How should we green roof a city?

Mo writes*

Everyone seems to be talking about climate change nowadays, it is becoming an increasingly important topic on the political agenda of several countries. Whilst one might typically think of carbon dioxide emissions, or other greenhouse gases, when thinking about action against climate change, there are several other ways in which countries are tackling climate change and its impacts. One of which is through green roofs!

Green roofs can be defined as an area squared off on the surface of a man-made structure to allow the growth of vegetation. Vijayaraghavan (2016) identifies several components necessary for the construction of a green roof: a vegetation layer, a substrate, filter fabric, drainage material, root barrier, and insulation. But don’t think you can’t get creative with it, different types of vegetation mean different drainage benefits as well as increased biodiversity.

If you live in a city in North America, you’ve probably come across a green roof. The chances double if you live in Toronto, which in 2016 had the greatest square footage of green roofing installed amongst cities in North America [pdf]. How did Toronto achieve this? This leadership in green roofs can be attributed to its green roof bylaw. This by-law mandates the construction of a green roof on new developments of industrial, residential, institutional, and commercial buildings if the surface area of the roof exceeds 2000 square meters.

North American cities are not the only places where you’d be able to spot green roofs. Several cities around the world are introducing green roofs as a means by which to mitigate climate change impacts on a local level (Vijayaraghavan, 2016). They provide a number of social and environmental benefits, but most importantly for cities, especially undergoing rapid urbanization, its provides the reduction in the urban heat island effect as well as providing better stormwater management (Vijayaraghavan, 2016). Not only do green roofs help cities mitigate and adapt to the impacts of climate change, but also brings in more green space into a built environment. The social value of living in an aesthetically-pleasing environment shouldn’t be overlooked.

Even different municipalities in The Netherlands have started their own ventures into getting green roof construction going throughout their respective cities. Most notably, the city of Rotterdam has shown greater interest in green roofs and the benefits it would provide [pdf]. In order to increase the square footage of green roofs throughout the city of Rotterdam, the municipality offers a subsidy for those constructing one. This subsidy currently offers 15 euros per square meter of realised green roof and will be available to residents until the end of 2020 or when the money runs out. Other than this subsidy, there doesn’t seem to be other incentives pushing residents or building owners to build green roofs.

This mechanism is quite different compared to how Toronto is green roofing its city. Both mechanisms are valid means of constructing green roofs, one relies on legislation and regulation, whereas the other makes use of a market mechanism. But a question emerges, which mechanism is the most effective in order for municipalities to increase green roofs throughout the city?

Bottom Line: Green roofs are one of the ways cities can locally mitigate climate change impacts and there are different mechanisms by which municipalities can get more green roofs throughout a city. What are the costs and benefits of the two mechanisms, and which is the most effective?

* Please help my Environmental Economics students by commenting on unclear analysis, alternative perspectives, better data sources, or maybe just saying something nice :).

Author: David Zetland

I'm a political-economist from California who now lives in Amsterdam.

2 thoughts on “How should we green roof a city?”

  1. It is a very fine idea that should be implemented more effectively if backed by govt support and awareness among masses. I think urban people are supposed to be more aware of this as compared to rural people as the urban sector is more susceptible to climate change. We could prioritize the urban sector but it doesn’t mean to underestimate the rural sector. In Pakistan, (where I’m from) I personally find rural people who are more conscious about the environment than the urban people. The factors could be MANY!

  2. Hi Mo,
    Very interesting topic and I love the overall concept – I very much agree the social benefits shouldn’t be overlooked. I haven’t heard of grass playing a large role in carbon sequestration, so I’m curious what the mitigation benefit would be of a green roof city? For example, as vegetation goes are there specific types which would be more beneficial in the role of carbon sequestration than others that would be appropriate for a roof?
    Another thought I had, if all the roofs were to be green, would that eliminate the possibility of these roofs having solar panels, or can they have both? If not, is it worth it or should a balance be struck between green roofs and solar, since transforming the energy sector will have a crucial role here and targeting cities seems like a high impact route to take (in terms of a solar initiative)? Perhaps these considerations could play into your BCA.

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