Demystifying Green Roofs by Tristan Rouse / by Sustainable

The City of Toronto also believes this idea because they have introduced the Green Roof By law that requires new commercial, institutional and residential development which are larger than 2,000 m 2 to implement a green roof of between 20-60% of the total roof area.

Types of Green Roofs


An extensive green roof is a system that is seen as an alternative to ballast layers such as gravel. It is a lightweight system that requires little to no maintenance they include sedums, herbs and some low lying grasses. They do not require irrigation systems and mostly rely on water and nutrients from nature. These green roof systems can range from about 100mm-150mm.


On the other hand, an intensive green roof can be viewed as creating a rooftop garden. They are usually accessible and can be used in many different ways based on the plantings. These green roofs are heavier because they need more soil for plants and trees to grow. Maintenance can include irrigation and it is more frequent than extensive green roofs. Plants such as grass, perennials, trees, shrubs and other plants can be included as well as ponds, patios and pathways. The depth of these systems can range from 150mm-2m depending on the types of plants.

There are different systems for green roofs depending on the manufacturer and system that is needed for the roof. The two main systems are a tray system and a continuous membrane system. The former is a system that interlocks trays of plants together to form the green roof. These trays can be switched and replaced if something is wrong with a specific area of green roof. The latter solution is a complete covering of the green roof area with a series of waterproofing and drainage membranes that is then covered with soil or sedum mats depending on the roof. Each project has different needs and each system has been proved to be reliable in green roof assemblies.

Why ‘Green’ your Roof?

Storm water is something that Toronto struggles to manage when there is heavy rainfall. Green roofs are a way to help this process by holding the rain water for a period of time to allow the storm water system to drain the water properly. Intensive green roofs can hold between 70- 135 L/m 2 of water. If 1,000 m 2 of a roof is a green roof, 135,000 L of water is stored and released into the system later than if the roof just had gravel ballast. Both types of green roofs also promote biodiversity and attract a flora and fauna, allowing bees, birds and butterflies, beetles and spiders to live on the green roof.