Friday, 30 March 2012

Green roofs

Saint Mary's University research green roof
Green roofs are important stormwater management tools that reduce the volume of stormwater  leaving the site by capturing and retaining rainfall and returning precipitation to the atmosphere through evapotranspiration. They are ideal for improving stormwater management in urban areas as they make use of existing roof space. To learn more about green roofs, I spoke with a group of Saint Mary's University grad students who are researching green roof plant communities (thanks Steph, Amy and Tobi!). 

There are two types of green roofs - intensive and extensive. Intensive green roofs have deep soils which can support a wide variety of plant species, including larger shrubs and trees.  They are used for both aesthetic and functional purposes. Intensive roofs require maintenance and care, much like a typical home garden. Extensive green roofs use less soil (layer can range between 2 - 20cm)  which makes them much lighter, but also limits the variety of plants that can be grown in such shallow conditions. Grasses, mosses and sedum species are commonly used.  Extensive green roofs require little to no maintenance. Some roofs may need irrigation for the first year while the plants are establishing, but they can then be left entirely on their own. 

Green roofs can be installed in three ways. Soil can be brought to the roof top and placed on top of the required waterproofing, filtering and insulation layers then planted with selected vegetation. More typically, green roofs are installed as small modules or vegetation trays that contain soil, plants and a shallow gravel layer to ensure the contents are secure on windy rooftop conditions. A third option, known as pre-cultivated vegetation blankets, are composed of soil and vegetation mats that can simply be rolled out onto the rooftop. 

Green roof modules

Diagram of a multilayer XeroFlor green roof system
Green roof layers including pre-cultivated vegetation blanket (
Along with improving stormwater management, other benefits associated with green roofs include increasing the life time of roofs, creating urban wildlife habitat, improving energy efficiency of the building, improving air quality and reducing the urban heat island effect. 

The city of Toronto has introduced a Green Roof Bylaw that requires the construction of green roofs on all new buildings of a certain size. You can read more about Toronto's bylaw here.


  1. The green roof is almost a fair substitute for trees, especially with the scarcity of soil and space in cities. Though this couldn’t absorb as much as the amount of water as trees could, it could still help in storm water management. It can temporarily store part of the rainwater, which will be taken up by plants and eventually, will be returned to the atmosphere as vapor. Plus, this isn’t the best of its feature yet as green roof technology is still being improved.

    Tiffany Larsen

  2. I agree with you Tiffany. If only urban places would embrace the idea, just imagine how eco-friendly New York City will become? If the whole city was filled with green roofs, I bet it’s going to be a sight worth remembering! =)

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