Remarkably, in a city as densely built as New York, roof space accounts for only 19 per cent of the city’s total area. Photo: Lynne Blundell

FAVOURITES – 6 MAY 2009 -The second and final part of an article on rooftop gardens by Liz Morgan.

What is a green roof?

Variously called green roofs, eco-roofs, rooftop gardens, vegetated rooftops, sky gardens, and so on, all rooftop green spaces are basically lightweight, engineered roofing systems that enable the growth of vegetation on conventional rooftops, whether factored in as part of the design of a new building or the retrofitting of an old one.

Green roofs come in all shapes and sizes. Perhaps the smallest in the world is a bus shelter roof in Sheffield, South Yorkshire at six square metres (although I am contemplating green roofing my letterbox, 240 square centimetres). The largest is the Ford Motor Company’s Rouge Manufacturing Plant in Dearborn, Michigan, covering an area of 4.2 hectares.

Landscape architects tend to put green roofs into two categories:

Extensive: These roof gardens, like that of Parliament House in Canberra and the Sydney Conservatorium of Music, are usually just a simple plant-covered surface. The growing medium is not very deep (2cm-7cm) and therefore only suitable for growing plants with shallow root systems. The roofs are lighter and cheaper to install than intensive systems; they (usually) require less maintenance and in general do not need irrigating. A possible option for new buildings and for retrofitting existing buildings.

Intensive: These roofs are generally what we would think of as a domestic roof garden or terrace: a place with tubs of shrubs, small trees, groundcovers, as well as paving and other hardscaping, and garden furniture. Other intensive roofs, however, resemble extensive gardens but with taller species of plants. Intensive roofs are charactersied as having a deeper growing medium, allowing trees and shrubs to be planted. However, they are heavier, thus often requiring extra structural design to the building, and usually needing expensive irrigation systems. Best suited for new buildings but possible in refurbishment projects. In Australia, one of the best-known and longest-established intensive green roofs is the heritage one that sits atop the Readers Digest building on Waterloo Street, in Surry Hills, Sydney, designed by landscape architect Bruce Mackenzie in 1967.

What are the benefits of green roofs?

  • REDUCING LOCAL TEMPERATURE “Toronto found that the green roofs could reduce the temperature by up to 2 degrees if they had 8 per cent green roof covering across the city”
  • STORMWATER MANAGEMENT Research from around the world indicates that green roofs can reduce annual run-off from roofs by at least 50%, and more usually by 60%-70%, contributing to urban drainage and flood alleviation schemes. Moreover, the rate of release following heavy rainfall is slowed, reducing pollution and other problems associated with storm surges.
  • ENERGY CONSERVATION AND GREENHOUSE GAS REDUCTION Green roofs can reduce the need for air conditioning in the summer and heating in winter, thus saving on energy use and lowering CO2 emissions. The Canadian city of Toronto reports annual savings of up to $C22 million from its green roofs and Chicago is said to save about $US150 million. In recognition of rising average temperatures, Tokyo has mandated that all new buildings with roofs of more than 1000 square metres must be greened, in an effort to moderate the city’s climate. The American Society of Landscape Architects says green roofs can reduce the heating and cooling costs for buildings by at least 10 to 15 per cent.
  • COST SAVINGS The American Society of Landscape Architects says green roofs conservatively can be expected to last two to three times longer than a conventional roof.
  • BIODIVERSITY AND HABITAT Green roofs can contribute to biodiversity and provide breeding and feeding sites for animals displaced by development. One project in Deptford, London was initiated to provide a breeding ground for the endangered black redstart, whose existing habitat was being cleared for development. Roof gardens can provide habitats for birds, amphibians, insects and important pollinators like bees.
  • QUALITY OF LIFE Green roofs contribute to a greener urban environment and quality of life for communities in high-density developments, and this can be seen in the high uptake of the green roof concept in Japanese and European cities. The benefits of green spaces to human well being are well documented, as is their effect on worker productivity and satisfaction.
  • CLEANER AIR/HEALTH BENEFITS Because plants take up carbon dioxide (a greenhouse gas) and exude oxygen they help purify the air. In Egypt, especially in vast, crammed cities like Cairo, the government is encouraging people to grow food on their rooftops to improve local air quality, as well as providing fresh food.

· GROWING FOOD Other than energy generation, I can think of no better sustainable use of rooftop space than growing food (and the two activities can coexist). It doesn’t get much fresher and local when the chef or home cook can pick vegetables, fruits and herbs from the roof of their building. In the island-state of Singapore, where almost all food has to be imported, a survey by polytechnic students identified about 212 hectares of apartment and commercial rooftops in four suburbs they estimated could be used to grow about 39,000 tonnes of hydroponic vegetables a year, worth about $S40 million a year.

“Imagine, for example, the effect of community kitchen gardens on the rooftops of New York City. All sorts of neighborhoods might become urban agricultural districts, the growing of food providing sustenance, conviviality, a relationship with nature and an opportunity to enjoy the fruits of ones labour. The gardens could also make visible the vital connection between water, soil, food and human culture. At the same time, they would create a network of living landscapes stretching across the ancient archipelago that is New York City. Add to that the energy savings a sea of green roofs might provide as they cool buildings in summer and insulate them in winter-according to The New York Times, those savings could amount to as much as $16 million a year.”

“A Field of Dreams”, an essay by William McDonough as introduction to Green Roofs: ecological design and construction.

What are the barriers to introducing green roofs?

  • Lack of awareness and knowledge about the concept and the technology.
  • Absence of external incentives to implement green roofs, e.g. state funding or tax breaks. A possible solution is to implement a scheme whereby a certain percentage of the total capital cost of publically funded projects could help meet the cost of installing green roofs or green walls in public buildings.
  • Higher costs at initial stages than building conventional roofs.
  • Risk associated with the absence of green roof industry standards and building code standards.
  • Green roofs are mostly unsuitable in situations where a pitched roof is the better design option.

Do green roofs actually make a difference?

The short answer would appear to be yes; there are proven stormwater management and energy saving benefits, not to mention aesthetics. How much of a difference is contestable. The most extensive modelling of the urban-scale benefits of green roofs in the United States has been done in New York. A study by the Center for Climate Systems Research (CCSR) at Columbia University’s Earth Institute estimates that in New York, fully 50 per cent of all roof space would need to be greened in order to have a significant impact on the city’s heat island. The multidisciplinary study group, which relied on data and expertise from Pennsylvania State, Michigan State, and Columbia University, settled on the 50 per cent baseline after deciding that 75 per cent coverage was an overly ambitious figure.

Modelling indicates that if 50 per cent of New York’s rooftops had gardens it would shave 1.4 degrees off the city’s heat island.

Their modelling indicates that 50 per cent coverage would shave 1.4 degrees off the city’s heat island, which ranges from 5-7 degrees. What accounts for the relatively small impact even at half coverage? Remarkably, in a city as densely built as New York, roof space accounts for only 19 per cent of the city’s total area (when seen from above as a single plane). While the difference between a 93- and a 94-degree [Fahrenheit] day may not feel significant, it can have a massive impact on energy use. According to estimates by CCSR for the New York State Energy Research and Development Authority, every degree of temperature increase outdoors triggers demand for an additional 60 gigawatt hours of energy per day.”

The CCSR concluded that because New York already has plenty of cool spots (110 square kilometres of parks, the Hudson River, etc) a faster way to tackle heat islands is to target green roofs in hot spots like the airport and industrial areas, rather than aim for a citywide introduction.

In Australia, there is a growing discourse about how we have taken our eye off the ball in regard to alternative, more sustainable technologies, solar power in particular: how whereas there once was a hope and an expectation that Australia would be a world leader alternative technologies, we watch as the best of our innovators move their operations offshore. People in European and Scandinavian countries scratch their heads in disbelief about how a continent with good sunshine nearly all year round still heats and cools its buildings with electricity generated largely from burning coal.

The Australian writer Thomas Kenneally, writing in the British Guardian newspaper recently about the horrific bushfires in Victoria, said “it seems to me that this calamity has shaken climate change sceptics”. I hope he’s right, and I hope many of those sceptics are in government and other positions of power and influence to enable change. In my view, the public is ready for sustainable change; they are just waiting for strong leadership.

In the public consultation process for the City of Sydney’s “sustainable Sydney 2030” vision, members of the public said they want a city that is “sustainable with a low-carbon footprint”; “that is clean, healthy, efficient”; “that supports design for self-sustaining buildings”; “that is creative, edgy and gritty”, and so on. The Sydney Metropolitan Strategy says: “The big issue for Sydney is ‘future–proofing’ it against the effects of climate change, and significant increases in energy and fuel costs that can be expected to result from our need to reduce greenhouse gas emissions on the one hand, and reduced supply and greater demand for oil on the other.”

Many Australian cities’ sustainability plans refer to the role the built environment can play but they focus solely on ground-level spaces, be they parks, squares or streetscapes. And while the sustainability strategies aim to improve energy efficiency and water management, they seem unable to see what is staring them in the face: the cities’ rooftops can help fulfill all these functions using existing infrastructure. As the Design for London leader said, the space above our heads is a city’s most under-used asset. Let’s shout it from the roof tops: Green Roofs for Sustainable Cities!

Further reading:

Cantor, S.L. (2008). Green Roofs in Sustainable Landscape Design. New York, London; W.W. Norton.

Earth Pledge Foundation (2005). Green roofs: ecological design and construction. Schiffer Publications.

Werthman, C. (2007). Green Roof: a case study. Michael Van Valkenburgh Associate’s design for the headquarters of the American Society of Landscape Architects. New York; Princeton Architectural Press.

Liz Morgan is a freelance journalist and a PhD student, researching food security and climate change, at Macquarie University.


From – Sidonie Carpenter – President, Green Roofs Australia.

Its with great interest that I read your article on Green Roofs are a Cool Idea, parts one and two, published in The Fifth Estate.

It is great to see and hear of so much conversation regarding green roofs finally happening in Australia. It’s definitely a very exciting area with huge potential and I believe an area worth considering in regard to the Australian built environment.

I am a little concerned with some of the facts that you state – in particular the depth of the extensive roof profile being 2–7 centimetres. Extensive roof profiles are typically 10–30 cm with the possibility of any green roof in Australia surviving on less than 20 cm being very slim

Yes there are some roof profiles in Europe and Nth America that are very thin – this would not be viable in Australia with our temperature and water issues.

The roof on Parliament house is over 100 cm thick – yes, simple because its only supporting turf but the soil profile is quote complex.

In regards to PV [photo voltaic] cells and green roofs there has never been such a perfect opportunity for integration of design.

The issue with PV in Australia is actually that its too hot. The optimum running (energy producing) temperature for current technology PV is around 23 degrees – most roofs in Australia, even in winter are more than double this temperature – rendering PVs into what is called a “brown out” and energy productions drops dramatically.

By installing PV on a green roof and dropping the ambient air temperature around the cells you can potentially increase its efficiency by up to 25 per cent.

Well done on your article. The issue of food production/security is a area of great interest and of course the huge opportunity to incorporate it into our urban areas on our roof tops.

I have just been on the judging panel for the Growing Up–Greenroof for Melbourne CBD competition and you will be pleased to hear there were three entries that incorporated food production into their green roof design – with one of the designs taking out third place.

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