How to Build a Rain Garden
Part 2 Planning My Rain Garden – Does Size Matter?
By Joy Smith
I am excited about the idea of building a rain garden to capture some of the runoff from our road, our driveway and our roof. I am energized by what I am reading, but of course the labor part of the project doesn’t thrill me as much. I will recruit help for that. I will use the Tom Sawyer method if possible. Remember how much fun fence painting is?
Right now I am thinking about where to locate my rain garden and figuring out how big it needs to be. To state the obvious, it needs to be between the source of our runoff and the lake. It needs to be located on our property because the Army Corp of Engineers does not look kindly on anything on their property, unless you have a permit.
What are our sources of runoff? Well now that I think about it, we have four: the roof and drive way in front; the roof in back; the shop roof; and the runoff from the road. I need to prioritize, so I am going to concentrate on the runoff from the road. It runs down the driveway, around the house, across the gravel path and into the lake. I am thankful it does not run into the garage, but maybe we can build a rain garden in a good spot that will allow at least part of the runoff to seep back into the ground.
The Extension Master Gardener Handbook has made it clear to me that I am talking about a LOT of water, not the small pond amount that I had in mind. This handbook says “A 1,000 square foot roof produces 630 gallons of runoff in a 1 inch rain storm”. That was an eye opener for me. I was not really aware of the volume of water that runs down our driveway toward our house and into the lake.
How do I figure out how big our rain garden needs to be? The online document “Size Your Rain Garden” provides a fairly easy formula for deciding the size rain garden you need to capture the water from a 1 inch rain event. In their example they want to capture the rain from the front of the house, the drive and the sidewalk which totals 900 sq ft of impermeable surface. They plan to build a 6 inch deep rain garden so they divide 900/6 and decide they need a 150 sq ft rain garden. In other words, the garden needs to be approximately 10x15 or 5x30. Of course the garden will be more attractive if it is not square, but we will talk more about that later.
According to houselogic.com, “a typical two-story, 2,300-square-foot house with a medium-pitch roof — has a roof area of about 1,500 square feet.” So if you are trying to capture the roof runoff from half that area you would want to plan a 125 sq ft (1500/2/6) rain garden that is 6 inches deep.
That is bigger than I was thinking, but my projects always tend to grow! Well it is raining outside and there is water running down my driveway. It is a perfect time to don my raincoat and follow the runoff back to its source. I will try to convince my husband that this sounds like fun. He will probably join me because he prefers to be outside almost all the time and there are no shovels needed… yet.
Check the next newsletter for updates on planning, designing and building our rain garden.
Resources Used
Size Your Rain Garden: https://wrri.ncsu.edu/docs/partnerships/bcwa/5.1.RG.SIZE.pdf
NC Extension Gardener Handbook, 2018 North Carolina State University: Designing for Water Conservation- https://content.ces.ncsu.edu/extension-gardener-handbook/19-landscape-design#section_heading_6156
Roofing Options (information on average roof size): https://www.houselogic.com/organize-maintain/home-maintenance-tips/roofing-guide-options/
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