Green Roofs

What are they?
Green roofs are a special roofing system that include a thin layer of growing plants over a waterproof membrane, and may include a drainage system. Green roofs are not the same as rooftop gardens, which consist of planters and pots arranged on a roof, primarily as a recreation area. Green roofs can include recreational purposes, but this is not their primary purpose. Green roofs have been used for centuries all over the world to insulate buildings, prolong the life of the roof and to reduce stormwater runoff.

There are two common types of green roofs:

1) Extensive System:

• Used when limited space is available; ideal for small residential and commercial properties.
• Sometimes not accessible for recreational use, like roofs with slopes.
• Usually used for stormwater retention and insulation.
• They are low cost and low maintenance, requiring less technical knowledge.
• May or may not include a special irrigation or drainage systems.

2) Intensive System:

• Commonly used when larger spaces are available; tend to be used on large commercial or industrial buildings.
• They can support bigger plants like shrubs and trees.
• Most often used for recreational spaces or common areas.
• Used for stormwater retention and insulation. Includes opportunities for wildlife habitat.
• Require irrigation and drainage systems.

How do they work?
Green roofs transform the impermeable rooftop of a building into permeable spaces. The plants absorb rainfall and clean it through the roots and soil. Water that isn't fully absorbed is directed off the roof to downspouts.

A properly designed green roof consists of several layers which help capture and infiltrate stormwater. These layers include:

1. The top layer is the growing medium of the plants. The soil used isn't average garden soil, but is a special synthetic soil that is lightweight and will not clog the drainage system.
2. A root barrier is laid under the soil to prevent plant root penetration to lower layers, but still allows water to pass though.
3. The next layer manages the infiltrating stormwater and prevents water from seeping into lower layers. A drainage system removes excess water.
4. An insulation layer is sometimes laid for energy saving benefits especially in cold climate environments like Toronto.
5. A protective layer is laid on top of the roof to protect its surface. It also doubles as a waterproofer.

What are the Benefits?

• Reduces roof runoff by up to 90%, as well as slowing and cooling stormwater.
• They can be installed on all types of buildings and roofs, flat or sloped.
• Areas on green roofs can be used as amenity space for leisure or gardening if accessibly.
• They reduce a building's heating/cooling energy requirements by acting as an insulator.
• The vegetation improves air quality by filtering dust, absorbing carbon dioxide and producing oxygen.
• By reducing the exposure to UV and weather, green roofs increase a roof's life expectancy by 200 or 300%.

• When considering a green roof, keep in mind the slope of your roof. Anything greater that 45 degrees becomes difficult to build on.
• Some roofs will need structural reinforcement depending on the existing roof and type of green roof selected.
• Proper green roof installation requires experienced professionals to plan, install and maintain.

Green roofs are permitted in Toronto, provided the roof meets municipal by-laws for height, privacy/overlooking (if accessible), the Ontario Building Code and Canadian Building Code for roof stability, snow loads and fire hazards etc.

Costs
Green roofs are very expensive, up to $50 per square foot - but the prices vary greatly according to the style and design of it. It is important to note that they prolong the life of a roof by two to three times, and they cut energy bills significantly.

More info
There have been several green roof demonstration projects in Toronto. The green roof at Toronto's Mountain Equipment Co-op is the largest green roof in the city. The City of Toronto installed 7 experimental roof garden plots to demonstrate and gather information about it's effectiveness in the city.

Resources

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