by Nick Gromicko, CMI®
"Green roofs" are areas of roofs of residential, commercial and industrial buildings that are utilized as space for growing vegetation. They are actually built onto the roof's
surface in contrast to a rooftop container-garden or rooftop garden, in which plants are kept in individual containers. Green roofs capture precipitation, use it for plant growth, and then return it to the atmosphere through evapotranspiration, as opposed to typical bare roofs, which repel moisture and heat. The use of the term "green" refers to the roof's association with the environmental movement, and does not necessarily refer to the color of vegetation being grown. Green roofs can range in complexity from a basic layer of soil to intricate, irrigated landscaped gardens frequented by the public for pleasure.
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?
It is possible to purchase green roofs as complete systems. However, many systems are constructed on site using a layering process. Waterproofing material is first laid directly on top of the roof. Next is a layer that inhibits root growth and acts as a barrier between the garden and the roof itself. In some cases, this is the most expensive component of the system. The root barrier may be achieved by the use of a copper laminate or other chemical additives. An insulation layer may be laid next, followed by a drainage layer. These are followed by the growing substrate (usually soil), which may be comprised of organic and/or inorganic matter. Vegetation is then planted on top. A protective cover may be added over top of newly developing plants to ward against excess wind and to encourage root establishment. Some systems implement rainwater harvesting as a means to irrigate vegetation during periods without rainfall. Others use municipal water supplies. Drip-irrigation may be utilized, especially in larger systems. Green roofs installed onto pitched roofs tend to follow a simpler design that requires less waterproofing and drainage.
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.
In summary, green roofs are an economical and environmentally friendly way to take advantage of unused roof space. Those considering installing them should enlist the aid of qualified landscape engineers for design options and maintenance requirements, as well as structural engineers familiar with local code requirements.