Our urban desires replaced native vegetation with a concrete jungle, the skyline now shaped by lanky apartment buildings and mixed-use developments rather than tall gums dotting vast fields. Once sprawling open spaces and farmland, urban areas now play host to around half of the world’s population. And Australia is a particularly urbanised country, with about 64 per cent of its population residing in inner-city dwellings.

This growth has seen our cities become so built up that in warmer seasons they mimic a baking oven – trapping and storing heat within their walls. And with climate change set to deliver hotter summers still, many city’s temperatures will continue to rise to abnormal heights not felt by outlying suburban and rural areas.

The progressive replacement of nature by heat-absorbing materials, such as dark-coloured roads, roofs and concrete has contributed to the formation of the Urban Heat Island effect (UHI). This process sees heat trapped within the building materials of a city, causing excessive rises in temperature within the CBD, which then snubs natural tendencies to cool once night has fallen. In fact, once dusk has faded to dark, the heat stored within these hot surfaces is released back into the atmosphere, creating uncomfortably warm evenings, while those folk skirting the city sleep peacefully in naturally cooler temperatures.

One solution to alleviate UHI is to design with heat resistance in mind, utilising higher albedo paints and reflective materials to reduce the amount of radiation absorbed into urban surfaces (which will be released back into the atmosphere as heat). The other solution is to design more green spaces for cities.

If you wander outside of your air-conditioned office tower during your lunch hour to discover sweat beads instantly appearing on your brow, take a quick jaunt to an inner-city park to recline under a tree and you’ll find the outdoor temperature is up to 8˚C cooler beneath your leafy shelter. Within green spaces, the soil stores excess moisture, while the trees absorb radiation and release water vapour back into the atmosphere in a process called evapotranspiration, which, in turn, cools the air.

While most cities currently maintain at least one major park or urban forest within their centre, there’s another layer to a city’s architecture we could be making more use of to conquer UHI – our rooftops. Real estate is a valuable commodity at ground level, yet the flat rooftops of many city developments continue to go to waste.

These underutilised spaces – particularly flat ones, with easy access and a water source – are prime positions for rooftop farms and gardens. Placed in full sunlight, rooftops can be the perfect site for helping to build a green city.

Whether used for growing food or merely creating additional greenspaces, rooftops gardens not only mitigate UHI, but also reduce the building’s reliance on air-conditioning, saving energy through natural sources. Rainwater can also be captured to water the garden – managing stormwater more effectively by reducing run-off entering waterways – and can be reticulated to filter throughout the building, reducing water inputs.

If more rooftops were transformed into vegetable farms, food could be grown locally in the heart of a city, lessening food miles, increasing food security, and contributing to a more sustainable city lifestyle.

In Melbourne there’s The Little Veggie Patch Co’s Pop Up Patch, which has transformed an unused car park into an edible gardening club. Abroad, Brooklyn boasts Brooklyn Grange, a space comprising two rooftop vegetable farms across 2.5 acres. New York is also home to Eagle Street Rooftop Farm and Gotham Greens, as well as the High Line, a previously disused aerial railway inspired by the almost 5-km-long Promenade Plantée in Paris – both of which transformed disused rail lines. In Rotterdam, rooftop farm Dakakker was created by architecture firm ZUS, and supplies local restaurants with fruit, vegetables and herbs. In Tokyo, City Farm is located atop shopping mall DiverCity Tokyo Plaza in Odaiba. Here locals can rent a little patch of the rooftop to grow their fruit, vegetables and rice. Despite sustainability concerns surrounding the high usage of water involved in growing rice, this farming system is a great aid within the inner-city, fostering the plant evapotranspiration process and cooling the air surrounding the building.

Meguro Sky Garden, also in Tokyo, is another example of environmentally conscious designers utilising normally neglected spaces in a positive way – and a concept that was actually planned into the original architectural design. Built above the Shuto Expressway/Ohashi highway junction in Meguro, the garden follows the shape of the circular highway, its loop slanting from 15–35 metres above street level. Winding your way from top to bottom, you’ll encounter more than 1000 trees and shrubs, including native sakura, pine trees and grapevines, as well as an expansive veggie patch at the base. The highway junction – linking the Shuto Expressway with the Central Circular Route – was built to aid traffic congestion. However, thanks to clever design, not a single car can be heard within the elevated oasis. Lush patches of grass are lined with park benches and picnic tables ready for city dwellers to rest and recline at, soaking up a moment of stillness while the bustle of the city lies within view.

While Promenade Plantée opened in 1993 – triggering the idea that gardens can be designed atop buildings and transport infrastructure – more than 20 years on, only a handful of examples exist globally, with the more recent developments instigated through the urban agriculture movement. But if we care to mitigate UHI, improve air quality, aid a building’s sustainability rating, and consider its overall effect within its surrounding city – not to mention enhancing the aesthetic value of the city – then whenever a new tower is drafted, rooftop gardens should be considered in the design process. Imagine flying over a city and looking down to a grid of raised green squares surrounded by lower-lying parkland, rather than grey concrete blocks – a city within a forest.

—