For the purposes of Toronto's Green Roof Bylaw and the Eco-Roof Incentive Program, a green roof is an extension of an above grade roof, built on top of a human-made structure, that allows vegetation to grow in a growing medium and which is designed, constructed and maintained in accordance with the Toronto Green Roof Construction Standard. A green roof assembly includes, as a minimum, a root repellent system, a drainage system, a filtering layer, a growing medium and plants, and shall be installed on a waterproof membrane of an applicable roof.
There are three main types of green roof systems:
- complete systems where all the different components including roof membrane are an integral part of the whole system
- modular systems that are positioned above the existing roofing system
- pre-cultivated vegetation blankets that consist of growing medium and plants that are rolled onto the existing roofing system with drainage mats and root barriers as required
Intensive/active green roofs have a deep growing medium that supports a variety of landscape design and growth. They are accessible and used as recreational space. An example of an intensive/active green roof is the Manulife Centre which is located over a parking garage. It is a well established green roof (25 years old) with mature trees that reach three stories high.
Manulife Centre - 44 Charles St. West, Toronto
By contrast, an extensive green roof has a shallow growing medium and the landscaping is designed to be more self-sustaining, requiring less maintenance than an intensive system. Extensive green roofs are less expensive than intensive systems, since they are lighter and require less structural support and need less frequent maintenance. The roof on Mountain Equipment Co-op in Toronto is an example of an extensive green roof, built in 1998.
Mountain Equipment Co-op - 400 King St. W., Toronto
Green roofs can be designed to be an integral part of a new building, or can be installed later on an existing building. When a building is designed with a green roof system, there can be several benefits. For example, the building is designed to provide the necessary structural support, and won't require reinforcement later. Also, the building can be designed to take advantage of the aesthetic value that a green roof can offer by providing viewing areas.
York University installed a 30,000 square foot green roof during the construction of the Computer Sciences Building in 2003. The green roof is not accessible. It has been monitored by the Toronto Region and Conservation Authority to determine the quantity and quality of stormwater and quantify other benefits of green roofs.
New green roof on York University Computer Science Building
An example of a retrofit is the intensive 704m2 green roof on ESRI Canada Ltd. 12 Concorde Place. It covers 100% of available roof space on a commercial building, and it hosts 52 plant types including sedums, grasses, flowers, herbs, shrubs and trees.
New green roof on ESRI Canada Ltd., 12 Concorde Place
Below is a brief description of a complete system that is more fully described in the consultant's report, Environmental Benefits and Costs of Green Roof Technology for the City of Toronto.
In a complete green roof system, all parts of the roof are designed to support vegetation growth. These systems provide the most flexibility in terms of the type and nature of growing medium, drainage and protection layers and type of vegetation. Complete systems vary in thickness and weight from as low as 50mm to 75mm (2 to 3 inches) in depth and 60 to 90 kg per sq. m, (12 to 18 lbs per sq. ft.) in weight. They can be installed with a variety of waterproofing membrane types.
Typical green roof assembly on conventional roof
Photo Courtesy of Sopranature
Below is a brief description of a modular system that is more fully described in the consultant's report, Environmental Benefits and Costs of Green Roof Technology for the City of Toronto.
Modular systems are essentially trays of vegetation in a growing medium that are grown off-site and simply placed on the roof to achieve complete coverage. They are available in different depths of growing medium typically ranging from 75mm to 300mm (3 to 12 inches). The variety of vegetation is typically more limited.
Photo courtesy of GreenGrid System
Photo courtesy of Green Roof Block System
Below is a brief description of a vegetation blanket system that is more fully described in the consultant's report, Environmental Benefits and Costs of Green Roof Technology for the City of Toronto.
A pre-cultivated vegetation blanket is a pre-grown interlocking green roof tile. The blanket shown below is available in a thickness of about 45mm (1.75 inches).
Photos courtesy of Elevated Landscape Technologies
Blanket systems are available in a variety of system designs. The most versatile system contains 25 mm (1 inch) of planting substrate. The result is a lightweight system ranging in weight from 40 to 60 kg per sq. metre.
The majority of the vegetation is made up of several varieties of sedum, a succulent plant (8.0 to 13.0lbs per sq. ft.) tolerant to extremes in temperature that survives with little or no irrigation while requiring very little maintenance. They are cultivated at ground level, then rolled and transported as a complete system on pallets or by crane.