So what is storm water? Storm water is rain that is derailed from the hydrologic cycle by the built environment. This is my interpretation, not the definition you will find in the dictionary. Rain is supposed to be absorbed into the ground, or into streams or rivers, but instead flows down your gutters onto your driveway, into the street, the nearest catch basin and finally the nearest storm sewer. When rain becomes a problem to manage, it becomes storm water.

Six years ago, I added a second-story addition to my one-story, 1950's ranch house in Downers Grove, Ill. The addition included a 12x24-foot increase in the overall footprint of the house.  Schooled in my professional practice to think about how building new area decreases the ability for rain to be infiltrated back into the ground water in vegetated areas, I added a few elements to my house to offset my larger footprint. I added rain barrels, decreased the lawn area in favor of perennial gardens and added a green roof over my one-story attached garage.

The garage roof structure was replaced with 2x12s at 16-in. on center to carry the 5-inch deep system of under drainage, growing media and plants. The roof has a 45 percent slope and, at the time, was the steepest green roof in the Chicagoland area. Slotted aluminum angles 5 inches deep were placed mid-slope on each gable and at the edges of the roof to allow the system to drain freely to the gutters along each edge of the roof. The total area of the roof is approximately 525 square feet and acts like a giant living sponge absorbing much of the rain that falls onto it.  Once the system has absorbed as much rain as it can soak in, water begins to trickle into the gutters and slowly filters and releases excess water.  In contrast, the gutters receiving water from the shingled roof fill and rush with water in time with the rainfall. Long after the rain is done, the green roof continues to sing with the quiet cadence of trickling water, a song that can last for hours in the blazing sun.  

Green roofs offer many more benefits than the ability to filter and absorb storm water, however, I've never seen an economic case to justify their cost. While it's hard to put a blanket cost per square foot on a green roof because there are so many variables (flat, sloped, varying depths, structural capacity of existing building, roof membrane, etc.), I'd estimate that an average green roof cost approximately 25 dollars per square foot.

Yes, the life of your underlying roof membrane will be 2 to 3 times longer than if it was not protected by a vegetated protective layer, and yes, your cooling costs for the conditioned area of a building located under a green roof can be nearly neutralized.  But, if you try and force an economic justification for your green roof, I'm afraid you will be left wanting.  Consider looking out of your window to a garden instead onto asphalt shingles.  You might consider the value of your roof as I do - worth it.

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