For over 4,000 years from the hanging gardens of Babylon to the Nordic Vikings, many cultures have used green roofs as method of construction for homes, barns, and communal buildings.  This vernacular tradition, strongly associated with Scandinavian countries, was very well suited to insulating dwellings in harsh climates and maintaining a more consistent internal temperature.  As technology has advanced in both architecture and landscape through the development of better materials and proprietary green roof systems, roof gardens have become more commonplace across the globe.  Green roofs are thriving in a range of climates from the sub-arctic to the equatorial tropics. 

The definition of green roofs has also evolved from the traditional sod roof and can be categorized under two common definitions—extensive and intensive. An extensive green roof predominantly utilizes shallow soil depths and low profile hardy plants.  These are more often for insulating buildings and reducing run-off and are generally not trafficable. Intensive green roofs, which tend to have greater soil depths and a wider variety of plant species including trees, may also incorporate accessible open space.

Photo 1 - Roof Garden


Photo 2 - Sod Roof

Photo 3 - Hanging Gardens of Babylon

One of the primary benefits of any style of green infrastructure is reduced energy consumption.  A green roof assists in reducing temperature transfer and in maintaining a more consistent temperature throughout the day and night.  In an Urban Environment, just as tree canopies assist in reducing urban heat island effect, so do green roofs providing a broader benefit to the community.  When used in conjunction with solar panels, an extensive green roof enhances the efficacy and energy production of the solar panels by providing a stable roof temperature and mitigating excessive reflected heat.

In terms of Green Infrastructure, green roofs assist in both reducing the speed and volume of building runoff as well as improving water quality.  Captured water from ground floor paved surfaces can be passed through roof top vegetated areas in order to reduce hydrocarbons, nitrogen, and phosphates. Another environmental benefit is the effect vegetation has for improving air quality, as well as carbon capture and oxygen production. This improvement in air quality can have an impact on health concerns such as asthma and other respiratory diseases.

Several studies have also shown mental well-being and physical health benefits may be derived from proximity to green space.   Biophilia is the innate human desire to be surrounded by plants. That access can be in a park, a natural environment, or a green roof, all of which can provide access to open space for recreational activities and respite from hectic city life at street level. 

Another benefit in cities where space is at a premium is urban agriculture on rooftops. New York, Singapore, and Hong Kong utilize their precious space to reduce the amount of food imported from outside of the city limits through urban farms.  These farms increase food security, reduce the city’s carbon footprint, and provide avenues for employment.  Aquaponics is an example of how to return up to ten times greater yield than traditional farming methods.

Green roofs also provide habitat and biodiversity.  Pollinators such as butterflies, bees, insects, and birds seek food, shelter, and water in these environments.  Providing safe refuge for these creatures that in return assist in pollinating our crops and ensures our own food security.

Photo 4 - Roof Garden

Photo 5 - Roof Garden

Last but not least, just as street trees improve a neighborhood’s property values, green roofs will provide an increase in a building or apartment’s value.  A Canadian study showed an 11 percent increase in property values for buildings with recreational green roofs.  This is one factor behind the boom in accessible open space on the roofs and terraces of apartment buildings in inner city areas.  Green roofs are also shown to extend the lifespan of a building’s roof by up to 40 years, by reducing the rooftop’s direct exposure to the elements and mitigating severe changes in temperature.

In Minnesota, there is an ever-increasing desire to incorporate green infrastructure into projects for many or all the above reasons.  Some examples include of green roofs in the include the Target Center, Minneapolis Central Library, and the Saint Paul Fire Headquarters. 

Sources and References:

Tomalty, R. & Komorowski, B. 2010, The Monetary Value of the Soft Benefits of Green Roofs, Smart Cities Research Services

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