Green roofs provide many benefits. They reduce cooling loads, by reducing the temperature of the roof surface by shading it and by evapotranspiration. The surface of a green roof, and thus the air directly above the green roof, will be significantly cooler than the surface of a traditional roof and the air above the traditional roof. This reduces the heat island effect seen in urban areas (Figure 25.1). A green roof protects the roof membrane from damage caused by sun and storms, and a green roof can create a park-like environment for people to use directly or use by looking onto the greenery.
Evapotranspiration is the process where plants draw water up through their roots, transpiration, and then evaporate the water as vapor from their leaves. The water is used along with carbon dioxide to create the molecules that the plant uses to construct itself. The waste product in this process is oxygen. The water evaporating into the air above the plants reduces the air temperature. On a Chicago roof with temperatures in the 90s, a green roof’s surface temperature ranged from 91 to 169 degrees Fahrenheit. A dark conventional roof on a nearby building was 169 degrees Fahrenheit. Also, the air temperature slightly above the green roof was 7 degrees Fahrenheit cooler than the air above the conventional roof (Hogan et al. 2008, 3).
Green roofs are classified into two categories. Extensive green roofs are not very deep and have hardy plants that can thrive on a roof top and can survive through low water times. Once established these roofs do not need much maintenance, and since the growing medium is only two to four inches thick, the structural weight of an extensive roof is not great. Extensive roofs can be located on sloped roofs up to about a 30 degree slope. Intensive green roofs have a growing medium that is somewhat deeper to a lot deeper than an extensive green roof. The depth of growing medium allows an intensive green roof to be more like a garden on the ground. The depth of growing medium also creates a considerable structural load, and there is more maintenance involved taking care of the plantings. Irrigation may be necessary.
FIGURE 25.1 The heat island effect over an urban area.
Source: United States Environmental Protection Agency, Heat Islands. www.epa.gov/heatisland.