SUSTAINABLE  CANBERRA                                           australian institute of landscape architects   AILA®

project introduction

Case Study # 3

Green Roofs

Green roofs for Canberra?

One could ask why in a city like Canberra, with copious green spaces, treed boulevards and wooded hills, we would need green roofs?  In this city, we have a least six green roofs, the most famous being the encompassing landscape over the top of Parliament House. 

Parliament House is a building with a whole lot of specially prepared soils on top.  Whilst the landscape, designed by the firm of landscape architects, Peter Roland & Associates (USA), is expensive to maintain, it provides environmental benefits to the building.

Looking out the window of any tall building located in Canberra’s central business hub, the viewer is provided with a panoramic view of the city’s well treed settings.  Look a little closer and more critically, there is a plethora of barren rooftops with their alarming air conditioning units.  As development in the city intensifies with pressure for commercial and residential sites, more green space will be consumed.  One key to dealing with global warming is that buildings and their landscapes (if they are lucky enough to have a landscape) should be working collaboratively with the environment rather than against it. 

Green roofs have multiple benefits.  They can provide insulation, reduce urban heat island effect, harvest rainwater, provide biodiversity, provide islands within wildlife corridors and even be used to grow food.  While their virtues have been recognised in Northern Europe, North America and Asia, they are slow to catch on in this country. 

Green roofs vary greatly from a thin layer of soil and waterproofing planted with prostrate succulents over a verandah roof through to the whole shebang, such as the landscape at New Parliament House.  The outdoor space at the House is an example of an extensive roofscape: a space for people to use in a garden setting.  In Singapore, a hospital has turned its disused roof space into a vegetable production area.  Tomatoes are grown on a trellis system and when harvested are included on the patients menu.

Japanese designers are canny at dealing with finite spaces.  Tokyo is one of the world’s densest cities: home to over 5000 people per square kilometre compared to around 400 per square kilometre in Canberra.  Japanese architects can manage to fit a house and garden into a 150 square metre site.  Many Australian designers would blanch at the prospect, even well informed greenies.  Our average block size in Canberra’s greenfields sites are around 500 square metres, and buyers still press for corpulent houses and garages. 


Roppongi Hills Keyakizaka

Japan is one of the nations leading the charge in green roof technology.  In Tokyo, local government authorities concerned about the lack of greenery and urban heat island effect (temperatures in the city have risen by three degrees over the last 100 years) are addressing this planning problem.  They have ruled that any buildings with a footprint of more than 1000 square metres, must include a minimum of 20% of the roof covered in vegetation.

At Roppongi Hills Keyakizaka, is one of the world’s most impressive green roofs. This complex is in Tokyo’s entertainment district and incorporates cinemas, motels and restaurants.

The developer, Minoru Mori desired a landscape that reflected both his rural upbringing and the seasons.  Covering 13000 square metres (over one hectare) this roof boasts a diverse environment.  Yohji Sasaki, the Japanese landscape architect has created a green roof with both intensive and extensive sections.  Soil depths vary from a mere 30mm to 1200mm.  Deeper soil profiles allow the growing of trees and large shrubs.

The landscape consists of a series of ponds populated with frogs, medaka (killifish), and dragon flies; a functioning rice paddy; organic vegetable garden with plants trained up bamboo poles; and a barbeque area for corporate functions.  Farmers are employed to instruct school children on planting the rice demonstrating this ancient cultural practice.  The paddy produces around 60kg of organic rice per year.  As in the old days, nothing is wasted.  The rice straw is stacked in sheaves to dry.  Once dry they are woven into traditional slippers.

The rooftop also serves another vital function.  It has been designed specifically to counterbalance the building mass during earthquakes.

One of the many benefits of green roofs is that they can capture, store and slow down the movement of rainwater.  At Roppongi rainwater is harvested and used for irrigation. 

In keeping with the clients brief, cherry blossoms mark Spring.  In summer, green rice shoots emerge from the paddy.  Copses of Japanese maple turn crimson, orange and gold in spring whilst the persimmon colours an intense orange.  While winter is marked by moon viewing through bare branches.    

However, the landscape represents more than a plant beautification scheme as it incorporates environmental, cultural and functional aspects.  As Graeme Hopkins, a South Australian landscape architect who visited an array of green roofs during 2005 stated:

This Tokyo rooftop demonstrates that a living ecosystem and a cultural rural ecosystem can work together with human recreational intervention.


Returning to Canberra from Tokyo, two cities as similar as chalk and cheese, we can ponder what role green roofs might play in a less densely packed city. Scientific evidence indicates green roofs can significantly reduce air temperatures assisting in reducing the urban heat island effects.    Given that with temperatures are predicted to rise up to six degrees Celsius by 2050 this could be of great benefit to Canberra.  Landscaped roofs not only look appealing but also act to insulate buildings from hot and cold extremes and reduce heating and cooling costs.

As the city grows, green spaces on the ground will be consumed by buildings.  The opportunity presents itself to incorporate green roofs into the proposed eco-development at Eastlake, on the shores of Lake Burley Griffin.   While planting cherry blossoms on these green roofs might be missing the point, these landscapes could trial local native plants.  Due to Eastlakes proximity to the Jerrabomberra wetlands and Lake Burley Griffin, green roofs could provide additional breeding sites for wetland birds. As well as exploring which plants might be able to cope with tough conditions on roofs, monitoring stations are required to demonstrate the environmental benefits of green roofs.  So like Tokyo, green roofs might all of a sudden start appearing on top of Canberra’s buildings.


 

Gardening Australia Magazine

Rooftop by Sue Barnsley Design (AILA)

American Society of Landscape Architects

Green Roof Project

Winston Churchill Memoria Trust Report

Graeme Hopkins FAILA

UK Green Roof DYI Guide

A Do It Yourself Guide for the amateur, small scale building / landscaping contractor, communities groups and all who want to enliven roofs with soils and plants.

Study on Green Roof Application in Hong kong

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