What is it?

The idea of using plants to insulate or waterproof roofs is nothing new. Many centuries ago, the inhabitants of Northern Europe covered their homes with a layer of turf to keep out the cold.

Dating from the 1980s, modern vegetated roofs, commonly referred to as green roofs, are inspired by these ancient techniques. With the introduction of favourable environmental policies, first in Europe and then in North America, they have gained a new lease on life. They have changed a great deal over the years and now have a wide variety of looks and functions

What is it used for?

Traditional roofs tend to re-emit the sun’s energy that they have absorbed during the day, contributing to the well-known problem of heat islands in our cities.

With green roofs, buildings are better insulated and use less energy. As a result, the issue of heat islands is alleviated. In addition, green roofs:

  • reduce the quantity of rainwater running into our sewers;
  • protect roofs and increase their life span;
  • do not produce any waste (asphalt, gravel, etc.);
  • improve air quality.

Green roofs can also have recreational and aesthetic functions and provide a home for greater biodiversity. As a bonus, they can be used to grow edible plants!

How is it made?

Green roofs can be installed over traditional roofing materials or underground load-bearing structures (like parking garages). They comprise several distinct layers:

  • a layer of vegetation;
  • a growing medium; › a drainage and water retention system;
  • a barrier to prevent roots from piercing the roof’s waterproof membrane.

There are extensive green roofs, which are lighter, and intensive green roofs, which are heavier. The latter have a greater variety of uses (for example, it is possible to walk on them) but require a stronger support structure.

In Quebec

The vegetated roof on Montreal’s Centre for Sustainable Development absorbs up to 23,000 litres of rainwater, reducing the incidence of wastewater overflows into the St. Lawrence River.2

In the world

The U.S. green roof capital is Chicago, where an innovative program has led to the installation of over 200 such roofs, including one on City Hall.3

The green roof that tops the California Academy of Sciences in San Francisco is one of the most spectacular in the world. Not only does it provide natural air-conditioning, but it also recovers rainwater, imitates the surrounding landforms and provides a home for 1.7 million local plants!4