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

Green roofs and gardens have the potential to revolutionise urban environments into the future as they boost biodiversity, clean polluted air, fight climate change and offer a pleasing 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. Today, nature is increasingly being introduced into inner cities, but to complement modern buildings rather than colonise their remains. Examples include green roofs, vertical forests, rooftop farms and green walls, all of which are most often encountered in urban areas.

Sod roofs originated with Viking dwellings in Scandinavia, and were widely used in the Middle Ages, when they were a feature of most houses. These roofs consisted of a layer of turf on top of layers of birch bark. Protecting roofs from the weather, in the cold winter climate they provided much-needed insulation. 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. The movement later spread to Europe, North America, Asia and other parts of the world. Europe however is still the green roof leader.

Benefits of green roofs

There are many advantages in having a green roof.

• 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. Green roofs also clean up stormwater by removing 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. This was the second case of rare orchids growing on the same rooftop.
• Increasing solar panel efficiency. Solar panels can be in competition with green roofs for roof space, but where both are installed together a surprising benefit has emerged following a study by Dr Peter Irga from the University of Technology Sydney. 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. Green roofs tackle the problem by having poor thermal absorption properties and a high albedo (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. Another climate benefit is improved thermal insulation, and less energy being needed to heat and cool the floor directly below the roof. 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, which can be useful 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. This works by physically protecting the roof from damage, UV radiation and wear from heating and cooling cycles.
• The increased value of a property with a green roof, partly as a result of anticipated operating cost savings.

Making it happen

Various countries and cities have progressive green roof policies designed to encourage their spread. These include mandates (more common in Europe), and subsidies and other financial incentives (more common in lower-regulation North America).

The Swiss city of Basel is the global green roof leader, with the inner city having 5.7 square metres of green roof space per inhabitant, more than anywhere else. In 2002, it was the first city in the world to mandate green roofs for new and renovated flat roofs. Second is Stuttgart in Germany, with 3.4 square metres, and third is Linz in Austria with 2.6 square metres.

A Ford plant at River Rouge near Detroit has a four-hectare green roof, one of the world’s largest. The decision to install it was driven by economics in addition to environmental considerations. 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 has a unique part-vegetated elevated walkway called the High Line that stretches for 2.3 kilometres through the city. This is a repurposed freight rail line that was saved from demolition, and is now a big tourist draw. 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. These are planted with wildflowers and sedum (a flowering groundcover), and are described as “bee stops”. This initiative would be easy to replicate elsewhere.

Planning for green roofs

Greening a roof works best for a new building, as it can be incorporated into the design. However, many projects are retrofits. A flat or gently sloping roof is a prerequisite.

Installing a green roof can be discouraged by a high upfront cost that may be double that of a normal roof. The payback period to offset this via cost savings may amount to decades. Financial incentives can help a lot, where they are provided.

Broadly speaking, there is a choice between two types, extensive and intensive. Extensive roofs have relatively shallow soil, and work well in combination with drought-tolerant plants that have shorter root systems. They are better suited to large areas. Intensive rooftop garden systems, meanwhile, have a greater soil depth, but can also be used for food growing and some trees.

Structural considerations need to take into account the weight on the roof, with intensive systems being substantially heavier. The combined weight of the soil, plant matter and other materials, coupled with the water that it could hold, together with human visitors, all need to be factored in. A green roof retrofit may require expensive reinforcement.

Among the most important layers of a green roof are a waterproof membrane and a root barrier. Water is a necessity, and in order to supplement direct rainfall a green roof will need an irrigation system that ideally utilises harvested rainwater.

Maintenance is an issue for green roofs, but fortunately, if perfect aesthetics aren’t an issue then most elements can be sidestepped, curbing the workload. One that cannot be avoided is the removal of unwanted tree seedlings. Intensive systems are higher-maintenance than their extensive counterparts.

Green walls

Meanwhile, the vertical equivalent of green roofs is also turning into a big trend. Green walls, also called living walls, have forerunners in the ivy-clad walls of old English houses, a fashion that later spread to America. These conjure up a sense of romance and convey an impression of historical 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 Cité des sciences et de l’industrie in Paris, the result of a partnership involving a botanist, an architect and an engineer.

A living wall can be internal or external to the building, using panel structures, stacked pots or growing mats. Water is often supplied via drip irrigation systems. More visually striking than green roofs, living walls have the potential to transform the interiors of large spaces from relative sterility into something more natural and appealing. The motivation for a corporation to go to the expense of installing a green wall is often to make a bold ecological statement, with such walls commonly located in entrance areas that see the most foot traffic. Benefits from bringing nature indoors in this way include improved air quality, relief for sick building syndrome, better aesthetics and improved employee productivity. Hospitals built with green walls have reduced the bed days of their patients.

External green walls can offer protection against heat. In one Spanish study, an exterior living wall was found to have a cooling effect on indoor temperatures of 4 degrees Celsius. Interior living walls can be incorporated into the cooling system of a building.

In terms of the number of green wall installations, London is a front runner, with numerous 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. As with green roofs, living walls have been found to enhance property values.

Vertical forests

Twentieth-century Austrian architect Friedensreich Hundertwasser had a number of unusual ideas in relation to multi-storey buildings. One of these was to devote a room to a “tree tenant”, which would find a way to grow out of the window. More recent developments in this field are less eccentric and more practical.

There is currently quite a lot of buzz about “vertical forests” incorporated into high-rise buildings. While many of these are at a concept stage, two have already been incorporated into twin residential towers in Milan. These were designed by an architectural firm, with input from horticulturalists and botanists.

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. They are periodically checked from inside and outside and are subject to regular pruning, and inevitably they require a substantial amount of water.

Rooftop gardens for food production

A more recent trend than the passive green roof is the rise of the elevated urban farm. At a global level, there is a limited amount of land available for food production, and historically the expansion of cultivation has impacted on biodiversity by encroaching on forests and other natural habitats. Proposals for vertical farms in high-rise buildings would be very energy-intensive due to the need for widespread lighting, but rooftops are a practical solution. Often the food is grown in raised beds.

For nearby restaurants, rooftop gardens offer very short supply chains, resulting in fresh ingredients that avoid the need for transport, storage and refrigeration. One very large rooftop urban farm supplying the restaurant sector is Agripolis’ Nature Urbaine in Paris, the world’s largest. Its intensive vertical growing model uses a soilless growing medium, and has been dubbed “aeroponics”. Some restaurants have even gone one step further, and built such gardens on their own rooftops.

In various places including Brooklyn, New Orleans, Montreal, Brussels and London, grocery stores and supermarkets have created urban rooftop farms. Elsewhere, such as Chicago, the harvest from these gardens is being used to feed the community.

Closer to home

Australia and New Zealand have some examples of green roofs and living walls scattered around, although progress in both countries has fallen behind many other parts of the world due to a lack of mandates and financial inducements. However, city councils in Sydney, Melbourne and Adelaide provide useful online resources for would-be green roofers.

CH2 (Council House 2) is a building in Melbourne that is used by Melbourne City Council, and which contains a roof garden that is used by staff for recreation. 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.