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What Makes A Roof Green?

Green Roof

The view from Mountain Equipment Co-op’s green roof.

Since January 31, 2010, green roofs have been mandatory on large building construction projects in Toronto.

Actually, Toronto was  the the first city to adopt a mandatory bylaw for green roofs in North America.

Even City Hall has a green roof now!

Interest in green building and energy efficiency has been growing all over the world.

Currently, traditional roofs are simply wasted space, sometimes called “tar deserts”. The green roof movement is working to change that.

Green roofs are growing in popularity and for good reason. Since green roofs can help solve environmental problems as well provide other helpful benefits, this type of roof solution just makes sense.

The History of Green Roofs

We often think of green roofs as a modern innovation, but the idea of green roofs has actually been around for thousands of years in one way or another.

Ancient ziggurats in Mesopotamia were built with rooftop gardens that provided cooling shade from the sun. The Hanging Gardens of Babylon are probably the most well-known example of ancient green roofs. These garden terraces were watered using a complicated irrigation system, enabling the gardens to remain lush and green while surrounded by the dry desert.

Much later, the Norwegians began using sod roofs to insulate and protect their homes from the elements. Modern day green roofs are much more similar to the Norwegian sod roof than ancient versions.

When we refer to green roofs now, we are generally talking about roofs that are covered with vegetation and include a waterproofing element. Rooftop gardens are not usually included in this category, although they also offer many environmental benefits.

The green roof as we know it today, was developed in Germany in the 1960s and has become increasingly popular across Europe, Japan, and the United States.

How Green Roofs are Constructed

A typical green roof includes several different layers. Although these layers will vary based on the manufacturer, they will generally consist of the following components: structural support, roofing membrane, membrane protection and root barrier, insulation, drainage, aeration, water storage, growing medium, and vegetation.

There are two types of green roofs—extensive and intensive. An extensive green roof has a few inches of growth and small plants, while an intensive green roof has larger plants and up to several feet of soil. The bottom layer of the green roof is comprised of a rubberized covering that protects the actual roof surface from moisture. A system of drainage layers are placed on top of the rubberized covering in order encourage the proper flow of water.

How Green Roofs Work

As rainwater falls, the soil of a green roof captures the moisture. A small percentage of the rainwater is released into the atmosphere while the majority of the moisture is saved in the layers of the green roof so it can be used later by the vegetation. When the soil is saturated, water will flow through the drainage system and down to the gutter. This process aids in reducing stormwater runoff and water pollution.

The Benefits of Green Roofs

Green roofs are an economically and environmentally sustainable approach to urbanization.

First of all, green roofs can save money on heating and cooling. In the winter, green roofs help prevent heat from escaping from the roof. In the summer, evaporative cooling reduces the need for air conditioning. Green roofs can also significantly extend the life a roof, sometimes doubling the lifespan.

Green roofs offer a variety of environmental benefits. One of the major environmental benefits is that green roofs help to reduce the “urban heat island” effect. This occurs in larger cities when traditional rooftops radiate solar heat back into the atmosphere.

Green roofs also help to improve air and water quality by filtering pollutants. In addition to these benefits, green roofs provide am attractive alterative to traditional roofs. Intensive green roofs can showcase beautiful garden-like vegetation that bring color and vibrancy to urban spaces.

A Green Roof Over the Gardiner Expressway?

The Green Ribbon proposes building seven kilometres of green roof over the existing elevated Gardiner Expressway. Not only would the Green Ribbon carry the same benefits of a traditional green roof (on a much larger scale), but it would also create new recreational parkland for pedestrians and cyclists. Click here to learn more about the Green Ribbon.

Have you spotted any green roofs around Toronto? Let us know in the comments!

Image Credit: Jackman Chiu

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