Living in a “stone jungle”? Try a green roof!

The twentieth century witnessed massive urbanization and urban growth. Urbanization effects on the environment include flooding, deforestation, urban climate modification, and more built surfaces. In urban environments, built surfaces exhibit higher temperatures than the surrounding air. This happens due to reduced evapotranspiration caused by a decrease in vegetated surfaces.

The heat signature exhibited by built surfaces in urban areas is termed as an urban heat island. This phenomenon has been observed in some major North American cities, such as Phoenix, Los Angeles, Houston, Atlanta and New York City. An example of an urban heat island is shown in the picture below, which is a thermal image of Atlanta taken by NASA through a specially outfitted Learjet in May 1997. Blue indicates cooler temperatures, red indicates warm temperature and white indicates hot pockets.

Caption: Typical example of UHI daytime thermal image of Atlanta. Photo courtesy of NASA/Goddard Space Flight Center Scientific Visualization Studio.

According to a study by Akbari et al., the summertime maximum monthly temperature of downtown Los Angeles has increased by 2.5^oC from 1920 to 2001. The increase of urban air temperature associated with urban heat islands leads to an increase in energy use of air conditioning in warm summer months. Is there a solution to abate the negative impacts of urbanization-induced built surfaces? In order words, is there a response to urban heat islands?

The answer is Green Roofs. In an urban environment, Green Roofs, or vegetated roofs, serve as an alternative to vegetated grounds by making use of the available open roof surfaces. Vegetation helps cool urban air temperatures and reduces urban heat islands by cooling the atmosphere through evapotranspiration. Hence, an increase in vegetated areas is a path towards greener and more ecological city development.

In fact, Green Roofs are nothing new. One of the oldest known Green Roofs were the Hanging Gardens of Babylon. They were built by King Nebuchadnezzar II in 650 BC to recreate the land of his wife including trees and plants in the hope to cure her homesick heart. Twenty-six centuries later, Germany is the pioneer in Green Roofs research and application.

In the past, Green Roofs were used for aesthetics. Today, in addition to aesthetics, such roofs are used for urban heat island mitigation, reduction in energy cost for air conditioning, and storm water management by reducing the runoff volume and improving the quality of water by filtering.

That said, there are two types of Green Roofs: extensive and intensive.

Extensive Green Roofs are shallow with a soil depth of 2 to 6 inches, planted with drought tolerant grass, mosses, sedums and thymes. Photo copyright: ZinCo.

Intensive Green Roofs have soil depth of greater than 6 inches and are planted with larger plants. Intensive green roofs require more maintenance and irrigation. Photo copyright: ZinCo.

Green Roofs…

•         reduce urban heat island effect,

•         reduce costs of summer air conditioning,

•         reduce the runoff volume and peak discharge,

•         serve as natural filters for bacteria and fungi and thus improve the quality of water going into streams and waterways owing to the natural filtering process of root system’s bacteria and fungi,

•         provide sound insulation,

•         have a longer life span when compared to conventional roofs,

•         increase evapotranspiration rates and have a positive influence on urban microclimate, and

•         look better than conventional roofs!

To sum it up, Green Roofs have numerous benefits and could play an important role in sustainable urban development.

Indumathi Jeyachandran (indu_jey[at]yahoo.com)

Geno Haus in Stuttgart, Germany. Photo by Peter Philippi.

Recommended reading:

Akbari, H., Pomerantz, M. and Taha, H., 2001. Cool surfaces and shade trees to reduce energy use and improve air quality in urban areas. Solar Energy, 70(3): 295-310.

Alexandri, E. and Jones, P., 2008. Temperature decreases in an urban canyon due to green walls and Green Roofs in diverse climates. Building and Environment, 43(4): 480-493.

Mentens, J., Raes, D. and Hermy, M., 2006. Green Roofs as a tool for solving the rainwater runoff problem in the urbanized 21st century? Landscape and Urban Planning, 77(3): 217-226.

Peck, S.W., 2001. Green Roofs: Infrastructure for the 21st century exploiting the last urban frontier. Interface (Raleigh, North Carolina), 19(11): 4-12.

Rosenfeld, A.H., Akbari, H., Romm, J.J., and Pomerantz, M., 1998. Cool communities: Strategies for heat island mitigation and smog reduction. Energy and Buildings, 28(1), 51-62.