You Got to Be Kidding Me!

Green Roofs Making Inroads

Posted in Environment, Urban Planning by Stacy McMahon on February 13, 2007

The Pennsylvania Farm News has an article detailing some green roof projects at Penn State. If you’ve never heard the term before, a “green roof” essentially means turning most or all of a building’s roof into a planter box. Once growth takes place, the roof acts as a catchment basin, filtering rainwater and releasing it in a controlled fashion, either to the ground or into the storm sewer system.

What makes the green roof “green” is the dense covering of plants and groundcover-like vegetation placed on top of the building, explains Berghage, associate professor of horticulture. “Typically a flat roof works best,” he says. “To have a green roof, a building must be designed to support the extra weight of the vegetation, 4 to 12 inches of planting medium, a roof membrane and a drainage layer.”

Those preparations were completed last fall on the 4,700-square-foot roof over part of the Forest Resources Building, and on the 4,500-square-foot roof over a root cellar near the greenhouses behind Tyson Building off Eisenhower Boulevard. Runoff from the green roof over the root cellar will be collected and piped to a basin where students in Berghage’s Eco-Roof Technology horticulture class can monitor the water quantity and quality, as well as plant growth and effectiveness of the green roof.

Alexandria’s nearly-complete new high school has an almost identical system, except that in its case the collected rainwater will be recycled into the building’s blackwater (toilet) plumbing. That means some of it will go into the city’s combined sewer system. CSSs are an obsolete type of sewer system where stormwater and sewage go down the same pipes. The problem is that during heavy rainfall, the system can exceed its capacity, causing a “bypass” condition where raw sewage is discharged into waterways.

Green roofs offer a number of benefits, explains Berghage. “Many older cities have sewage-treatment problems, and their sewage systems often overflow with stormwater,” he says. “Green roofs absorb and hold some of that water. Every drop of water that hits a roof must go somewhere, and usually that place is a stormwater basin. Not only do we lose valuable land by setting it aside for stormwater-retention basins, but they become an eyesore and a breeding ground for mosquitoes …”

Alexandria is slowly phasing out its CSS, but it could take literally centuries before the last pipes are separated. In the meantime, green roofs make it almost as if large buildings were actually parkland. Greeny-ness aside, that’s a very slick engineering solution, especially in a city as open space-poor as Alexandria.

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5 Responses

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  1. Christina said, on February 13, 2007 at 4:01 pm

    Berlin does this. It’s pretty cool, if you get high up, you can see all the roofs covered in grass and wildflowers.

  2. tgaw said, on February 13, 2007 at 7:31 pm

    Oh neat– it reminds me of the pillboxes we saw in Normandie– where they planted grass on top of them to camouflage it.

  3. Christina said, on February 14, 2007 at 9:34 am

    I was thinking more about this last night and remembered something that was really cool about my old workplace in Berlin. Instead of having parking on the top level of the parking deck, it was planted with grass and the day care center was located up there. The kids had a nice, safe, green place to run around and play in in the middle of the city and they even had a little wading pool up there (I had a nice view of it from up in my 9th floor office). The parks there are full of junkies and punks, and space for a day care center with land around it is limited, so I think this was a really great idea. It’s also a good way to control who can come in and out of the day care center.

  4. Clint said, on February 15, 2007 at 10:21 am

    How do you keep the roots from penetrating your roof?

    Plants are pretty industrious. I have weeds that have literally penetrated the foundation of my house, and sprouted in a concrete-floor utility room. What’s to keep the reverse from happening to my roof?

  5. Stacy said, on February 16, 2007 at 8:40 am

    Clint, I don’t know the specific answer to that, but my guess is that the planting is done in a series of boxes supported above the actual roof and connected by drain pipes to carry the water away. If nothing else you’d at least have the chance to see the roots trying to cross the gap, and stop them before they reach the building.


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