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Anne Vazquez

Landscape Architects Put Green Roof To Test

In May 2006, the American Society of Landscape Architects (ASLA) began to monitor the performance of a green roof installed on its Washington, DC headquarters building.

By Anne Vazquez

In May 2006, the American Society of Landscape Architects (ASLA) began to monitor the performance of a green roof installed on its Washington, DC headquarters building. The Society decided to embark on its “Green Roof Project” when its 10 year old headquarters building was due for a roof replacement. Environmental values and an increasing interest in green (vegetative) roofs spurred decision makers at ASLA to specify this type of roof for the project.

photo Image: Michael Van Valkenburgh Associates, Inc.

“Environmental stewardship has been part of the core mission of ASLA since its founding,” says Nancy Somerville, executive vice president and CEO of the Society. “So when faced with a deteriorating roof, we knew that if our building structure would be able to handle it, we wanted to do a green roof. It would also be a positive way to address some of the urban environmental issues faced by communities. We wanted it to be a very visible demonstration project.”

The 3,300 square foot roof, designed by Michael Van Valkenburgh Associates of New York, NY, features both intensive and extensive plantings (rendering shown here). An intensive green roof is thicker and can support a wider variety of plants; however, these add more weight and require more irrigation and maintenance. Extensive green roofs feature a soil layer that is thinner (usually 4" or less) than intensive models. Both types of plantings were used in the ASLA project so managers would be able to evaluate the performance of each approach.

In addition to plant performance, primary areas of focus were: stormwater retention; quality of stormwater runoff; energy saved in heating and cooling the building; and reduction of the urban heat island effect.

Stormwater retention:

When a facility is able to reduce the amount of stormwater that leaves its location and enters the municipal sewer system, it lightens the load on public infrastructure. Green roofs have been touted as one way to achieve this objective, because the plants absorb some of the rainwater. It was found that, between July 2006 and May 2007, the ASLA roof retained nearly 75% of rainfall there, which totaled 29". This prevented 27,500 gallons of water from entering the municipal sewer system.

According to the data recorded, the proportion of water the green roof was able to retain depended on the amount of rainfall during a given event. For instance, researchers found that the roof typically retained 100% of a 1" rainfall, but with heavier rain, the percentage of retention decreased. Additionally, it was found that the roof retained a higher percentage of water during the plants’ growing season.

Quality of stormwater runoff:

The absorbent nature of green roofs means that much of the ground (or roof) surface pollutants that would otherwise get picked up by rain (headed for the sewer system) are instead trapped in the green roof soil. For its roof, ASLA reported that, because of the amount of water retained, the roof provided a reduction in the amount of nitrogen (a nutrient) introduced back to the watershed (via the municipal water supply).

There were, however, higher amounts of other nutrients (e.g. phosphorus) and heavy metals (either from roof materials or particulate matter in the air) in the stormwater runoff. ASLA notes amounts were below EPA standards and below levels expected from street runoff. Such runoff is typical of “young” green roofs, notes the ASLA report, and based on other research, nutrient levels are expected to decrease.

Somerville explains that these comparisons involved the quality of the runoff from the green roof versus the quality of rainwater that had not hit a surface. ASLA plans to compare runoff water quality from its green roof with that of conventional roofs in the future.

Energy Savings:

The engineering analysis for the project found that the green roof created a 10% reduction in ASLA’s building energy use during winter months.

In summer months, energy use reduction was “negligible.” This was surprising to the team, and further analysis showed the roof should contribute a 2% or 3% savings in the summer. As with many sustainability issues, there were multiple forces at work. The team looked at overall heating and cooling usage and identified the probable cause of this deficit: overcooling of the building. ASLA will alter the cooling schedule this summer with further monitoring.

Urban heat island effect:

Another widely recognized benefit of green roofs is reduction of the urban heat island effect. Occurring in densely built up areas, this condition manifests itself in urban locations in the form of average temperatures 5°F to 7°F higher than non-urban areas. This is caused by a lack of vegetation (which helps to cool air temperatures) and is compounded by the waste heat expended from facilities running air conditioning to combat outside heat.

Using data loggers on its roof and several nearby buildings, ASLA determined its green roof had been as much as 32°F cooler than conventional black roofs; temperature differences were the greatest on the hottest days. The ability of vegetation to reduce air temperatures was evident, with areas with thicker plant growth being coolest.

As more data on its green roof is made available by ASLA, fms can refer to the findings to decide if such a roof should top their buildings.

Research for this article included an interview with Somerville along with existing information on the ASLA Green Roof Project. To learn more, visit www.asla.org/land/050205/greenroofcentral.html.

Pondering a green roof? What are your concerns? Send thoughts to avazquez@groupc.com.

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