Save energy and reduce stormwater runoff. Install a green roof.
With their blanket of living plants, green roofs provide shade and remove heat from the air through evapotranspiration, reducing the temperature of the roof surface and the surrounding air. On hot summer days, the surface temperature of a green roof can be cooler than the air temperature, whereas the surface of a conventional rooftop can be a stifling 90 degrees warmer, producing what’s known as the heat island effect. Heat islands can affect communities by increasing summertime peak energy demand, air-conditioning costs, air pollution and greenhouse gas emissions, heat-related illness and mortality, and water quality.
Many communities are taking action to reduce urban heat islands, including installing green roofs. Green roofs can be added to a wide range of buildings, from industrial facilities to private residences. They can be as simple as a 2-inch covering of hardy groundcover (known as an extensive green roof) or as complex as a fully accessible park complete with trees (an intensive green roof). Green roofs are becoming popular in the United States, with roughly 8.5 million square feet installed or in progress as of June 2008.
Starting from the top, an extensive green roof has a layer of plants. These are typically low-growing, shallow rooting, drought tolerant plants called sedums. There are many different varieties of sedum, with different coloration and different flowering characteristics so a roof can have a varied appearance. The plants are supported by a shallow layer of growth medium, an engineered mixture of lightweight soils, vermiculite, and other materials that provides a good environment for the sedum. The minimal depth of the soil aids in keeping weeds from establishing themselves on the roof, but there may be some local plants that can survive in that environment, and they may take root on the roof as well.
Underneath the soil are several membrane layers. There is also a drainage layer (to allow excess water to move freely rather than carry the soil off the roof in a mudslide) and a root barrier layer that keeps the roots from penetrating the roof. The roof membrane sits on the roof deck, insulation or structure of the building, much like a conventional roof.
Installing the simpler extensive green roof may cost around $10 per square foot, and annual maintenance costs will run around $0.75 per square foot. Intensive systems are more expensive. Toyota recently introduced green roofing tiles--modular, interlockable grass tiles that are much lighter than other systems and easier to install. They're more expensive than other green roof products, but prices are likely to drop as demand increases. While the initial costs of green roofs are higher than those of conventional materials, building owners can help offset the difference through reduced energy and stormwater management costs, and potentially by the longer life span of green roofs compared with conventional roofing materials.
Click here learn how to install a green roof garden.
See NRDC's Greening Advisor for more information on reflective and green roofs.