Green roofs: Gardens are going up and away


UNIVERSITY PARK, Pa. — Green roofs seem to be sprouting all over the country these days. From New York to Chicago, from office buildings to schools, many communities are getting excited about the potential of green roof technology.

And they should be, said Robert Berghage, associate professor of horticulture at Penn State University and director of the Penn State Center for Green Roof Research.

“There’s a lot of cool stuff going on with research and development in green roofs,” he said. “Especially at Penn State.”

Stormwater management

The concept of green roofs actually began 30 years ago in Germany as a way to approach stormwater management.

North America, with Penn State being one of the leading institutions, has been conducting its own research for the past 10 years.

In fact, Penn State was the first institution to offer a class where students can design and construct green roofs and living walls on real structures.

Researchers have found that instead of piping stormwater away from drains, treating it and then reusing the water source, a properly installed green roof can capture the stormwater and use it to nourish plants.

Current research is focusing on stormwater runoff quality and quantity. Berghage said a typical green roof in central Pennsylvania can retain about 50 to 60 percent of the annual rainfall.

Researchers are also measuring how stormwater runoff quality varies through different roofing systems — green, metal and rubber.


Another large part of the research is concentrated on types of plants grown and types of soils and media used.

Berghage said he and his colleagues are constantly trying to determine what plants are the most effective, less temperature sensitive and easiest to maintain in a green roof environment.

Benefits of going green

In addition to stormwater management, green roofs can provide other advantages:

* Roof longevity. Berghage said that the cost to install a green roof varies from $8 to $30 per square foot, but it is more expensive than traditional roofing. The average costs for a shingle roof and a rubber roof are $2-2.50 and $5-6 per square foot, respectively.

However, he said, when you factor in the longevity with green roofs, that cost decreases dramatically. A typical rubber roof will need replaced in 15 to 17 years. If that rubber roof is protected with a green roof, Berghage said it can last at least twice as long.

* Energy benefits. A green roof acts as a thermal mass that decreases air conditioning bills in the summer, and, if the rest of the building is properly insulated, can help even out the temperature swings of the winter.

* Biodiversity. Berghage said many people are using green roofs as a wildlife habitat to attract birds.

Given Penn State’s efforts in the colony collapse disorder of honeybees, Berghage said the roofs also provide a great place for pollinators.

* Growing potential. When it comes to green roofs, the possibilities are endless.

“You can grow anything on a rooftop that you can grow on the ground,” Berghage said.

That is, he added, with the proper growing conditions and care.

“There’s no such thing as a no-maintenance green roof.”

But people are putting in the effort. From sky-high community gardens to aesthetically pleasing roofs complete with observation decks, people can get quite creative with green roofs.

For example, the 4,700 feet of green roof of Penn State’s Forest Resources building, completed in 2006, is able to be enjoyed from an observation deck, accessible from the second floor of the building inside.

Can your roof go green?

Before you start dumping soil and trying to make things grow on the roof of your office or home, Berghage recommends you consider these factors.

1. Make sure the building can support the green roof. Berghage advises that you have a structural engineer assess the strength of the building and roof.

A typical green roof designed mainly for stormwater management will require at least 4 inches deep of soil and will weigh an extra 30 pounds when fully wet.

Berghage said a good rule of thumb for a simple roof using a lightweight media is about 7 to 7.5 pounds per square foot of roof per inch of media depth.

Green roofs designed to grow vegetables or plants that require more management may also need more soil, causing the roof to be that much heavier.

Many older buildings were not designed with that much tolerance, Berghage said. The last thing you want is your green roof coming down through your ceiling.

2. Decide your roof’s purpose. Do you want your roof strictly for stormwater management purposes or is its function to be beautiful and provide a place for you to relax? Are you installing a green roof to help with energy costs or to provide a wildlife habitat?

Berghage said one roof may be able to accomplish all of these functions, but the more specific you are with your goal, the more the design will vary. He added that the more complicated the roof is, the more maintenance it requires.

3. Work with the design team. When installing a green roof, it’s important to think about all of the steps that need to happen ahead of time, Berghage said.

For example, if you need to install windows, make sure that’s done before laying down soil. Berghage said it is becoming easier and easier to find experienced contractors who have installed green roofs and understand the process.

4. Protect the base. Before installing the green roof, the base roof must be waterproofed and tested. If it has not been tested or did not pass, an additional root barrier is required.

Drainage is also needed to divert extra stormwater runoff away from the roof. Drainage can be granular material such as gravel or a synthetic layer such as filter fabric.

5. Select media and plants. The media should be tested and meet the FLL guidelines developed by Germany, and accepted as industry standards, for the planning and upkeep of green roofs.

Berghage said most media combine a lightweight aggregate like clay, slate or shale with organic compost. Less costly materials like fine gravel and sand can be used, but they are heavier.

Plant selection goes back to the purpose of the roof. Also consider the climate, irrigation, depth of media and maintenance constraints.

Images of Penn State’s green roofs and green roof displays

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