Green Roofs are Cool

... and can make a major dent in the "urban heat island" effect in our cities -- that is, the increased temperature due to lots of paved surfaces that absorb the sun's heat.

As part of this February's Smart Growth Conference, a group of us had the chance to visit the green roof atop the headquarters of the American Society of Landscape Architects (ASLA) in their historic downtown Washington, D.C. offices.

But before Nancy Somerville, ASLA's Executive Vice President, shepherded us up to the roof, she told us about some of the benefits -- and challenges -- of installing the green roof.

Retrofitting a green roof on an older, historic building was not easy -- or cheap. One of the major expenses was the need to construct a new stairway to the roof, so visitors wouldn't have to clamber up a ladder. The roof also includes a grate walkway so people can circumnavigate the roof and view the varied plantings. And given that this roof is under the stewardship of landscape architects, it's not surprising that the roof's design is quite attractive and the plantings diverse.

The ASLA green roof has a unique feature: it is shaped into two "waves" or mounds. They're formed with thin layers of styrofoam insulation, and serve to hide the HVAC systems. As Somerville explained, they also allow the plantings to be at eye level, helping create a more attractive amenity for visitors. [Click on the drawing to view it at larger size].

But you have to bear in mind that ASLA's green roof is a model / demonstration project, a living

laboratory to help landscape architects learn about the functioning of an urban green roof. So it wouldn't be fair to hold it to a strict cost-benefit accounting.

In fact, as Somerville (above left) noted, one of their goals is to carefully monitor and measure the impacts the roof has on temperatures and rainfall retention, and to also see which plants thrive best.

Which brings us to potential benefits. Somerville ticked off several, from lowering urban temperatures, to reducing stormwater runoff, to improving air quality, to reducing energy use inside the building, to creating new biohabitats.

Perhaps the key reason for the growing popularity of green roofs lies in the fact that between 15 and 25 percent of the surface area of our cities is comprised of roof tops. Since most rooftops today consist of impervious, often dark-color surfaces, they do nothing to reduce either temperatures or stormwater runoff. But add a few inches of soil and plantings, and you get some dramatic differences.

The first year results for ASLA's green roof, for example, showed that it retained 74 percent of all rainfall (some 29 inches) -- including 100 percent of all rainfalls of less than 1 inch. Reducing urban stormwater runoff can yield environmental benefits, as well as save cities costly infrastructure investments.

How much of a soil layer do you need on a green roof? There's no single answer. As Somerville pointed out, what are termed "extensive" green roof areas typically consist of just 2 to 6 inches of soil or growing medium, and are quite light in weight. "Intensive" roof areas can have depths of 6 inches or more, and will support a broader range of plants. Part of what is possible, depends on the roof's structural capacity. On the ASLA building this varies in different parts of the roof. For example, more intensive planting are possible above the elevator shaft which offers greater structural support.

Before we left Somerville reminded us of the "amenity value" of having a green roof. With wonderful views and cooler temperatures, it can offer a great place for employees to take a break. Not surprisingly, green roofs can increase a building's real estate value.

Can't visit the ASLA's green roof? Take a look with their rooftop webcam! And visit their green roof project web site for more details and performance data on the roof.