Green roofs have come a long way since the Hanging Gardens of Babylon.
Green roofs are being recognised as a critical addition to our urban areas as more and more man-made structures are squeezed into cities to keep up with population demands.
The environmental benefits of green roofs are well documented: absorbing carbon, reducing energy consumption, helping with stormwater management and offering acoustic support. They also offer opportunity for bio-diversity and food production in urban areas.
Another key benefit to green roofs is that they mitigate the urban heat island effect. A chapter study by the United States Environmental Protection Agency (EPA) reported that green roofs were substantially cooler than their dark or white counterparts.
Comparing two roofs in Chicago, the study found that on a 32 degrees celsius (90 fahrenheit) day in August, the green roof temperature ranged from 33 to 48 degrees celsius (91 to 119 fahrenheit) while an adjacent dark conventional roof dark temperature was 76 degrees celsius (169 fahrenheit).
“The near-surface air temperature above the green roof was about four degrees celsius (seven fahrenheit) cooler than of the conventional roof,” the report read.
A similar study in Florida found that the average maximum surface temperature of a green roof was 30 degrees celsius (86 fahrenheit), while the adjacent light-coloured roof was 57 degrees celsius (134 fahrenheit).
Green roofs have also been shown to have healing properties. Humans love greenery and research demonstrates that the presence of plants can directly contribute to health and well-being. Healing gardens in healthcare spaces help patients to recover more rapidly, while office workers become more productive when working around living plants or enjoying a green view.
The roofs also offer a green community to cities and building inhabitants. There is nothing quite like retreating to a lush canopy sky garden atop a skyscraper or visiting a roof terrace during office hours to escape from the glare of digital technology.
Ecological artist Lloyd Godman is pleased that “greening cities” has become a mainstream movement and anticipates more government interaction in the future, and offers some suggestions that can help ensure greening success.
Godman always recommends using plants native to the region – it’s their best chance of survival.
For example, the recently opened Sky Garden in London atop 20 Fenchurch Street (the Walkie Talkie building) has utilised drought resistant Mediterranean and South African species.
According to Sky Garden’s website, it is London’s tallest green roof and is housed beneath a canopy. However, the plants have been specifically chosen to work in harmony with the particular quality of light found under the canopy so flowers can flourish all year round.
Godman also encourages architects to look at building materials when selecting plants.
“If we look at our cities, the building materials that we use for all the multi-storey buildings is a refined geology: rocks, glass, steel and all kinds of concrete,” he said. “They’re like a raw mountain which can struggle to grow plants and unfortunately people look to just keep putting substrate on top of it that will retain water and grow plants that need a lot of water. Why? Because it looks good.”
Godman suggests using plants that are naturally evolved to grow on cliffs - Tillandsia plants and similar plants that have soft tissue.
Tillandsias (air plants), succulents, cacti and some orchids and bromeliads are great for green roofs thanks to their CAM cycle, which means they absorb CO2 at night and remain more dormant during the day.
“The cell – the little stomata takes the carbon and releases oxygen," he said. "It also transpires water and that these plants have evolved to do is they close that cell during the day so they retail all their moisture when it’s hot."
"At night, they will open and absorb carbon making them great for thriving in this environment."
Godman is currently conducting an air-plant experiment atop Melbourne’s Eureka Tower at level 92.
“Air plants in particular can go on buildings with little maintenance," he said. "They’re particularly good for seriously high buildings as they can be installed at the development stage.”
Godman noted that there is also an issue related to soft vs. hard tissue plants.
“If a soft tissue plant falls of a building, it’s not going to kill anyone,” he said. “It is similar to a leaf coming down a building.”
“Hard tissue plants on the other hand are bits of wood, branches etc. Should you have a tree on rooftop and amid a storm with 100 kilometre winds a branch breaks off, it will not float like a leaf, it will go directly over the edge of a building and hit whatever is below."
Godman is concerned with safety on certain projects, such as Milan’s Bosco Verticale, the recently completed twin apartment towers which house approximately 900 trees, as well as the planned Clearpoint Residencies apartment building in Sri Lanka.
Clearpoint Residencies will feature plantings on terraces on each of its 46 floors tall, and residents will be able to grow their own fruit – including mango trees.
Godman sees this as being potentially “disastrous” should a tree fall of a roof or terrace.
Maintenance, unfortunately, is not generally discussed beyond the measurable substrate, beautiful flora or irrigation system. The fact is, green roofs require maintenance – a lot of it.
The recently completed One Central Park building in Sydney has six full time gardeners something Goodman said is necessary.
“It costs approximately 20 to 30 per cent of your installation cost to maintain a green roof or vertical garden,” he said, noting that many are decommissioned because maintenance costs are too high or plant selection wasn’t deeply considered.
In his recent trip to Paris, Godman found that many Parisian buildings have additional plants growing behind the building ready to replace those that have died on roofs.
“This isn’t a problem,” said Godman. “We just need to be honest about it and know that plants have a limited life, they do need maintenance. I don’t like the term 'self-sustaining. I just don’t see it that way.”
From an economic point of view, this does show promise for the landscape employment sector and rising opportunities for garden maintenance jobs.
In terms of systems, most green roofs use hydroponic systems and damaging nitrates and pink phosphates are being used to fertilise plants and keep green roofs alive.
Godman sees this is a future issue as phosphorous is a finite resource.
“There’s no alternate to phosphorous and we’re placing great demands on it through our agricultural system that we’re likely to run out in 30 or 40 years time,” he said.
When it ends up in waterways, phosphorous can also damage algae.
Godman suggested a phosphorous urine catcher as a possible option.
Last year, in a bid to find a natural fertilising system, Amsterdam’s water company Waternet announced an experiment that would see them collecting urine from public urinals as an alternative to phosphates to irrigate the green roofs of the city.
This experiment also prompted the question of its viability for many agricultural uses.
According to an article in People, Planet, Profit, Waternet is constructing a facility that will make 1,000 tons of fertilizer from the wastewater of 1 million people.
Godman offered a few final final predictions regarding the future of green roofs.
- Buildings without plants on them are eventually going to look old fashioned - buildings need to become an extension of our parks.
- City councils will bring in bylaws like they did with insulation where builders will have to cover a certain percentage of a building's surface - roof or façade - in plants.
- Echoing the words of Phillip Johnson, he noted that “If you select the right plant, that will work in this aspect then you will have success.”
“It’s the only way we are going to cool this planet down,” he said.