by Andrew Speak, PhD candidate in Geography
There are some exciting, positive changes going on in some of the world’s cities and most people don’t even know it is happening. That’s because it is happening above their heads! I’m talking about green roofs. A green roof is basically replacing conventional bitumen or concrete roof surfaces with a layer of plants. The main type is known as an extensive green roof and consists of a thin layer of soil, which supports a mat of Sedum plants. Sedum is a succulent plant that comes in many varieties, and has pretty flowers, but importantly can withstand the harsh conditions on a rooftop – periods of drought and high winds for example. At the other end of the scale is an intensive roof which has a thicker soil layer that can support a wider variety of plants such as small trees, shrubs and even vegetables.
Urban vegetation has many benefits, which are increasingly being recognised by city planners. Street trees possess these benefits, but there is generally a lack of space at street level for tree planting schemes, so the space afforded by rooftops is a perfect site for urban greening. There are a number of specific benefits:
- Reduced solar energy gain by building materials, through shading and replacement of concrete surfaces. This lowers the need for air conditioning in summer which can lead to huge financial benefits. Plants reflect more radiation than conventional urban surfaces. Vegetation also has a cooling effect from the process of evapotranspiration which uses incoming long wave radiation to change water from liquid to gas. The altered thermal budget of cities leads to a reduction in the Urban Heat Island phenomenon, which can make cities very uncomfortable places to be in summer.
- Plants act as passive filters of urban air pollution by providing a larger surface area for deposition. Pollutants are then washed off in rains.
- Replacement of impervious urban surfaces with soil can reduce the pressure on urban drainage systems by acting as a storage buffer in rainfall events. The water retained by green roofs is then returned to the atmosphere by evapotranspiration. There is some evidence that pollutants can be retained within the soil layer as well, thus reducing the impact on receiving water bodies.
- Green roofs can provide habitats for birds and insects, thus replacing the biodiversity lost to urban sprawl. Using native plants on green roofs is frequently promoted.
- Urban green space has a strong aesthetic quality and has been shown to reduce stress and promote feelings of well-being.
- By protecting roof membranes from huge diurnal temperature extremes and UV radiation, the lifetime of the roof is extended, thus adding another long term financial incentive.
The ability of green roofs to counteract high urban temperatures is being promoted as a form of climate change adaptation. Work done by Manchester University’s Ecocities group has demonstrated the usefulness of green roofs to keep the city cool under future climate projections.
So if they are so beneficial, why aren’t UK cities full of them? Currently, a lot of green roofs in this country tend to be ‘showcase’ roofs on National Trust visitor centres, garden centres and art galleries. One inescapable reason is that green roofs do have a fairly high initial construction cost and intensive roofs can also have considerable maintenance costs. Plus, not all existing buildings can support the extra weight that a wet or snow-laden green roof would add to the structure. But this hasn’t stopped countries like Germany, Austria, and more recently the US and Japan, changing their googlemaps satellite street views from grey to green.
The contemporary green roof movement started in German-speaking countries. One theory is that they sprouted spontaneously from flat roofs in Berlin that had been covered in sand as a fire-proof method after the war. Deliberate roof garden construction was a large feature of the modernist movement, with flat roofs seen as an extra space to be utilised for enjoyment of healthy outdoor lifestyles. The environmental movements that started in the 70s ensured growing numbers of people would start to look for alternative ways to live more sustainably. Germany, Austria and Switzerland have always been very proficient at incorporating verdant elements into urban design, as beautifully demonstrated by the architect Hundertwasser. Perhaps, it is something unique about the German appreciation of nature that has influenced the design of cities with a desire to bring nature into them. Whatever the reason, Germany leads the way in green roofing with 5 square miles of green roofs being built every year, helped by government subsidies for construction costs, and policies that state new builds of a certain area with a flat roof MUST have a green roof.
Ubiquitous green roofing also exists in Scandinavia, where the turf roof dominates. These roofs serve the purpose of acting as insulation from extreme winter cold, and have been in use since Viking times. A recent trip to Norway opened my eyes to the possibilities of turf roofs, with everything from car garages to bin-sheds supporting mini-meadows.
The UK lacks a definite policy at the moment with regards green roofs. A number of architects install them on new builds, with the motivation being mostly driven by meeting BREEAM sustainability standards and getting an A or B on the Building Energy Rating, but there are no legal or carrot-and-stick methods to ensure green roofs are factored into new building designs. Some new living roofs are even criticised because they are high-profile and well-publicised, which has led to accusations of them being a form of green-washing of neoliberal construction projects.
There are signs that the UK is catching up though. The Green Roof Centre in Sheffield is doing great work at promoting green roofs and carrying out research on suitable plants and substrates. They have also drawn up a UK specific code of best practice for green and living roof installation. The Centre have been involved in a number of projects on schools, bus shelters and university buildings, helping Sheffield towards having the highest number of green roofs. London is also unveiling more and more green roofs of various sizes and types, often thanks to the influence of charismatic urban ecologist and green roof fanatic, Dusty Gedge. Here in Manchester there are a number in the city centre, such as Number One First Street, The Hive, Spinningfields Apartments, Whitworth Art Gallery and MMU’s All Saint’s building. There are a couple of notable roofs in the suburbs as well, such as the roof vegetable garden at Hulme Garden Centre and the intensive green roof on Unicorn Grocery in Chorlton which even has a pond on it! And small DIY green roofs are popping up all over the place in people’s gardens.
So the ball is rolling, albeit slowly here in Britain. Whether the motivation is to reduce air conditioning bills, attract wildlife, lower the burden on the city’s drains, or just have a conversation piece on the garden shed, more and more plants are sprouting up in the urban roofscape.
Andy Speak is a 3rd year Geography PhD student, investigating a number of environmental benefits of green roofs in Manchester. Watch Andrew talking about his research in this video.
Further info on green roofs: