By Bud Coburn
There are two basic types of green roofs: intensive and extensive. The term “intensive” is used because these types of systems generally require a great deal of care and maintenance. Intensive green roofs integrate a large variety of different types of plants, such as vegetables, flowers, trees and shrubs. They require deeper soil and are heavier than extensive designs, and are intended for use by the public. Extensive rooftop gardens are less elaborate, featuring fewer plant species and requiring less maintenance.
Facts and Figures
- Green roofs are becoming increasingly popular in America. Many U.S. cities, such as Chicago and Washington, D.C., have begun offering incentives for their implementation. According to the EPA, roughly 8.5 million square feet of green roofs had been installed or were in progress as of 2008.
- Many well-known buildings, including an assembly plant of the Ford Motor Company, have incorporated green roofs.
- The city of Copenhagen, Denmark has passed legislation that requires the implementation of vegetation on all new roofs that have less than a 30° pitch, according to the EPA.
- Green roofs installed on just 20% of the buildings in Washington, D.C., could add an additional 23 million gallons of water storage, and reduce outflow to the storm sewer system by an average of just under 300 million gallons per year, according to the EPA.
How is a green roof implemented?
Advantages of Green Roofs
- Green roofs mitigate the problem of stormwater runoff and the precipitated water pollution it causes by diverting rainwater away from
municipal sewer systems.
- Green roofs provide insulation, thereby decreasing wintertime heating and summertime cooling needs, leading to energy savings.
- Green roofs reduce ambient air temperature, thereby combating the “heat island effect,” which is the term used to describe how urban centers generally retain excessive summertime heat.
- Pollutants, such as carbon dioxide, are filtered out of the air, lessening the associated health risks, such as asthma.
- The green roof layer may create natural soundproofing, minimizing the lower and higher frequencies of the audible noise spectrum.
- Installing a green roof extends the service life of the roof by protecting it against the sun’s UV rays.
- A building’s property value may be increased by implementing a green roof.
- Green roofs provide a source of relaxation and pleasure, especially in urban centers where local green space is typically limited.
- Intensive systems require little maintenance.
- Green roofs are appropriate for a wide variety of building types and climate zones.
- Many flat roofs are already sufficiently strong to be retrofitted with green roofs without additional structural support.
- Green roofs are less prone to leaking than conventional roofs, according to the EPA.
Disadvantages of Green Roofs
- The initial startup cost of a green roof may be high, depending on the intricacy of the system.
- Some roofs may need additional structural support to bear the weight of an intensive system.
- All green roofs require some maintenance.
- Sufficient waterproofing is always required.
- Many insurance providers do not offer coverage for issues arising from the use of a green roof.
- Extensive green roofs are not conducive to public access.
- Expert knowledge of architecture, engineering, landscaping and botany are required of those responsible for designing and installing green roof systems.
- Roofs exposed to high winds may not be suited for many plant species.
- Water bills might rise to meet the demand of non-native plants in dry climates.
Hazards of Green Roofs
- Using regular garden soil may be hazardous, as it could contain unknown pathogens and weeds.
- If vegetation becomes overly dry, it may pose a fire hazard.
- Green roofs may cause leaks if improperly installed.
- Green roofs should only be installed according to local municipal ordinances and guidelines. Special attention may need to be paid to stormwater runoff policies, especially in urban areas.
- Trained engineers should be consulted in the development of green roof systems so as to ensure the structural integrity of the roof and its weight-bearing capacity.
- When considering maximum weight, the estimated load should take into account saturated soil conditions.
- All system elements must be properly installed, especially root barriers and waterproofing, so as to avoid leaks.
- Drainage should be unimpeded.
- A clear perimeter around the roof, along with easy access to a water supply, should be maintained in case of fire.
- Ornamental grasses and any other extremely flammable plant life should be avoided. Succulents may be planted instead because they retain water and are less flammable.
- Plants should be suited for local climatic conditions, when possible.
- Weight should be evenly distributed across the roof.
- Intensive green roofs should use at least 6 inches of soil.
- Sloped roofs should include a stepped design to avoid soil erosion and runoff.