a good overview.  Simply input your zip code and scroll down to Average Precipitation.  For Moscow, Idaho this is 23.7 inches. 

Imagine a roof of 400 square feet with 23.7 inches (1.97 feet) of water sitting on top of it.  This is our potential rain resource.  In order to get to something we understand better we must convert to gallons.  To do this, we must convert those numbers to a volume like cubic feet by multiplying our area by the depth of water.  Everything must be in feet to do this so the equation looks like this:  Area (in square feet) x depth (in feet) = cubic feet.  Our example: 400 square feet x 1.97 feet = 788 cubic feet.

Now we can convert this cubic foot measurement to gallons.  Looking this up we find that there are 7.48 gallons per cubic foot.  Multiplying we get a grand total of 5,894 gallons falling on our roof. This is a lot more than most people can easily store and this is for a small roof.   

What do we do with this number? We start the design process of storage and useage.  In our next article we’ll talk about the design process and walk through the steps.  I’ve created a spreadsheet that looks at monthly rainfall and useage to assist in this process.
I think it is common to think we live in a dry area and that rainfall isn’t a significant source of water for us on the Palouse.  The math doesn’t lie and it will become quite apparent after doing the math that we do have a good resource by which we can offset our use of precious groundwater.

Mike enjoyed the cool summer and can be reached at

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