Using Green Roofs to Boost Biodiversity

How many roofs would a woodchuck chuck if a woodchuck could chuck roofs?

The answer, to the best of my knowledge, is one. That slightly modified alliterative classic popped into my head recently while I was lazing with my hound in a meadow by the river. We were watching a mama and her kits mow that meadow in their rapacious rush to prepare for the winter. They were not the least bit afraid of us and approached quite closely, giving me the opportunity to see them tearing the grass out with their paws and eating it much like a child would.

Green Roof Those critters triggered a long buried memory of a similar experience I had in 1994 while working on the then brand new semi-intensive green roof on the environmental science building at Trent University. At that time, it was one of a very few green roofs in the country and every one was excited. That day, a couple of us were up on the roof investigating the planting and overwintering success of the vegetation. It became apparent that it was successful because there, right in front of us, was a mother and her kits happily munching away. In the event that you are wondering how they arrived on that roof, the architect had cleverly provided a walkway ramp for just such a possibility.

Trent now has 5 green roofs, over 3500 M2, mirroring the adoption of the technology and philosophy growing in the Province of Ontario. In 2009-10, Toronto, the Provincial capital, instituted the first North American by-law directed at facilitating the adoption of this technology. The city provides grants up to $7.50 per square foot. In 2012, the city began enforcing the by-law and there are now over 440 green roofs in service; over 260 having been installed in the past 6 years. A perusal of installed project data indicates that several green roof companies have successfully reduced the cost of an extensive green roof to below $25.00 per square foot.

The By- Law

The by-law is a component of a comprehensive greening policy included in the master plan and reflects a commitment to Provincial programs such as the highly successful greenbelt program. These planning approaches are aimed at stabilizing the negative emergent events associated with urban sprawl such as habitat destruction and fragmentation, deteriorating water quality, and pollution resulting from runoff. Toronto, like many older cities, has an aging storm water and sewage system. A single storm in 2013 resulted in over a billion liters of untreated sewage entering Lake Ontario. Beaches are often posted as unsafe.

Toronto By-Law

The Toronto by-law requires all new construction, commercial, industrial, and residential, with a gross floor space over 2000 M2 to install a green roof. The area of green roof required ranges from 20% to 60% based on gross area. Whether the roof is being constructed voluntarily or as required by the by-law, guidelines must be followed. The by-law is comprehensive, but in its original 2010 form it provided no guidelines for biodiversity.

The majority of green roofs in the city are extensive and although this is a decidedly better approach than traditional roofing, they are a far cry from meeting the intent and spirit of the by-law or the technology. Extensive green roofs, defined here as those with 100 mm (4 inches) of medium or less, are prone to a number of problems. For example: winter mortality, summer drought problems (the by-law requires adequate irrigation systems be installed but this may not be occurring), poor habitat for indigenous fauna, and inadequate carrying capacity for a diverse biota.

Key differences between extensive and intensive green roofs

Extensive & Intensive Green Roofs

In 2013, the city responded to the undesirable preponderance of extensive green roof installations by developing the Biodiversity guidelines and incorporating them into the by-law. Thus facilitating the implementation of policy objectives outlined in the official plan. For example: Policy 3.4.1 “To support strong communities, a competitive economy and a high quality of life, public and private city-building activities and changes to the built environment, including public works, will be environmentally friendly, based on: b) Protecting, restoring and enhancing the health and integrity of the natural ecosystem, supporting bio-diversity in the City and targeting ecological improvements”.

Specific aspects and techniques to achieve desired biodiversity include: depth, topography, variation of growing medium composition and distribution, vegetation diversity, and adoption of native species adapted to the harsher conditions of roof top locations. For example alvar, fen, tall grass prairie, sand barren species, structures, bird perch and nesting opportunities, a water source, and provision of different micro-climates. Furthermore, the guidelines outline the objective of supporting adjacent ecosystems. A green roof located near the shoreline of Lake Ontario should reflect that ecosystem; one adjacent to the rouge valley or other forest system should reflect or support the forest.

Objectives of design of bio-diverse green roofs

Design of Biodiverse Green Roofs

Featured image courtesy of Wikipedia


Chris is a plant biologist and forester who has dedicated over two decades to plant ecology research in addition to acquiring almost 20 years of experience in the garden center and florist sector. He graduated from Trent University with an Honours degree in plant biology and from Sir Sandford Flemming College with a diploma in Forestry. As an ecologist, Chris has been involved in many projects, large and small, from government directed natural resource management audits and climate change research with carbon sequestration in permafrost peatlands to natural vegetation restoration following disturbance. As an garden center and florist business owner in the Lower Mainland of British Columbia, he has born witness to the tremendous change that has occurred in the horticulture industry over the past twenty years.

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