Several key factors need to be considered when designing a green roof, says Barbara Denson, director of origination, Global Markets, for Weston Solutions, in an article written for EHS Journal. This includes weight, ranging from 18 to 22 pounds per square foot for an extensive roof and 30 pounds or more per square foot for an intensive roof. Maintenance and irrigation options also need to be considered in the choice of plants, and the potential for high winds also needs to be factored into the design, she says.
Extensive roofs consist of six inches or less of growing medium depth, along with lower weight, lower plant diversity, and lower cost and maintenance, while intensive green roofs have more than six inches of growing media and typically feature higher plant diversity, higher weight, and higher cost and maintenance, reports The Green Daily.
A simulation by Environment Canada found that a typical one-story building with a grass roof and 3.9 inches (10 cm) of growing medium results in a 25 percent reduction in summer cooling needs, while field experiments in Ottawa found that a 6-inch extensive green roof reduced heat gains by 95 percent and heat losses by 26 percent compared to a conventional reference roof, reports The Green Daily.
If a building is applying for the U.S. Green Building Council’s Leadership in Energy & Environmental Design (LEED) credits, the depth and percent coverage of the roof should be considered, says Denson.
Benefits from green roofs range from filtering pollutants in the air and creating a habitat for wildlife to reducing the heat island effect and cutting heating and cooling bills. It can also reduce storm water runoff, decreasing the size of the water retention pond, says Denson.
She says one of the major design considerations in a green roof system is its storm water retention capability to help mitigate flooding. She cites studies that show that in a typical storm, Weston’s 4-inch-deep GreenGrid Modular Green Roof System reduces storm water runoff by approximately 75 percent of the first 1 inch of rainfall.
She says modular systems like the GreenGrid are easier to install and make it easier to repair the rooftops.
Green roofs filter stormwater due to the plants and growing media, which improves the quality of the runoff, reports The Green Daily. The Website also says that a green roof can boost solar panel efficiency by reducing the ambient temperature on a roof, while solar panels can help protect the roof from high winds.
The Website recommends using a certified Green Roof Professional (GRP) when designing your green roof. These certified professionals are knowledgeable in the areas of design, installation, waterproofing, drainage, plants and growing media.
A recent study shows that Chicago, Washington D.C., and Baltimore are among the cities in the U.S. with the most green roofs installed.