They are rarely seen from the streets, but rooftops around the world are indeed becoming green. Toronto’s skyline is no exception. The first city to adopt a green roof bylaw in North America, Toronto has witnessed a dramatic rise in the size, scale and number of green roofs across the city over the past decade. That success can be attributed to both the bylaw that sets out a requirement for green roofs on new buildings, but also programs like the City of Toronto’s Eco-Roof Incentive Program that play an important role in helping to offset costs and encourage green roof construction on existing homes and buildings.
Green roofs help support their natural surroundings, conserve energy and enhance the comfort of the building’s occupants. Among the different categories of green roofs, biodiverse green roofs, termed as such, are specifically designed to support plant and animal life, including birds, butterflies, and other pollinators. Biodiverse green roofs, therefore, help promote healthier and ecologically beneficial urban environments while also providing aesthetic and structural enhancements to their buildings.
Biodiverse green roofs help provide:
- A home for many: Creating a welcome and nourishing environment for pollinators, butterflies, and other insects maintains and enhances biodiversity in an urban setting. This helps to mitigate species loss by creating much-needed habitat.
- A breath of fresh air: Biodiverse green roofs assist in cleaning the air by converting carbon dioxide into oxygen and filtering particulates from the air.
- Shelter from the rain: The plants, substrate, and drainage layer of a green roof act as absorbing materials for rainwater. In so doing, they mitigate the amount of water that makes it into sewage systems and lowers the risk of flooding.
- Cooler heads: Green roofs are great at absorbing heat, contributing to cooler surfaces and lower indoor temperatures. While they will not provide a complete substitute for air conditioning, they can help take the pressure off of HVAC equipment and contribute to lower overall temperatures. Additionally, by cooling the roof, green roofs can enhance the efficiency of rooftop solar panels.
- Quieter homes: Green roofs do more than absorb heat and rainwater. They can also serve as a natural sound buffer.
- Longer lifespans: Roofing components and materials can age quickly if left exposed to the elements. They are also prone to more damage from occupants. A green cover provides a layer of protection from these external threats, significantly extending the life of the roof and saving maintenance expenses.
- A valued addition: Cooler buildings with longer-living roofs fetch higher value in the real estate market. Green roofs are also becoming increasingly popular among eco-conscious occupants and investors with a focus on sustainability.
Building a biodiverse green roof
There are several factors to weigh when installing a biodiverse green roof. Looking to get started? Here are some of the most important considerations:
- What to plant: The vegetation layer of a green roof plays a significant role in fostering biodiversity. Maximizing the diversity of plant species and plant life forms (e.g., succulents, herbaceous perennials, woody plants, coniferous, deciduous, etc.) has many benefits as it increases opportunities for pollination, food, shade, nesting, perching and nutrients.
- Composition: Varying the type of growing medium throughout your roof will also support a greater diversity of plant and animal/insect habitats. Consider how various soils, different granule sizes, and mixtures will encourage various visitors.
- Growing media: Plants survive longer in growing medium depths of 4 to 6 inches. That doesn’t mean the whole roof needs to be covered to that degree, but it is worth considering when plotting out what and where to grow.
- Green roof topography: Flat green roofs might look nice, but they limit the potential for biodiversity. Consider ways to switch up the shape of your green roof by adding slopes, changing soil depths, and adding structures. Varied topographies give rise to greater biodiversity by creating different microhabitats within the same green roof.
- Creating microclimates: Consider using building features (e.g., wall shade, elevations, etc.) and natural structures (e.g., branches, rocks, etc.) to create various microclimates throughout the green roof. This helps enable the growth of a greater variety of plants and offers a comfortable landing point for insects and birds that thrive in different conditions (e.g., dry heat, moist vegetation, cool habitats, etc.).
Incentivizing biodiverse green roofs and other eco-roofs
Creating a biodiverse green roof certainly has many advantages. While the upfront costs associated with the installation can sometimes be a barrier, some grants and incentives programs can help get building operators started.
For example, Toronto’s Eco-Roof Incentive Program (ERIP), provides grants that help offset both the cost of a structural assessment associated with green roof construction as well as a rebate-style grant payment upon green roof completion proportional to its total vegetated area.
Launched in 2009, the ERIP provides $100 per square metre of installed green roof area and up to $1,000 for a structural assessment. In cases where green roof construction may not be feasible, the ERIP provides $2-$5 per square metre for cool roof area on newly installed cool roofs. Over the past decade, these incentives have helped create over 952,000 square metres of eco-roof area (the equivalent of 117 Canadian football fields).
To discover additional benefits of eco-roofs, visit the City of Toronto’s Eco-Roof Incentive Program or contact their program staff at ecoroof@toronto.ca.
