Small changes for a greener city

BY MATTHEW LEVINSON
Sydney could save $185 million in energy costs over five years by adding green roofs to city buildings. And it can be done with simple changes to planning guidelines, according to a report prepared for the City of Sydney Council.
Covering flat concrete rooftops with lush gardens or ‘green roofs’ can improve building insulation, reducing the need for air conditioning on hot days.
These rooftops can house green energy sources such as solar hot water and photovoltaic panels. Depending on the plants used, they can also provide a green corridor for birds, butterflies and other native fauna, and even grow food for residents.
Tone Wheeler, a leading environmental architects and co-author of the Green Roof Retrofit Design Model report, said there are a “pitiful” number of rooftop gardens in the city now, probably less than 10.
“Between 60 and 65 per cent of all buildings in the Sydney CBD could have some form of green roof,” Mr Wheeler said.
In mature cities like Sydney, researchers have shown that buildings consume up to half of the city’s energy. But the journal BioScience estimated last year that green roofs could cut air conditioning costs by 25 per cent, and halve electricity use.
There is no data on the benefits of green roofs in Australian conditions, but research from Toronto, Canada, suggests Sydney residents could save as much as $125 million initially, followed by annual savings of up to $15 million.
A search of development applications to the Sydney City Council shows just five of these projects underway.
Tone Wheeler’s firm, Environa Design, along with landscape architects Material and a number of water, environment and economic experts, received a $48,000 Sydney City Council grant 12 months ago, to find ways for the city to encourage green roofs.
They built an experimental green roof on top of a Pyrmont apartment building. They delivered the results in June this year in a design reference manual and accompanying strategy.
Green roofs provide an undisputed opportunity for the city, according to the report’s authors. But at more than $200 per square metre of green roof, building owners need encouragement.
The report recommends five steps to spur new green roofs in Sydney: promotion, data collection and economic modelling, case studies, planning changes and incentives.
Changes to planning guidelines would provide a relatively cheap and effective incentive for building owners, Mr Wheeler said, compared to cash incentives such as rate reductions.
“A minor amendment to the planning laws can encourage better buildings,” he said.
“This could be done by giving [building] owners more ground space or an extra floor in the building, which can be offset by the whole of the roof being accessible to grow plants or food.”
Structurally, the main issue with vegetated rooftops is managing the weight load and avoiding corrosion or decay, according to structural engineer Bala Muhunthan.
“The waterproofing needs to be guaranteed and the roof designed for the load,” Mr Muhunthan said.
“And it needs to be maintained to make sure no water gets through to the membrane.”
The report’s authors found that the main weight load associated with green roofs ‘ soil ‘ is unnecessary because grasses and low-lying plants can actually grow in a thin carpet or felt-like material.
They also showed that ‘portable’ garden beds, which can be moved with a crane or fork lift, would help cut the risk of corrosion or water damage.
“This means the most serious engineering concerns can be marginalised,” Mr Wheeler said.
A spokesperson for the City of Sydney said the report has been received and is being reviewed.