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How Can You Make Choices that Treat the Earth Well

Green Roofs

Cool and Insulate Buildings

Hectares of New Space

Frequently, city roofs are black They are covered with waterproofing material to make sure they don’t leak. Like our city streets, they offer a black surface to the sun’s heat. Black is a heat-absorbing surface colour. The combination of black pavement and black building roofs makes downtown areas absorb a great deal of heat during hot summer days. This heat absorption often makes temperatures in the urban core rise higher than those of the surrounding countryside. The higher temperatures in the city are sometimes referred to as “urban heat islands.”

As global temperatures warm with the effects of climate change, the “heat island effect” in some cities is becoming stronger. This temperature effect is clear in Toronto. It is also particularly notable in Tokyo, where every summer dating back to the Edo period, Tokyo has been attacked by a fierce heat wave. Tokyo’s average summer temperature has risen by an amazing 3oC over those of 100 years ago, five times faster than the rate of global warming. This excess heat absorption has become a serious problem to the health and comfort of residents.

To mitigate their heat island problem, Tokyo’s government has mandated that roof space on top of new buildings larger than 1,000 square meters must feature at least 20 percent green roof. A flat, black tar rooftop can get as hot as 185oF on a summer day. The same roof covered with vegetation, would register a mere 75oF. In Germany, a estimated 10% of buildings have green or “eco-roofs,” totaling more than 13 million square metres (2004). Some cities in Germany now require green roofs on flat-roofed buildings. In Shanghai, China, the largest roof top garden in the city was recently completed on top of a slaughterhouse. The greenery was added to help deal with the smell from the installation, and to make the environment more beautiful. The Shanghai roof garden includes the creative feature of a fish pond installed on a glass ceiling, that allows people working inside the slaughterhouse to enjoy a shadowy view of fish moving about right above their heads.

The green roof approach to buildings is still quite a new trend in North America, but is catching on quickly. The city of Portland, Oregon, has encouraged eco roof projects since 1994, in large part due to concern over storm water runoff The city of Toronto has added an experimental green roof to its City Hall, that is open to the public. It’s known as the Green Roof Infrastructure Demonstration Project.

From Babylon to High Tech

Putting plant materials on roofs is not a new idea. Some experts believe that the custom of planting hanging gardens on arches or roofs evolved in the Orient. As early as 900 BC there are reports of roof gardens in the Orient. In the 6th century BC, the legendary Hanging Gardens of Babylon were built by Nebuchadnezzar. His magnificent roof garden was considered to be one of the seven wonders of the ancient world. The roofs of wealthy patrician Roman houses were also sometimes lavishly decorated with plants.

The more practical roots of roof greening in our northern latitudes can be said to be based on the positive effects of spontaneous vegetation that grows on roofs without cultivation. Scandinavian and Icelandic earth houses featured roofs covered with grass sod, or “grass roofs.” These traditional grass-covered roofs provided highly effective insulation and heat storage for their occupants during the harsh northern winters. They are still used today, with a more modern under-layer to hold up the soil and sod.

Governments, architects, businesses, horticulturalists and engineers are recognizing the potential of vegetated roofs for the 21st century. They are joining forces to update an old technology into a new one that they call “green infrastructure:” using soil and living things to increase energy efficiency and healthier city living.

The Green Roof Advantage

The benefits from putting vegetation on top of buildings are impressive. Members of the growing green roof industry educate about the many modern advantages from the incorporation of plant material into roofing. What can these “green roofs” do that normal roofs can’t? Their promoters list the following benefits:

  • beautification of the environment and increased social well-being
  • reduction of city “heat island effect”
  • improved air quality
  • temperature regulation reduction of air conditioning and winter heating costs and energy consumption
  • reduction of carbon dioxide impacts
  • lessens nitrogen pollution in rain
  • lengthened roof life
  • water storage and filtration (reducing runoff to sewer systems)
  • new habitat and space for biodiversity
  • safe, protected green spaces in cities

. Green roofs are very practical and durable: they can last up to twice as long as conventional roofs, providing savings of both money and materials. Studies have found that they also reduce the energy and heating and air-conditioning equipment requirements needed. With today’s rising energy costs, this is a major advantage for owners of both new and renovated buildings. Environment Canada research showed that a one-storey building with 10 centimetres of growing medium topped with grass could reduce summer cooling needs by as much as 25%.

In a test done by Environment Canada on two “greened” roofs on Toronto buildings during the summer of 2002, it was discovered that a six and a half million square metre greened space led to a reduction of 1-2 degrees C in urban heat island. A 1% reduction in temperature allows for a 5% reduction in cooling and refrigeration needs for all the buildings saving on energy generation and green house gases. The study stated that, “Green roofs insulate buildings resulting in additional energy conservation totaling an estimated $1 million in annual energy cost savings and a reduction of 2.18 Mega Tonnes of greenhouse gas (GHG) emissions.” http://www.greenroofs.ca/grhcc/background.pdf. An impressive action victory for climate change!

Green roof insulation helps to make city buildings more sound-proof, reducing noise inside the building. They can also provide attractive new spaces for building residents. Green roofs are being discovered as delightful new outdoor spaces for daycares, meetings and recreation.

About Green Roofs

There are two main types of green roofs. One is designed primarily for environmental and insulation purposes, rather than for access. This type, called “extensive” by experts, typically only has two to ten centimeters of soil, and does not add much weight to the building. It is likely to be covered with grasses and low-lying herbs, and is largely self-maintaining.

A second type, designed to allow for larger plants such as trees and shrubs, needs a greater depth of soil, adds more weight to the building, and requires more maintenance. These “intensive” green roofs are designed to be accessible to the people in the building. They may be penthouse gardens for apartment buildings, new green spaces for civic buildings, or school roof gardens. Urban schools with limited space on the ground are discovering the advantages of gardening on the roof: plenty of sunshine, a secure space and new “learning grounds” and a living laboratory for urban ecology, biology, and a whole exciting range of evolving “green technologies.”

A green roof requires a specially designed waterproof, root repelling membrane and a drainage system. In northern climates, thermal insulation to keep heat in the building in the winter and out in summer may be added above this layer. Then a lightweight growing medium (rather than heavy soil which can pose a weight problem on a rood) is added, followed by plants appropriate for the size and depth of the green roof.

Moving Ahead

The many advantages of green roofs have made the transformation of building tops into a growing industry. Turning a roof from black to green can involve the skills of a wide variety of professionals, including architects, engineers, manufacturers of green roof systems and accessories, roofing and landscaping contractors and consultants, energy experts, landscape architects, horticulturalists, governments, non-profit organizations, and educators. This new use of existing space is one example of how new “green” jobs can be created. And as enthusiasm grows for the many kinds of learning that green roofs can provide, educational institutions are also looking for ways to invest in making turning their roofs into valuable new resources.

Dealing with climate change is brining new players on board to seek constructive solutions to reduce greenhouse gases, increase energy efficiency, protect biodiversity and invest in healthy, sustainable communities. Green roofs promise to be an important contribution to climate change solutions in the 21st century.


What do Green Roofs Have to Offer?

Eco-Design Resource Society Green Roofs for Healthy Cities

Images – Green Roofs Before & After, and the dramatic “heat-island effect” of urbanized areas on hot summer days.

A History of EcoRoofing



  1. a) What are the contributing factors that cause the ‘heat island effect’ in cities?
    b) Why is this a problem? Hint: See 3.2.6
  2. Describe some of the advantages of ‘Green Roofs’.
    Name 3 buildings and their locations in Ontario that have a green roof.
  3. Explain the difference between an ‘extensive’ green roof and ‘intensive green roof.