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Green roofs – the eco-urban trend we need to embrace

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Green roofs – the eco-urban trend we need to embrace

Green roofs, gardens, and other urban features have the potential for a revolution in urban planning. They can help to increase biodiversity, combat climate change, and provide a pleasant aesthetic.

In the 1988 Talking Heads song Nothing But Flowers, David Byrne envisaged a world in which civilisation had decayed and was replaced by luxuriant vegetation. Nature is being increasingly introduced to inner-city areas, though not as a way of colonizing their remains but rather to enhance them. For example, green rooftops ,vertical forests, roof farms, and green walls are all common in urban environments.

Sod roofing was first used by Viking homes in Scandinavia. They were popular in the Middle Ages when most houses had them. The roofs were made up of layers of birch bark and turf. They protected roofs against the elements and provided insulation in cold climates. From the 18th century onwards, these grass roofs were gradually replaced with tile roofs.

The modern green roof movement was pioneered by Germany starting in the 1960s, and featured a more sophisticated approach to construction. Today at least 12 per cent of flat German roofs have been greened. Later, the movement spread to Europe and North America as well as Asia. Europe is the leader in green roofing.

Benefits from green roofs

Green roofs offer many benefits.

  • Improved aesthetics and lowered stress, if people are able to see green roofs or can walk around roof gardens.
  • Stormwater retention and curbing storm-water runoff. Studies have tended to find stormwater discharge being reduced by around 50 to 80 per cent, in turn curbing its contribution to water pollution as it makes its way to the drains. Stormwater can also be cleaned up by green roofs, which remove pollutants and heavy metals.
  • Cleaning the air, by filtering pollutants and capturing harmful particulates.
  • Increased biodiversity, including indigenous grasses, wildflowers, birds, insects and butterflies. In one remarkable example, a plant thought to be extinct in the UK was found on a London green roof in 2021. This involved a colony of 15 small-flowered tongue orchids discovered on the top of the London branch of Nomura bank. It was also the second instance of rare orchids being found on the same roof.
  • Increasing solar panel efficiency. A study done by Peter Irga, University of Technology Sydney has shown that solar panels are not only a competition for green roofs when it comes to roof space. However, when they’re installed in conjunction there is a remarkable benefit. Because solar panels are less effective at higher temperatures, panels installed above a green roof were found to be up to 20 per cent more efficient, with an average improvement of 3.6 per cent. Those panels above a green roof generated 69 megawatt-hours (MWh), compared to 59.5 MWh on a regular roof, a 16 per cent increase.
  • Ameliorating the urban heat island syndrome, which causes cities to heat up a few degrees higher than the surrounding areas. The problem is solved by green roofs, which have poor thermal absorption and high reflectivity. Welsh researchers looking at nine cities around the world have found air temperatures reduced by nearby green roofs by 3.6 to 11.3 degrees Celsius.
  • Tackling climate change by absorbing atmospheric carbon in the plants and the soil. A green roof provides thermal insulation and reduces energy consumption by cooling and heating the floors below it. However, a 2018 study found that in the short to medium term a green roof is a net carbon emitter, due to a carbon payback time ranging from 6.4 to 15.9 years under different scenarios.
  • Sound insulation can be used under airport flight paths.
  • EMF protection, with up to a 99.4 per cent reduction in electromagnetic fields under a 100mm-deep green roof.
  • Increasing the lifespan of roofs by about threefold, according to US estimates. It protects the roof against UV radiation, damage and wear due to heating and cooling cycles.
  • A green roof can increase the value of your property, partially due to anticipated operating costs savings.

Making it Happen

Various cities and countries have green roof policies that encourage the spread of progressive green roofing. They include mandates, which are more common in Europe, and financial incentives such as subsidies or grants that are common in North America with lower regulation.

The Swiss city of Basel has the largest green roof area per capita, at 5.7 metres. In 2002, it was the first city in the world to mandate green roofs for new and renovated flat roofs. With 3.4 square meters, Stuttgart, Germany is second, followed by Linz, Austria, which has 2.6.

The green roof at the Ford factory in River Rouge, near Detroit is four hectares. It’s one of the largest. It was a decision made for economic reasons as well as environmental concerns. This saved the company from having to invest US$50 million to clean up toxic stormwater, while costing only US$18 million, a sum that includes its own rainwater system.

New York boasts a unique elevated walkway that runs 2.3 km through the city. It is part-vegetated and characterized by a high line. It is an old freight train line, which was repurposed and saved from being demolished. The many native plants growing along the line are inspired by the native species that grew naturally when the disused track was left to its own devices for 25 years.

The British city of Leicester has added living roofs to 30 of its flat bus shelters. They are called “bee-stops” and have wildflowers planted on top. It would be simple to duplicate this initiative elsewhere.

Planning to install green roofs

Greening roofs works well for new buildings, since they can be integrated into the design. Many projects can be retrofitted. It is important to have a flat roof or a gently sloped roof.

The upfront cost of installing a green roof may discourage you from doing so. It could be twice the price of a regular roof. This may be offset by cost savings that could last for decades. If financial incentives are available, they can be very helpful.

There are two kinds of roofs: intensive and extensive. The soil is relatively thin for extensive roofs, so they work well when combined with plants with shorter roots that are drought-tolerant. These plants are more suited for large areas. Rooftop gardens with intensive plants have deeper soil, and can be used to grow food or trees.

Structural considerations must take into account weight of the roof. Intensive systems are significantly heavier. It is important to consider the combined weights of soil, plant matter, and other materials as well as the potential water it might hold. A green roof retrofit may require expensive reinforcement.

The waterproof membrane and the root barrier are two of the most critical layers in a green roof. A green roof must have water. To get the most out of direct rainwater, an irrigation system will be required.

Maintenance can be a problem for green roofs. However, most elements can be ignored if they are not an aesthetic issue. This will reduce the work load. Unwanted tree seedlings must be removed. The intensive systems require more maintenance than the extensive ones.

Green walls

Another trend is the vertical version of green roofs. Living walls are also known as green walls. They were inspired by the Victorian ivy walls that lined old English homes. This trend later spread to America. They evoke a feeling of romance and give off an air of historic antiquity.

The first living wall system was patented in 1938 by Stanley Hart White, a US landscape architecture academic who did not really develop it at the time. Only in 1986 did the first indoor living wall appear, at the Cite des sciences et de l’industrie in Paris, the result of a partnership involving a botanist, an architect and an engineer.

A living wall is an internal or exterior structure that can be used to support plants, growing mats, or stacked pots. Drip irrigation is used to provide water. Living walls are more visually appealing than green roofs. They can transform large interior spaces from one of relative sterility to something natural and attractive. A green wall can be a big statement for an organization. These walls are often located near the entrances that receive the highest foot traffic. This brings nature inside, which can have many benefits, including improved air quality and relief from sick building syndrome. It also improves aesthetics, employee productivity, as well as improved worker productivity. Green walls in hospitals have been shown to reduce the number of patients who stay there.

External living walls may offer heat protection. An exterior living wall had a cooling effect of four degrees Celsius in a Spanish study. You can incorporate interior living walls into the cooling system.

London leads the pack in terms of green wall installation, boasting many examples. In Sydney, one of the world’s tallest living walls is on the side of the 34-storey One Central Park building where plants are suspended using cable systems. Living walls, like green roofs can increase property value.

Vertical forests

Twentieth-century Austrian architect Friedensreich Hundertwasser had a number of unusual ideas in relation to multi-storey buildings. One idea was to give a room to “tree tenants”, who would grow out of the windows. Recent developments are more pragmatic and less bizarre in this area.

There is a lot of talk about vertical forests being incorporated into high rise buildings. Two of these have been integrated into two residential towers in Milan, while many others are still at the concept stage. They were created by an architect firm with the input of botanists and horticulturalists.

Impressive greenery grows on every floor, and for each resident there are two trees, eight shrubs and 40 other plants. Together these absorb about 30 tonnes of CO2 a year. These plants are regularly inspected from the inside and out and require water.

Rooftop gardens for food production

The rise of the urban farm is a more recent phenomenon than passive green roofing. Globally, the amount of land for food production is limited. In addition, historically, cultivation has had an adverse impact on biodiversity, as it has led to the destruction of forests and other natural habitats. Due to the requirement for extensive lighting, proposals for vertical farming in high rise buildings are very resource-intensive. However rooftops offer a viable solution. Many times, the food is grown on raised beds.

Rooftop gardens are a great option for nearby restaurants. They have short supply chains that allow fresh ingredients to be available quickly, which eliminates the need for transportation, storage, and refrigeration. Agripolis’ Nature Urbaine, Paris’ largest rooftop farm, supplies the restaurant industry. The intensive vertical growth model, which uses an inorganic growing medium to grow vegetables, has earned it the name “aeroponics”. Some restaurants even built these gardens from their rooftops.

Many cities, including New York, New Orleans and Montreal, have urban rooftop gardens. These gardens are also used for community food in places like Chicago.

Closer to home

Australian and New Zealand both have examples of living walls and green roofs, but they are behind other countries due to lack of financial incentives and mandates. For those who are interested in green roofing, the city councils of Sydney, Melbourne, and Adelaide offer useful resources online.

CH2 is a Melbourne building that the Melbourne City Council uses. It also contains a rooftop garden which is used for recreational purposes by staff. The Victoria Desalination Project at Wonthaggi has 98,000 Australian native ground cover plants, and at 2.6 hectares it is the largest in the southern hemisphere.

Across the Tasman in New Zealand, the Westfield shopping centre at Newmarket, a suburb of Auckland, has 750 square metres of living wall, divided across seven separate square wall areas split between two nearby buildings. The New Zealand Insurance Centre HQ in Auckland has
a 500-square-metre green roof featuring a variety of plant species.

The group Green Roofs Australasia was launched in 2007, and is actively involved in a range of activities including education, advocacy, research and professional development.

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