One of the most frequent complaints of urbanites everywhere is the significant lack of plant life and green spaces in many large western cities. Cities are places where as many people are crammed into as little space as possible and which serve as nexus points for our societies’ economic activity. Moreover, most of the world’s largest urban areas were developed in times were citizens and politicians were less environmentally-minded. All these factors combined mean that many of them are severely lacking on the green front. Over the last few decades this has begun to change, with many cities adopting new, eco-friendly urban design practices and making an active effort to become more green. While ground-level green developments are fairly common, one area that is still under-exploited and offers a lot of space for green growth are the tens of thousands of unused rooftops in every major city – these can be transformed into green roofs, small green oases in the middle of the concrete jungle that have many benefits.
What are green roofs?
Green roofs are exactly what the name suggests: rooftops covered in plants. They are made by installing a layer of substrate on top of the roof’s waterproof layer and seeding it with different species of plants – from grass to flowers, bushes, small trees, and even some types that produce food.
There are two types of green roofs, extensive and intensive. Extensive green roofs are the lower maintenance variety. They have lower soil thickness, they require less care, maintenance and irrigation, and they admit fewer and less varied plant species. Extensive green roofs typically look like grassy fields. Intensive green roofs are the opposite, producing the more picturesque rooftop garden many have seen. The substrate in these is thicker and they require more irrigation and care. They can also be used to grow more varieties of plants, including some vegetables.
Green rooftops have been around for a long time but they have been growing significantly in popularity over the last few years. In 2002 the city of Basel began requiring that all new developments with flat rooftops have green roofs, and in 2009 the city of Toronto passed a similar law requiring green roofs on all new commercial and residential buildings. Just last year, in 2015, France joined the trend by approving a law which mandates that all new commercial building developments be built with green roofs or with solar panels.
How are green roofs helpful for the environment?
The two most important benefits of green roofs from a climate perspective are that they provide effective insulation for buildings, helping them remain hot or cool depending on the season, and that they improve air quality in places where it is most needed.
It is well known that heating and air conditioning are two of the biggest consumers of energy in buildings. Better insulation, preventing heat loss and helping buildings stay cool, can significantly reduce energy expenditure and thus a building’s carbon footprint. According to a study by Canada’s National Research Council, extensive green roofs can reduce a building’s daily energy demand from air conditioning by up to 75% in the summer. The rooftop is the point where most heat transfer occurs in buildings, and covering it in plants has been shown to mitigate this effect to great benefit.
The other major benefit is improved air quality. Plants can capture airborne pollutants such as ozone and particulate matter, effectively cleaning the air. With smog in cities becoming a global epidemic, transforming rooftops into spaces that help clean the air could offer great benefits. Furthermore, as a byproduct of their temperature-regulating function, green rooftops can also reduce greenhouse gas emissions from power plants through lower energy expenditure, which also contributes to improving air quality.
Another area in which green roofs can help, especially places with severe flooding concerns, like the UK, is by reducing rainwater runoff. The soil substrate of a green roof can absorb 25% to 90% of the rainwater that falls on it depending on the season (more in the summer when it is drier, and less in the winter when it is damp). This water is then absorbed by the plants and returns to the atmosphere by evapotranspiration instead of just draining into urban sewer systems. A 20 cm layer of substrate can hold between 10 and 15 cm of rainwater, reducing and delaying storm water runoff.
Green roofs can also be used to grow fruit and vegetables. Rooftop vegetable gardens are becoming a craze in many cities around the world. They receive plenty of sunlight and, in the UK, also plenty of rain to grow many types of food. Producing food on our rooftops isn’t going to wean us off our reliance on industrial agriculture, but if we can start obtaining some of our vegetables and fruit from proximate sources (and what is closer than walking up a few steps?) it will help reduce our environmental impact and our carbon footprint.