Green Spaces on City Rooftops

Reposted in full from G Magazine, 29 June 2010

The 2800-m2 roof garden atop the M Central residential complex in Pyrmont, Sydney, was completed in 2005 by landscape architects, 360°. It features native grasses, succulents, vine arbours and mature specimen trees alongside cascading water features, timber boardwalks and sculptures

'An aerial view of most major Australian cities shows only the occasional green patch, stark against an endless procession of grey concrete roofs, paths, car parks and bitumen roads.

As our climate grows inexorably hotter, it’s not unusual for city dwellers to experience daily temperatures more than 7°C higher than those living outside the built-up areas. And such temperature rises in our urban spaces can’t all be blamed on climate change. We have created urban heat islands in our cities by removing the best natural air-conditioning: plants. When we erect concrete jungles, we trap heat rather than regulating it through our flora.

Now gardeners are joining forces with architects to restore a little greenery to our cities by taking advantage of a little-used space – the roof.

“Interest in green roofs is growing substantially around the world,” says Australian landscape architect Jock Gammon.

Living, breathing air-conditioners

One of the most significant benefits of green roofs is their immediate cooling effect, which goes beyond the insulation properties of a layer of soil and plants.

“As the plant transpires water, it reduces surrounding air temperatures and cold air drops down the face of the building. With enough roof gardens, there can be a significant effect on cities,” says architect Paul Downton, who is the co-creator of Adelaide’s groundbreaking Christie Walk eco-development.

Gammon says that a green roof also enhances the performance of solar panels by reducing their operating temperature, making them more efficient.

Canada’s Ryerson University predicted that the city of Toronto could reduce its ambient air temperatures by up to 2°C if around eight per cent of the city’s buildings had green roofs – a total area of around 5,000 ha. They estimated a resulting annual energy use reduction of 114 MW and a corresponding saving in

greenhouse gas emissions of 56,000 tonnes.

Green roofs can also improve air quality. A 1996 NASA study found that a single fig tree purifies 10 cubic metres of air a day; other research showed that one square metre of grass removes about 0.2 kg of particles from the air each year.

In cities, clean rainwater quickly becomes dirty stormwater run-off, but this can be captured instead by green roofs. While plants release some water back into the atmosphere, the rest can be naturally filtered for use by building residents.

Downton names some further benefits: “As well as being quite beautiful, visually, green roofs bring back habitat and increase biodiversity in an area.”

Different cities, different solutions

The most substantial benefit of green roofs in Australian cities will be the evaporative cooling that occurs, says Ben Nicholson, a Melbourne-based town planner, who has toured the world’s best green roof projects.

“Green roofs can reduce the temperature of a roof’s surface by 50°C – it’s really significant. The reduction of heat transfer into the building elements depends on a number of factors, but it also makes a big difference on the surrounding air,” he says.

Overseas, many cities acknowledge the benefits. Tokyo, for example, now requires green roofs to be installed on 20 per cent of the city’s roof surfaces, and many US cities are introducing regulations designed to promote green roofs.

At 600 metres in the sky, one of the world’s highest green roofs is on Chicago’s Willis Tower, formerly called the Sears Tower, which was the world’s tallest building when it was completed in 1973. As part of a green retrofit to reduce the building’s energy use by an estimated 80 per cent, an experimental green roof featuring mountainous plants like sedum was installed a year ago on a 90th-floor rooftop, with further rooftop gardens planned.

Australia catching up

Sidonie Carpenter, a landscape architect and president of Green Roofs Australia, admits that the rise of the green roof has been a little slow to take off in Australia, despite our iconic lawn-covered Parliament House.

She points out that roofs in Australia, unlike those in Europe and America, are not built to withstand snow-loads and so retrofitting often involves very expensive structural engineering. Little data exists about the number of green roofs in Australia at this stage, but according to Carpenter:

“There’s real momentum in Australia now; overseas experts have been blown away by the attitude and ability of the people involved in the industry here.”

Future plans

City of Melbourne Councillor Cathy Oke chairs the Eco-City Committee and says she’s excited about the potential of green roofs to transform the city. “Cities have huge heat islands; they are hot and getting hotter and green roofs and vertical gardens are definitely a way to mitigate this,” she says.

Although many cities in Europe and North America have made green roofs mandatory, Oke says that there’s a long way to go before Melbourne follows suit.

Ben Nicholson believes that a strategic policy approach informed by good local data is critical for the success of green roofs. “Engineering, safety and privacy are key town planning issues.

He says there needs to be policy developed across federal, state and local governments including incentives and planning mechanisms to encourage multiple green roof developments in Australia’s cities.

“One green roof will make a difference to a building – but a thousand green roofs will make a difference to a city,” he says.'