Green Roofs

A green roof is one that’s been covered with a layer of soil and vegetation. Growing plants on rooftops can replace some of the vegetation that was removed when the building was constructed. Installing green roofs reduces the negative impacts of development while providing numerous environmental, economic, and social benefits. Green roofs can absorb rainwater, provide insulation, create habitat for wildlife, contribute to a more attractive urban environment, and reduce the urban heat island effect.

Hanging Gardens of Babylon. Source: Wikipedia.

The earliest documented roof gardens were the hanging gardens of Babylon in modern-day Syria, considered one of the seven wonders of the ancient world. Germany was one of the first countries to embrace green roof technology in modern times and constructing green roofs has grown in popularity throughout the world.

There are two different types of green roofs: intensive and extensive. Intensive green roofs include trees, shrubs and are limited to flat roofs. They are high maintenance and high cost.  Extensive roofs include herbs, grasses, mosses and drought tolerant plants. They are generally lower maintenance and cost less. However, they may not provide the same aesthetic benefits.

How Green Roofs are Built

Building a green roof can be a bit of an engineering challenge; it’s important that a building is first inspected to make sure it can structurally accommodate the extra weight. If so, a waterproof layer and a root resistant layer is first placed down on the rooftop. Excess water should be drained into the roof’s gutters, and the angle of sunlight should be appropriate for plant growth. Due to wind exposure, intense sunlight, moisture stress, severe drought, and elevated temperatures, green roofs can be a challenging location for some plants to grow. Plant selection must take into consideration site, micro-climate, and aesthetic factors.

Typical layering used in constructing green roofs.

Benefits of Green Roofs

Green roofs are associated with a variety of benefits. Due to root uptake and surface storage, excessive water problems can be reduced. Stormwater can be absorbed by the vegetation and released slowly over several hours, reducing the impact of flooding at ground level.

The vegetation provides habitat for birds, insects, and animals, creating small ecosystems and microclimates. The flowering plants allow pollinators such as bees to be introduced into the urban environment as well. While these ecosystems are small, they can increase the biodiversity found in cities.

A green roof on Chicago’s city hall. Credit: National Geographic.

Green roofs contribute to improvement of urban air quality and can even help cool the urban environment. In urban areas, vegetation can reduce the impact of the urban heat island. Vegetation has largely been replaced by impervious, man-made surfaces in built-up areas. These surfaces retain heat, leading to an increase in temperature compared to the suburban and rural areas, especially at night. This effect can be reduced by increasing the reflection rates of incoming radiation or by increasing vegetation cover. A regional simulation model using 50% green-roof coverage distributed evenly throughout Toronto showed temperature reductions as high as 2°C in some areas (Bass et al. 2002).

Another advantage of green roofs is their ability to provide aesthetic and psychological benefits for the urban populations. They can provide visual improvements, spaces for relaxation and restoration, and promote physical and psychological health. They can also be used for urban agriculture with food production providing nutritional, economic and educational benefits.

Green roofs reduce sound and air pollution by absorbing sound waves and harmful gases from the traffic below. The vegetation can filter out atmospheric pollutants before they reach ground level. Plants can also act as a sound buffer, preventing noise from above such as airplanes from reaching the residents on the top floor.

Finally, green roofing can be cost effective. Due to the waterproof membrane covering, the vegetation shields the roof from UV light and physical damage. This can extend the lifespan of a roof by over 200%.

Green Roofs Around the World

Germany and Scandinavia have been the forerunners at implementing green roofs. Green roof have received government supported since 1970’s. The US, however, is beginning to catch up. Chicago has more green roofs than any other US city with a total of approximately 7 million square feet of coverage.

Washington D.C has set a goal of 20% green roof coverage by 2020. Toronto is the first city to mandate green roofs on any industrial or residential building with over 21,500 square feet. By doing so, it is estimated that the city would save $37 million a year in savings on storm water management, energy bills and costs associated with the urban heat island effect.

San Francisco passed a bill in January 2017, which will require between 15 and 30% of roof space on construction projects to be devoted to green roofs. In parts of the US, including New York, incentives for green roofs in the form of tax benefits have been issued to promote the installation of vegetation on buildings.

Sources:

Bass, B., Krayenhoff, ES., Martilli, A., Stull, RB. and Auld, H.2003 The impact of green roofs on Toronto’s urban heat island Pages 292 – 304 in Proceedings of the First North American Green Roof Conference: Greening Rooftops for Sustainable Communities; 20–30 May, ChicagoToronto (Canada) Cardinal Group

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