Patches of Green in a Concrete Jungle: The history and future of green roofing and rooftop gardens

By Hayley White

Reading time: 12 minutes

As cities extend into farmlands and things slowly become less green, the dreaded (or for some, beloved) concrete jungle is becoming more widespread. With urban cities growing and houses being built throughout New Zealand, I wonder how we are going to lessen the environmental impact.

In a perfect world, there would be trees everywhere, with gardens, flowers, and greenery interspersed throughout our cities. But without enough space, this does not seem possible. What many may not realise however is that green space can be created. At home, this can often look like a little potted garden on the balcony of one’s apartment. And a very select few might grow a garden on the top of their dwelling.

By Hayley White

Reading time: 12 minutes

As cities extend into farmlands and things slowly become less green, the dreaded (or for some, beloved) concrete jungle is becoming more widespread. With urban cities growing and houses being built throughout New Zealand, I wonder how we are going to lessen the environmental impact.

In a perfect world, there would be trees everywhere, with gardens, flowers, and greenery interspersed throughout our cities. But without enough space, this does not seem possible. What many may not realise however is that green space can be created. At home, this can often look like a little potted garden on the balcony of one’s apartment. And a very select few might grow a garden on the top of their dwelling.

How many romcoms have you watched where the broody, lovestruck young woman walks to the top of her worn-down apartment building to wander among the flowers of her rooftop garden? One of the most iconic rooftop garden scenes is in the film Just Like Heaven (2005) with Reese Witherspoon. Rooftop gardens and green roofs are now moving beyond romance to become more and more popular around the world.

The concept of growing a garden atop a building is not a new phenomenon by any means. In fact, numerous cultures throughout history have utilised flora and fauna – sometimes even entire trees – to beautify and add important infrastructure to buildings.

The earliest evidence of any kind of green roof was noted way back with the ziggurat structures of ancient Mesopotamia. Built in the courtyards of temples in major cities from the fourth millennium to 600 BCE, these ziggurats were stepped pyramid towers. On the landings of each step or terrace, trees and shrubbery were planted to give climbers some well-earned shade and relief from the Babylonian heat.

Numerous other nations carried on this spectacular display of craftsmanship, transforming both land and home.

In ancient Rome, historian Pliny the Elder wrote that specific trees were imported to be planted on the roofs of institutional buildings and mausoleums. And in the Middle Ages, inverted sod layers using birch bark as waterproofing were planted atop a water retention membrane supported by roof boards and wooden beams. The layered birch bark was used to drain water in a way that kept roofs from collapsing and made sure that people living in the houses stayed dry.

This practice was most commonly seen in Scandinavia. Indeed, there were actually two main forms of green roof buildings created by the Vikings during the Middle Ages. One of those forms consisted of large, multi-roomed structures constructed of a timber framework, with turf sods lain on the roof and around the walls. These buildings were massive – housing 70 to 90 people – and had a central hearth for gathering. The other kind simply had a bit of sod turf on top of the timber roofing.

The modern-day green roof system was born in 19th-century Germany. Because of rapid industrialisation and urbanisation, the Germans sought to reduce their buildings’ fire hazards, but it was not until a roofer named H. Koch thought to add sand and gravel substrate (a surface on which plants can live) that the early beginnings of green roofing started to take place. From there, it was only a matter of time before seeds were planted and gardens bloomed. Green roofs became a big thing for the English, too, especially during the war when turf was used to camouflage their military airfield hangars during the 1930s.

The benefits of green roofing – especially for the environment – have been established and utilised throughout history.

Great insulators, green roofs have the ability to keep homes warm during brutal winters and cool during hot summers.

During the industrial era, the Urban Heat Island effect (UHI) became an increasing problem. UHI occurs when natural flora, fauna, and forestry are replaced by pavements, concrete buildings, and other surfaces that absorb and retain heat. The effect makes heat-related illnesses more likely and causes people to use expensive air conditioning and refrigeration to reduce indoor temperatures. UHI has a major impact on almost all aspects of the outside environment as well, and greater pollution in the ozone layer caused by the heat from paved surfaces, in turn, decreases air quality.

Green roofing, however, is known to improve the condition of air. It is common knowledge that plants are able to filter air by absorbing carbon dioxide and letting out oxygen. Rooftop gardens also help trap and remove airborne pollutants that are created by the UHI effect and pollution from air conditioning, among other things. This works brilliantly for big cities where air pollution is a big issue.

Added plant life helps urban wildlife as well. The sedums, herbs, grasses, and various other choices of vegetation that can be used for green roofs all offer different habitats and food for birds, butterflies, insects, and bees. Greenroofers.co.uk in The Benefits Of Green Roofs tells us that in Switzerland, “a study of 11 green rooftops found there to be an incredible 172 separate species”.

Another important environmental benefit of green roofs is stormwater management. In an urban landscape, less textural greenery and more smooth, flat surfaces mean that rain runs off buildings and straight into gutters. It then flows into storm sewers and either through water facilities, or straight into natural waterways, such as lakes, rivers, or streams, picking up dirt or contaminants in the process. In cities, overflow of even one sewerage system can mean sewerage gets into our waterways.

Green roofs can prevent this ecological hazard. As plants mature and grow larger roots, they have the potential for a greater amount of water retention, reducing the amount of stormwater runoff into sewers, reducing flooding and potential waterway contamination. A study by Pennsylvania State University showed that green roofs capture around 80 per cent of rainfall during rainstorms, compared to the usual 24 per cent of rainfall captured by common flat roofs.

Green roofs or living roofs can also beautify an otherwise ugly building.

Will Thorne has been installing and producing green roofs for buildings since he founded his company in 2005. Two of his biggest jobs were the green roof on the Waitakere Civic Centre, installed in June 2006, and the NZI Building in Auckland, installed in May 2009. The Civic Centre is believed to be the first green roof on a civic building in New Zealand and recently won the Award of Excellence for Sustainability. The NZI Building’s green roof helped the construction achieve its five-star Green Star NZ rating.

Photos: greenroofs.co.nz

Will tells me that there are two different types of green roof: extensive and intensive. They mainly relate to the depth of the growing medium (either soil or substrate) which, in turn, dictates what kinds of plants are more appropriate to grow.

Extensive green roofs are not usually recreational, unlike a rooftop garden. Because they don’t function as a leisure garden, the aim is for the plants themselves to be self-sustaining and require little to no maintenance. For an extensive green roof, planting suitable greenery should not require too much structural modification of the roof.

In comparison, the main purpose of an intensive green roof is as a rooftop garden for people to enjoy. Intensive green roofs take the same amount of management as a garden based on soil, and can necessitate major architectural adjustments, because the building needs to be able to support the added weight.

When I ask Will if there is any limit to what someone could plant on their roof, his answer is that it depends on the depth of the growing medium. “If money is not an issue and weight is not an issue, then you can do a green roof with a growing medium that is 400mm (deep). For example, bananas trees, Pohutukawa, and things like that, can be planted on top of that level of depth,” he explains. “But generally, the shallowest roof we can do has a growing medium of 75mm. That’s pretty limited to sedums, succulents, and some ground covers. If we can go up to 150mm, we can start introducing native plants, grasses, and tussocks.”

He also tells me that green roofs are actually not all that hard to maintain. General maintenance is upheld by his company for the first 12 months as a part of the initial installation. During that time, his team will make regular visits to ensure the plants are growing and to see if any replacements are required. “We find that, obviously, it does need some maintenance. You can’t just put a green roof up and leave it there, otherwise, nature would take its course,” he laughs.

Aside from what they can do for our environment, cultivated rooftops also provide a nice break from all the concrete in our inner-city landscapes.

Unfortunately, there are not a lot of green roofs here in New Zealand compared to other countries such as Italy in Europe, Japan, and the United States. Will says that until local councils get more on board with the concept and resource consents are sped up, green roofing might still remain a niche market in New Zealand. Despite that, he says that there is potential for the green roofing industry to get a lot bigger. “I just think, one day I’d like to be able to go up the Sky Tower in Auckland, look down, and just see a sea of green roofs. That would be pretty cool.”

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