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Green roofs and walls – a growth area in urban design

As the demand for greener and cooler cities increases, new “green infrastructure” technologies, such as green roofs and walls, are coming to the fore. But what are they? Put simply, green roofs and walls…

This green roof demonstration has 14 different combinations of substrate, depth, plant type and irrigation. John Rayner, University of Melbourne

As the demand for greener and cooler cities increases, new “green infrastructure” technologies, such as green roofs and walls, are coming to the fore. But what are they?

Put simply, green roofs and walls are landscaped building surfaces. Australia has been relatively slow on the uptake of this movement, with much of the available expertise, systems and materials based in the northern hemisphere.

Last month saw the release of Growing Green Guide: a free guide to green roofs, walls and facades in Victoria – a collaborative local and state government project, which I contributed to.

And this weekend at the Melbourne International Flower and Garden Show there will be a pop-up garden display by the Melbourne School of Land and Environment demonstrating many of the ideas and techniques behind green infrastructure.

But for those who are not in Melboure and/ or won’t be able to give over their weekend to the delights of Australia’s urban horticulture industry, here’s a snapshot and some important design factors.

The impact of green infrastructure

A trellis-based facade in Copenhagen, Denmark. John Rayner

Green roofs, as mentioned already, are a landscape constructed on a roof and include a range of different types. They can be a small, container beds two metres deep supporting trees and shrubs, or lightweight configurations of only a few centimetres in depth and more than a hectare in size covered in succulent and herbaceous vegetation.

Green walls fall into two very different types. Living walls support plants through irrigated, vertical containers or felt-based structures fixed to a wall surface. Green facades use climbing plants to provide green coverage over a wall, either directly on the building surface or more commonly using a steel trellis or cable system, with plants grown in-ground or in containers that are supported across the building facade.

The evidence of benefits associated with green roofs and walls is increasing as research efforts (from the US, UK and Australia) expand across the globe. Green roofs have been shown in a number of studies to reduce building energy budgets, slightly reducing winter heating costs but providing more significant reductions to summer cooling.

A modelling study in Toronto on the implementation of green roofs across low-rise, flat roofs greater than 350 square-metres concluded that ambient air temperatures across the city could be reduced by up to 2C.

Millenium Park, Chicago. John Rayner

The benefits of retaining stormwater on green roofs are well established and tend to form the basis of many city green roofs tax or incentive schemes, particularly in North America.

Green roofs act like a sponge on a roof, absorbing and storing rainfall and allowing re-use form plants, in turn reducing urban water run-off. One study involving a green roof with a 60mm deep substrate (soil substitute) showed rainfall retention of almost 83%.

Green roofs and walls have been shown to increase property values, partly because people like to view them, but more because they provide more recreational and amenity uses. In one New York study rentals were shown to be 16% higher in buildings with green roofs compared to those without.

The dramatic growth in green roofs and walls across the US over recent years, despite generally poor economic conditions, suggests demand for these installations will continue.

Why not try this at home?

Chicago Cultural Centre extensive green roof.

Naturally, green roofs and walls need to be planned and designed properly to ensure they are successful. The most significant issue to consider is weight loading. A roof or wall must have the structural capacity for the mass of a green roof, wall or facade installation – both at construction and over time.

Not all plants will succeed on the sometimes hostile and elevated environment of a roof or wall; and vegetation growth is strongly limited by the depth, available volume, air, water, nutrition and environmental conditions present at the site.

Low growing succulents, such as sedums, and drought-tolerant perennials including many Australian plants, are widely used because of these constraints.

Waterproofing is another key issue and most structures will require additional specialised treatments to ensure the integrity of the structure is not compromised by the installation. Excess water will need to be drained away, particularly from green roofs.

Most modern systems, such as those produced by German green-roof company Zinco, incorporate lightweight plastic drainage cells to provide this outcome.

The High Line in Manhattan, New York City at West 20th Street. Wikimedia Commons

While the Growing Green Guide was written to increase local understanding and skills amongst architectural and design professionals in Melbourne, it is also relevant to a wider Australian audience.

Who knows? Australia might even have its own version of New York’s aerial greenway High Line one day.

Are you an academic or researcher working in urban design? Contact the Arts + Culture editor.

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7 Comments sorted by

  1. Michael Shand

    Software Tester

    Great article, really interesting stuff, hopefully it takes off.

    Unfortunately the proposed buildings across the railway tracks at flinders street included one proposal that had a green roof that you could walk on and was rejected over a design that had a wanky curved wavey roof

    1. MSLE

      logged in via Twitter

      In reply to Michael Shand

      Agreed, excellent article. Thanks John. The University of Melbourne display garden showcasing green infrastructure at the 2014 Melbourne International Flower and Garden Show was also an outstanding showpiece for where urban horticulture is headed.

  2. George Naumovski

    Online Political Activist

    If it’s not mage profitable, then they will not even look at it. Cities are being built up instead of wide because from one piece of land you make more profits from multiple stories of offices or apartments. If you want to do something like this “green cities” then you are labelled a greeny tree huger to put it mildly but the way things are going, the heat factor from population growth/squeezing more people into spaces will make people sick and costly to cool down and as the cost of living sky rockets as it has, aircon will be only for the rich. This needs to be seriously looked at.

  3. Michael Lardelli

    logged in via Facebook

    Well I suppose if the government is going to force population growth in Australia leading to increased densification of cities that raises temperatures and run-off due to increased hard surfaces then green roofs and walls may help to counter it. But it would be cheaper simply not to increase the population size. Also, in drier areas (such as in many of the hotter parts of Australia) the water will need to be pumped up into buildings which is a very energy-intensive exercise. So really, the only thing "green" about this infrastructure is its colour.

    1. John Doyle


      In reply to Michael Lardelli

      If one takes a long view our cities are going to look a lot different from today. All our modern cities are built on cheap energy, oil in particular.
      This is not sustainable. Maintaining energy hungry buildings is not affordable for much longer. This means skyscrapers will be left to moulder and only low rise will be sustainable. In such cases a green roof, likely growing food will be useful. Vertical gardens on skyscraper walls will not be viable as they require too much energy to sustain. and the idea we could have them made into multistorey farms is fanciful.
      We don't yet know what the future will be but we can be certain of a decline in our civilization as energy gets expensive. Then we'll have to see. What we have now cannot continue for much longer.

    2. John Rayner

      Senior Lecturer in Urban Horticulture at University of Melbourne

      In reply to Michael Lardelli

      Thanks Michael. Water can readily be harvested from hard surfaces for reuse - and from buildings for that matter. In fact even in drier Australian cities we have more than enough rainfall that could be harvested to support a range of 'greening' options - green roofs and walls included. We also know we can sustain succulent vegetation without irrigation - more brown than green over summer - but still beneficial.

  4. Terry Reynolds

    Financial and political strategist

    I developed a large residential development Beach Road, Hampton in 1994/5 with six townhouses and 46 apartments with carparking under the buildings and about 50% open space. On the top of each of the six terraced townhouses and the twelve apartments with a rooftop, I substituted a pitched roof for a flat solid concrete roof which was tanked and covered with screed and "terra cotta" tiles.

    All apartments had balconies and Paul Bangay did the landscape design for our gardens which look magnificent…

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