Save energy, reduce stormwater runoff, start a green roof.
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Vegetated roofs, or green roofs have a layer of living plants on top of the structure and the waterproofing elements. Green roofs provide shade and remove heat from the air through evapotranspiration, reducing temperatures 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°F (50°C) warmer, creating 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 installed on 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 extensive green roofs) or as complex as a fully accessible park complete with trees (referred to as intensive). 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, which are typically sedums. These are low-growing, shallow rooting, drought tolerant plants. There are many different varieties of sedum, with different coloration and different flowerings, so that a roof can have a varied appearance, rather than looking like an entire crop of a single variety. The plants are in a growth medium, an engineered mixture of lightweight soils, vermiculite, and other materials that provides a good environment for the sedum. The shallow depth of the soil aids in keeping weeds from establishing themselves on the roof, since most weeds cannot survive in the arid and shallow soil conditions on a vegetated roof. Local plants that can survive in that environment may establish themselves on the roof, as well. Underneath the soil are several membrane layers, rather than just a single membrane roof. There is also a drainage layer (to allow excess water to move freely, rather than lifting the soil and having it flow off the roof in a mudslide, and a root barrier layer, which 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, while 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 a lot lighter than other systems and easier to install. They're more expensive than other green roof products, but prices will likely 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 lifespan of green roofs compared with conventional roofing materials.
Click here to learn how to install a green roof garden.
The EPA has excellent resources for those interested in learning more about heat island effect and green roofs.
