Feature

The rise of rooftop gardens

G Magazine

No room for a big backyard? A roof garden is the perfect way to maximise space and bring more green beauty into the city.

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Credit: iStockphoto

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Credit: iStockphoto

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Skyscrapers topped with trees and shrubs. Apartment blocks with lids of lush green. Inner-city homes with rooftop living spaces. It’s an idyllic vision but it’s fast becoming a reality as landscapers and architects in cities around the world embrace the benefits of green roofs. In Australia, roof gardens on commercial buildings have been around for years, while homeowners have only recently begun to embrace the idea.

“It’s really grown,” says Jock Gammon from Junglefy, an Australian company specialising in roof gardens. “Just in the last three to six months we’ve had a lot of requests from architects wanting to put green roofs on renovations. The main market is for roof gardens on extensions onto houses, maybe overlooking a new kitchen out the back.”

Apart from the obvious visual appeal, what are the environmental benefits of roof gardens and what’s involved in adding one to your home?

Eco appeal

Roof gardens bring serious environmental benefits to the building they’re above and the surrounding area. “They clean the air, attract wildlife and insects... filter and clean water and lower the stormwater run-off,” says Amanda Morphett of Green Gateway, a company based in Dunsborough, WA. This last property would explain the popularity of roof gardens in some American cities, such as Chicago and San Francisco, where people are taxed extra for stormwater run-off.

“[Green roofs] can also lower the electromagnetic radiation from other buildings, and can increase your solar panel production by reducing the reflective heat coming off the roof so the panels absorb better,” Morphett adds.

Another quantifiable benefit of a roof garden is the way it improves a building’s insulation, cutting the need for extra heating and cooling. According to community group Urban Ecology Australia, the layers of moist soil, mulch and plants act as a thermal sink, stabilising the building’s temperature, whatever the weather.
Roof gardens also protect the roof membrane beneath, resulting in a roof that lasts around twice as long as
a conventional one, Junglefy estimates.

Some councils look favourably on the inclusion of a roof garden when weighing up building applications.
Gammon explains that many local governments insist on a certain amount of soft landscaping (that is, vegetation) in outdoor spaces, not just decking or paving. “Some councils will now consider green roofs and green walls as part of that soft landscaping. Green roofs slow the rate the water runs off the roof so people might not have to put huge retention tanks in their backyards.”

Structural basics

There are two main types of green roofs. Extensive roof gardens have a relatively lightweight, shallow base of about 10-30 cm, low-lying planting and are really just for looking at. Intensive gardens, on the other hand, are deeper (up to 50 cm) with taller plantings. They can be used as an extra outdoor living space, just like a courtyard, patio or backyard.

Roof gardens can be filled entirely with plants, or a combination of paving and other hard surfaces with garden beds or planter pots. Bigger-scale roof gardens may include additional features such as ponds or swimming pools.

Most domestic roof gardens are built on new homes or as part of a renovation, but they can be retrofitted onto existing roofs, depending on the structure of the home and the materials from which it’s made.

“The first step is getting a structural engineer involved, to make sure the roof is suitable for the kind of garden you want,” says Gammon.

With their thinner, lighter layers of soil, extensive roof gardens rarely require extra structural support from
the building. For this reason, they are more easily built onto existing structures than intensive roof gardens,
as are vertical gardens.

Whatever kind of roof garden you have in mind, consider your home’s structural loading capacity, existing materials, the nature of existing drainage systems, waterproofing and the electrical and water supply in place.

“The type of garden you want can vary depending on the building type and how old it is,” says Morphett. “You can decide then if you need extra supports and what parts of the roof will take more weight than others.”

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