Gardens in the sky

As city living becomes denser and warmer, innovative ways to keep cool, increase green space and keep urban areas attractive are always in demand. 

Since 2008, researchers at the Melbourne School of Land and Environment (MSLE) have been studying the best conditions to grow plants on Australian roofs – known as Green Roofs – to make the most of these ‘natural air conditioners’.

A team led by Nick Williams and John Rayner has now established the Burnley Green Roofs Project, which demonstrate many of their research findings – showcasing the best plant species and specially designed soils to use on city roofs and demonstrating how green roofs can recycle storm water and reduce building energy use. 

The project comprises three roofs located on the main campus building, which cover 300 square metres at an average of seven metres off the ground. The green roofs were designed by a multidisciplinary team including MSLE staff and led by design practice HASSELL. The main focus of the roofs is to facilitate small group teaching, undertake research and demonstrate the variety of green roofs available to the building and development industry.

Dr Williams says Green Roofs have a range of environmental benefits that could help adapt Australian cities to climate change, as well as social and economic benefits that could make denser cities more liveable and attractive.

“Cities suffer from the urban heat island effect which makes them up to four degrees warmer than surrounding areas. Green Roofs can help overcome this by reflecting the sun’s radiation and providing shade or evaporative cooling as well as dramatically reducing a building’s energy costs, trapping dust and pollutants and dampening noise. They also provide significant benefits in reducing stormwater runoff,” Dr Williams says. 

The first of the three Green Roofs is a large demonstration garden consisting of 14 different green planting zones including grasses, flowering plants and a vegetable garden, all linked by a distinctive red line. This main roof has a range of growing and irrigation treatments to showcase how beautiful and multi-functional green roofs can be. 

The second roof is dedicated to research that aims to quantify the environmental benefits of Green Roofs and plant performance. The third is a biodiversity roof, comprising a range of habitat features to encourage and sustain local wildlife.

In 2008 the research team at the Burnley campus was the first to build a Green Roof for research in Australia, which, although significantly smaller revealed a 38 per cent reduction in summer energy use.

“Green Roofs act as a sponge reducing storm water runoff and potential flooding while lowering local temperature through evaporative cooling,” Dr Williams says.

“We also want to use the biodiversity roof to understand how city roofs can be used to provide habitat and act as ‘stepping stones’ to link fragmented urban wildlife habitat. This roof includes local native plants that are food sources for caterpillars, an intermittent stream and sticks for native bee habitat.”

Despite the many benefits of green roofs, and their popularity overseas, they are not common in Australia.

Mr Rayner from the Melbourne School of Land and Environment says the aim of the Burnley Green Roofs Project was to illustrate what is possible, from non-irrigated succulent beds to deeper, productive vegetable gardens.

“Green Roofs are functional and beautiful spaces that can be built new or retrofitted to existing buildings. Planning and design are key components to successful Green Roofs and the Burnley Green Roofs are an example of this,” he says.

“We wanted to display a range of plants that can be successfully grown, but many need a certain depth of growing medium. Given we can’t use a heavy soil on a roof we developed lightweight growing media, together with foam building blocks to create raised beds.

Much of the growing media is recycled, including crushed roof tiles and ash waste from power stations as well as scoria, a lightweight volcanic rock.

“We also wanted to maximise the plants and vegetation on the roof while maintaining access, so there are shallow planted areas beneath grated metal walkways.”

The main demonstration roof is approximately 166 square metres and includes over 3000 individual plants from more than 200 different species. For over a year team members including Sue Murphy, Claire Farrell, Jenny McCoy and Annette Warner selected and raised plants in the Burnley nursery to ensure a wide variety would be ready for use on the roof. 

Such a significant roof structure required inventive design solutions that were provided by the design team at HASSEL and the landscape contractors Junglefy.

Many elements of the roof were pre-fabricated and test assembled off-site, then transported and reassembled on the rooftop – representing innovation in design, research and construction process, according to HASSELL’s Stephen Tan.

Ongoing research and the design lessons from the Burnley Green Roofs will now be used to provide technical knowledge for architects, landscape designers and public policy makers to install green roofs in Australian cities.

www.land-environment.unimelb.edu.au/expertise/sustainable-cities/girg/