Green Roofs: Does a City Good

We've seen them pop up in cities over the last few years, new hip buildings with lush green gardens on the roofs. I assumed the trend was to make better use of urban space for gardening. Well, you know what they say about assuming. It makes an ass of you and me. Or maybe, just me? Turns out there is more to those beautiful rooftop gardens. Way more.

These urban green spaces provide great solutions to food production and to urban environmental sustainability. Rooftop gardens, also known as green roofs, allow city dwellers to grow food for their own homes or as urban farmers. They can be installed on sheds and homes, as well as on large commercial and industrial buildings.

What I didn't know was that green roofs thrive on (and filter) rain water, decreasing the amount of polluted stormwater run-off draining into our waterways. And, like all plants, the process of photosynthesis creates oxygen and absorbs carbon dioxide (1). Here in Victoria, B.C., the City of Victoria Rainwater Rewards incentive program offers stormwater credits for green roofs; up to 40% reduction on the stormwater fee is available. For more information, see www/victoria.ca/stormwater

On top of it all, all of those layers of soil and growth also provide insulation to the building they are planted on, cooling it in the summer and heating it in the winter, decreasing energy consumption. According to a study conducted by the National Research Council of Canada, an extensive green roof can reduce summer energy demands by more than 75 per cent. By lowering air conditioning demand, green roofs decrease the production of associated air pollution and greenhouse gas emissions.

Green roofs also help mitigate the heat island effect, which can make a city one to three degrees Celsius hotter than the surrounding area during the day and warmer at night. Glass-and-concrete cities absorb a lot of heat during the day, and they’re much slower to cool down at night as a result. According to Health Canada, the best way to reduce the urban heat island effect is through plants. Through photosynthesis, plants absorb greenhouse gases and sunlight while cooling the air around them, making them an extremely useful tool for fighting the urban heat island effect.

As if all that wasn't enough, green roofs also increase biodiversity by sustaining a variety of plants and invertebrates, and provide habitat for various bird species. "By acting as a stepping stone habitat for migrating birds they can link species together that would otherwise be fragmented," according to Green Roofs for Healthy Cities, a non-profit which promotes world-wide education on green roofs.

Canadian cities have so much to gain with green roofs but momentum is growing. Toronto became the first North American city with a green roof bylaw in 2009, requiring large new builds to feature between 20 and 60 per cent green-roof coverage. As our cities continue to warm, we'll hopefully see a lot more of those green rooftops, with food production being only a small slice of the pie 😉

(1) Scientific American, 'Advice on Creating a Rooftop Garden'