Sustainable By Design: Landscape Architects Put Green Roof To Test

By Anne Cosgrove
Published in the March 2008 issue of
Today’s Facility Manager

InMay 2006, the American Society of Landscape Architects (ASLA) began tomonitor the performance of a green roof installed on its Washington, DCheadquarters building. The Society decided to embark on its “Green RoofProject” when its 10 year old headquarters building was due for a roofreplacement. Environmental values and an increasing interest in green(vegetative) roofs spurred decision makers at ASLA to specify this typeof roof for the project.

photo Image: Michael Van Valkenburgh Associates, Inc.

“Environmentalstewardship has been part of the core mission of ASLA since itsfounding,” says Nancy Somerville, executive vice president and CEO ofthe Society. “So when faced with a deteriorating roof, we knew that ifour building structure would be able to handle it, we wanted to do agreen roof. It would also be a positive way to address some of theurban environmental issues faced by communities. We wanted it to be avery visible demonstration project.”

The 3,300 square footroof, 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 ofplants; however, these add more weight and require more irrigation andmaintenance. Extensive green roofs feature a soil layer that is thinner(usually 4″ or less) than intensive models. Both types of plantingswere used in the ASLA project so managers would be able to evaluate theperformance of each approach.

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

Stormwater Retention. Whena facility is able to reduce the amount of stormwater that leaves itslocation and enters the municipal sewer system, it lightens the load onpublic infrastructure. Green roofs have been touted as one way toachieve this objective, because the plants absorb some of therainwater. It was found that, between July 2006 and May 2007, the ASLAroof retained nearly 75% of rainfall there, which totaled 29″. Thisprevented 27,500 gallons of water from entering the municipal sewersystem.

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

Quality of stormwater runoff. Theabsorbent nature of green roofs means that much of the ground (or roof)surface pollutants that would otherwise get picked up by rain (headedfor the sewer system) are instead trapped in the green roof soil. Forits 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).


Therewere, however, higher amounts of other nutrients (e.g. phosphorus) andheavy metals (either from roof materials or particulate matter in theair) in the stormwater runoff. ASLA notes amounts were below EPAstandards and below levels expected from street runoff. Such runoff istypical of “young” green roofs, notes the ASLA report, and based onother research, nutrient levels are expected to decrease.

Somervilleexplains that these comparisons involved the quality of the runoff fromthe green roof versus the quality of rainwater that had not hit asurface. ASLA plans to compare runoff water quality from its green roofwith that of conventional roofs in the future.

Energy savings. Theengineering analysis for the project found that the green roof createda 10% reduction in ASLA’s building energy use during winter months.

Insummer months, energy use reduction was “negligible.” This wassurprising to the team, and further analysis showed the roof shouldcontribute a 2% or 3% savings in the summer. As with manysustainability issues, there were multiple forces at work. The teamlooked at overall heating and cooling usage and identified the probablecause of this deficit: overcooling of the building. ASLA will alter thecooling schedule this summer with further monitoring.

Urban heat island effect. Anotherwidely recognized benefit of green roofs is reduction of the urban heatisland effect. Occurring in densely built up areas, this conditionmanifests itself in urban locations in the form of average temperatures5°F to 7°F higher than non-urban areas. This is caused by a lack ofvegetation (which helps to cool air temperatures) and is compounded bythe waste heat expended from facilities running air conditioning tocombat outside heat.

Using data loggers on its roof andseveral nearby buildings, ASLA determined its green roof had been asmuch as 32°F cooler than conventional black roofs; temperaturedifferences were the greatest on the hottest days. The ability ofvegetation to reduce air temperatures was evident, with areas withthicker plant growth being coolest.

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

Researchfor this article included an interview with Somerville along withexisting information on the ASLA Green Roof Project. To learn more,visit

Pondering a green roof? What are your concerns? Send thoughts to