Could green roofs offer a solution to urban environmental challenges?

A green roof in London.

Could green roofs offer a solution to the challenges urban environments face, asks Peter Dowdal

IT’S a buzzword in many urban planning circles around the world but what does the term “green infrastructure” actually mean? Green infrastructure provides the ingredients for dealing with environmental challenges by building with nature. If you look at the environmental challenges facing most urban areas at the moment, they include: urban heat, biodiversity loss, storm-water flooding, noise pollution, poor air quality, and lack of green amenity spaces.

The good news is, as always, that nature and the garden offers a solution to nearly all these problems. If only we would stop discounting the green environment and looking upon it as something to be controlled and concreted over and instead work with it then we would be far more successful in our endeavours. Green roofs not only counteract the challenges listed above, they actually have proven positive results in relation to all of them.

A green roof is simply a roof which is planted. They can be broken down into three categories: extensive, semi-extensive and intensive. An extensive green roof can be created on the smallest of spaces, even on the roof of a garden shed. Needing only 5cm-15cm of growing medium, they are typically self-sustaining, needing little or no watering or weeding, and are available in ready-grown mats which can simply be laid out.

There are layers to be included before any green roof can be installed such as waterproofing, insulation, weed barrier and drainage. Planning permission and engineers reports may be necessary for larger projects so it is important that you use a contractor who knows about green-roof technology.

Semi-extensive green roofs require a slightly deeper layer of growing medium, about 10cm-20cm to sustain perennials but not trees and shrubs and, finally, intensive green roofs need at least 30cm of growing medium which needs to be organic matter and will thus require a much stronger structure.

Could the hare’s-foot clover, Trifolium arvense, solve Cork city’s flooding problem? Dusty Gedge, a member of the Building a Green Infrastructure for Europe committee and a world leader in green-roof design and installation, has been promoting the use of green roofs as a mainstream building covering for over 20 years. This Trifolium is his favourite plant to use in green roofs for its aesthetic value as much as anything else.

Dusty has found that conditions on roofs are similar to those of coastal areas. Rooftops are, of course, very windy and whilst they may not have to contend with salt, they do have pollutants in the air and thus plants which do well on the coast such as alpines, and low-growing plants like the Trifolium and also Saxifraga, Sedum, Armeria and some grasses, thrive in these conditions.

In the City of London there is now 5.8m of green roof per person. Brownfield sites too, meaning disused industrial sites, are also rich in biodiversity. Battersea power station in London contains seven protected species and will be the biggest green-roof project in London in 10 years’ time.

An established green roof can reduce storm water run-off into drainage systems by up to 80%. Why then are local authorities and planners not insisting on them? Well, many of them are doing just that now but unfortunately not in Ireland.

Cork is seeing many large-scale developments in the city and suburbs at the moment including hotels and office developments, and the cranes are moving ever closer to the docklands area. How many of these developments have had to specify green roofs or any green infrastructure elements to ensure planning?

An urban roof garden space is one example of green infrastructure
An urban roof garden space is one example of green infrastructure

Cllr Mary Rose Desmond believes it’s time for Cork to start “greening up”. “We are way behind both nationally and locally with our approach to incorporating green planning initiatives,” she says. “There is no downside for any planning agency with this, the figures speak for themselves, particularly when Cork is a city fraught with flooding issues. We need to step up as an expanding authority and incorporate these policies as standard.”

In Wales, all new developments need to provide an increased amenity in terms of green space and biodiversity. In London, all new tall buildings need to provide a publicly accessible green roof in order to get planning and on April 22, the New York City Council approved the Climate Mobilisation Act. In what is being referred to as the “New Green Deal” for New York, all new residential or commercial buildings must cover roofs with plants, solar panels or mini-wind turbines or a combination of all three.

In Ireland, it seems, we are still struggling even to see the importance of trees in the urban landscape. We really are light years behind where we need to be in our thinking of green infrastructure. Using urban greening as infrastructure and not just as decoration, which is what London and now New York are doing, is essential in order to create sustainable cities which are urgently needed. Where Cork is coming up with divisive measures to control flooding at the point of it being too late, no thought is being given to source control.

It’s very difficult to stop the flood once it has started but if you can slow the source of the water then the flood is slower to form and in many cases, it will not happen at all. Cork is not unique in this regard; various German studies that date back to the 1980s have shown that green roofs can intercept 50% of annual rainfall and that roof gardens with much deeper soils can intercept up to 90% of rainfall. This means that the water either doesn’t enter the drainage system or if it does, it happens at a much slower rate.

Apart from all the benefits outlined above, green roofs can also help us with climate instability. We have too much carbon in our atmosphere, and not enough in our soils. A typical figure for carbon sequestered in a green roof is 375g per square metre. In London they are now looking at ways of integrating charcoal and materials that will store carbon into green roofs in order to store carbon.

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