Maximise green space and reduce cooling costs with a garden on the roof
Australia’s population has passed 23 million, which means more people squeezing into cities and towns designed for far more modest numbers.
About 90% of Aussies live in urban areas, most in freestanding homes, and modern block sizes are being scaled down to accommodate us.
In the 1950s, the average block size was 810 to 910 sqm, dropping to 600 sqm by the 1980s, then 350 to 450 sqm by 2000. As of last year, new lots were as small as 187 sqm, with an average of 200 to 350 sqm.
Fortunately, a shrinking block size doesn’t have to mean a corresponding reduction in green space.
Melbourne-based company Fytogreen (fytogreen.com.au) has been addressing the issue in an innovative way for the past decade by establishing roof gardens on public, commercial and residential buildings.
National sales manager Stuart Tyler says the concept of gardens going up, which originated in northern Europe, is gaining traction here as engineers, architects and horticulturalists work together to create rooftop oases that last as long as the building itself.
Weighing up the costs
As with other energy-efficient additions, such as solar panels, installation costs are outweighed by the long-term benefits.
In addition to looking good and reducing energy costs, the gardens delay stormwater runoff, provide filtered rainwater and create habitats for birds and insects.
‘Costs can range from less than $200 a square metre up to $300, depending on the profile depth of the roof, but if you have a roof of less than 50 square metres, that cost is going to increase quite dramatically,’ says Stuart.
‘The infrastructure, the design process, getting everything there then lifting it onto the roof all has a bearing on the price,’ he adds
What to plant
The right plants for a green roof depend on climate and exposure to elements such as harsh sunlight and strong winds.
LAY trafficable turf and appropriate drought-tolerant grasses such as couch or kikuyu.
SELECT plants based on profile depth, water needs and weather tolerance as well as appearance.
ADD hardy species like succulents, sedums and native groundcovers.
PLANT trees but take the size into account as it affects structural costs.
USE drip irrigation for establishing plants and for keeping plants alive during prolonged dry periods.
For more information visit the Green Roofs Australasia website.
Types of garden
Fytogreen installs two main types of roof garden for different applications.
Extensive roof gardens have a thin growing medium less than 200mm deep and can be retrofitted on existing homes or sheds.
Intensive gardens support a soil depth of up to 1500mm or greater and substantial vegetation.
Says Stuart, ‘Extensive roof gardens maximise the outdoors, reduce energy emissions and create enhanced landscapes.
‘Any shaped roof is suitable, up to a 60º pitch if needed, and as long as you have the right structural support beneath it, you can clad the garden with as little as 25mm of plywood.
‘Intensive roof gardens are ones you can walk around as they have hard surfaces like paving, built-in seating, garden beds, a recreational area or maybe even a barbecue.’
An extensive garden is generally installed for its insulation and aesthetic properties.
The aim could be to deflect glaring light and radiant heat if located near a metal roof, or the owners may want to create an appealing outlook.
‘Extensive roof gardens are mostly all garden and you can’t walk around them as they have no hardscape areas like concrete and paving.
‘This type has a much thinner profile, is much lighter and typically you only go on it when you’re doing maintenance,’ Stuart says.
Plantings at the Holt and Hart rooftop garden in Sydney's Surry Hills
How it works
Before considering a roof garden, ensure the structure can hold the saturated weight loading of the entire profile, including mature plants. The surface also needs to be prepared with proper waterproofing and drainage installed.
Fytogreen uses a growing medium called Hydrocell, a lightweight, water-retentive soil enhancer that enables fast plant establishment and lasts for up to 10 years.
An extensive roof garden with a thin profile has a blanket of air created by the foliage above the soil.
There’s also air within the soil layer and an air gap with the drainage layer underneath.
The air in gaps provides the insulation and protection against temperature changes.
As water transpires in and out of the plants the composition of the layers changes. Water is held in the soil and flows out of the drainage layer.
Whatever the reason for the installation of a rooftop garden, the happy result is a marked reduction in cooling and insulation costs.
‘One of the true benefits of a green roof is it keeps hot air out of the building,’ Stuart explains.
‘In Japan I’ve seen an example where a green roof installation resulted in a 40% reduction in air-conditioning use. If you apply that to Australia, especially during our warmer times, there’s a huge energy cost benefit that can be achieved.’
Many countries consider green roof design an essential part of the national landscape, Stuart says.
‘It is far more accepted in northern Europe and in countries like Germany. In Dusseldorf, government incentive programs have led to a huge uptake of green roof construction.’
Stuart says although a green roof costs more, it pays for itself.
‘The idea is to last the life of a building or a home. We start work knowing it’s going to last a minimum of 20 years.’