Green roofs are touted as the ecological answer to heavy rainfall and vanishing habitats. Richard Stirling takes a look at what is becoming a chic cultivated covering for our conurbations.

FRUSTRATED with the lack of green space in cities, planners are developing green roofs on the tops of urban buildings, which they hope will improve the lot of people living in densely built-up areas and also attract wildlife into otherwise sterile environments.

But green roofs are more than handy gardens stuck on top of buildings; they may hold the answer to problems with drainage and arid urban environments.

Some planners claim green roofs will help to buffer the effects of climate change by regulating the flow of rainwater to ease congestion on the drainage network.

A growing trend
From water authorities to ecologists and even the Mayor of London, there is a growing movement to break up harsh urban landscapes with high-rise vegetation.

The movement for green roofs received a boost a couple of years ago when the Mayor of London Ken Livingstone teamed up with Lord Richard Rogers to promote green roofs in the capital. Although their campaign was aimed at creating green space for Londoners with a comparatively low standard of living, the Greater London Authority did highlight a gap in the construction industry’s expertise; that it was much more attuned to low-rise landscaping than creating rooftop recreational space.

The organisations said: “There are no specific standards for green roofing in the UK, so architects and housebuilders who wish to provide green roofs often have to start from scratch, thereby increasing short-term costs.”

Livingstone’s and Rogers’s crusade to create green space in the sky was as much to do with giving Londoners breathing space in their already cramped living conditions as improving the environment down in the street below, but their efforts have drawn attention to a much overlooked and vitally important part of the building – the roof.

Stemming the flood
Lack of space in heavily built up areas can cause chronic problems when it comes to building infrastructure to deal with rainwater and wastewater.

Speaking at a recent Ciria conference, Dr Virginia Stovin of the Pennine Water Group said: “The investment programmes on these projects runs into several billions. Where do you put big new concrete storage chambers? In Sheffield they’ve been going into public parks which causes a lot of disruption for the people who use them.”

She said green roofs provide an alternative to the water management and flood alleviation systems that engineers are currently developing beneath the ground. “My big motivation for getting involved with green roofs is to have a 21st Century solution which the engineers aren’t thinking about now. They’re building bigger combined sewage overflows instead.”

She added that developers’ decisions can be influenced by what machinery is available for the project. Thames Water is proposing to build a 7.2m-wide, 34.5km-long storage and transfer tunnel at a cost of £1.5bn. Dr Stovin said the design was influenced by another large project in the area. “If anyone’s wondering why it will be 7.2m-wide, think about where all the redundant machinery from the Channel Tunnel will have to go.”

The flow of water is much less of a strain on the sewage network if it is managed from the top down, according to Dr Stovin. “Runoff from green roofs is much slower compared with conventional roofs,” she said. “That must have an impact when considering whether sewage overflows will be enough to cope with the volumes of water. Green roofs can be used to reduce the flow in other storage elements.”

Roots of the green roof
Exponents of green roofs claim they are a 21st Century answer to problems of drainage and flooding, but they have been around for thousands of years, at least since the Hanging Gardens of Babylon.

Modern green roofs can be traced to Berlin in the 1880s. Just like the accidental discovery of penicillin, so it was an architectural gaffe that gave us the green roof.

Cheap “Mietskasernen” housing was constructed using an inexpensive but highly flammable tar on the roof. When it came to fireproofing, the builders took the jerry-builtroute and shovelled sand and gravel over the tar. The roofs became naturally colonised by plants and some of these have survived for more than 100 years.

Reinhard Bornkamm was influenced by the Mietskasernen buildings and pioneered green roofs in post-war Germany. He published scientific work on green roofs in the 1960s and started green roof research in Berlin in the 1970s.

Green roofs underwent further development in the 1970s with the development of new waterproofing and membrane technologies.

They are roughly divided into two categories; intensive and extensive. The former uses a deep growing medium, which will support the growth of shrubs and trees. The city parks within the Canary Wharf estate are intensive green roofs. They can, however, require extra structural design to the building to support their extra loads.

Extensive roofs popularly use sedum moss. They are porous mats with 2cm of growing medium, which supports the sedum, although the mats can be replaced with 7cm of crushed brick. Brown roofs also fall into this category. These are covered with recycled aggregates from the site and left to colonise naturally.

Here comes the rain again
The current concerns about climate change include worries over changing weather patterns. These don’t involve apocalyptic flooding, but rather shorter bursts of heavier rain. In other words, it won’t rain more, but we will get wetter.

This change in rainfall will create a greater strain on the drainage system, which could end up in backlogs, blockages and flooding.

Dr Stovin said green roofs act as sponges to release water at a manageable rate. They soak up rainwater and can achieve between 45% and 70% reductions in runoff a year. Highly efficient green roofing in Berlin’s Potsdamer Platz allows only 1% of rainwater to run off the building.

“Green roofs are particularly attractive in dense urban areas,” said Dr Stovin, “they really do get Suds off the ground.”

Heat of the city
Green roofs don’t just help to stop flooding, they also help to reduce unusual hotspots in built up areas. Densely built-up areas create hot arid areas, or “urban heat islands”, which force the occupants of the surrounding buildings to turn up the air conditioning to maintain comfortable conditions. Dusty Gedge, of Livingroofs.org, said: “In London, about five years ago, the mayor was talking about fuel poverty. Now we’re talking about cool poverty.”

Approximately 90% of London is hard surfacing, which mostly consists of roofs, he explained. “It is predicted that London’s temperature can go up by 2020 by 1oC or 2oC. To combat the effect of climate change we need 10% green space in our cities.”

Heat is absorbed into the soil and is released slowly, which has a buffering effect on the surrounding air. Water within the substrate also absorbs heat from the building. The reflective properties of the plants on the green roof, usually sedum, also play a part in keeping buildings cool; the ratio of light reflected from the surface, or “albedo”, is greater than that reflected from the roofing membrane.

Gedge said green roofs help to buffer the effects of these urban heat islands. Studies in Ottowa found fluctuations in conventional roofs of 45oC, whereas green roofs were just 6oC. Green roofs can even extend the life of the building, he said. “A lot of councils are now putting green roofs on old stock to counter the effects of overheating, which extends the life of the roofing membrane. On the continent it is accepted that it doubles the membrane’s life.”

Green roofs also create ideal temperatures to generate solar energy; a green roof maintains the ambient temperature around a photovoltaic cell at 25oC, which is the optimum temperature for its function.

A good habitat to get into
As well as their important role in averting flooding and countering the effects of urban heat islands, green roofs create valuable habitats for birds and insects.

John Newton of the Ecology Consultancy warned planners will deny permission to developers and civil engineers unless they encourage biodiversity in their projects. “The construction industry is interested in good practice,” he said. “People want to be seen to be responsible and doing something and biodiversity is very much part of that.”

One of Britain’s rarest birds, the black redstart, benefits greatly from green roofs, as does another bird not usually associated with the urban environment, the lapwing.

Newton said cities can support a rich variety of birdlife and he is currently working on a 3,000m2 green roof in Greenwich Millennium Village to encourage skylarks. “Birds aren’t fussy, they just don’t like to be disturbed that much,” he explained.

Green roofs don’t have to be limited to layers of sedum moss, which has proved most popular among developers; they can contain other plants and features. Newton explained:  “We can put other elements such as rotting wood on green roofs to create different environments which invertebrates are going to love.”

Newton said developers should also accommodate wildlife into new roofing designs. “We’re losing the swifts from our cities because the old roofs are being replaced by flat roofs. All we have to do to rectify this is to put some boxes up. Swifts only need these for a couple of weeks in the year. There should be a law to say you should do it. There’s no loss, only gain.”

The sky’s the limit
Is the uptake of green roofs in towns and cities a realistic option for the construction industry? And what are the limitations of cultivating green spaces in high-rise conurbations?

Gedge said fitting green roofs should not pose much of a problem for most planners, although buildings with large areas of roofing can prove difficult for contractors. “The first thing to know about retrofitting green roofs is to understand what the loads are,” he said. “The weight that is quoted is 120kg/m2, which includes the snow loading.”

Dr Stovin agreed and added that retrofitting green roofs can prove a headache with some kinds of buildings. “A lot of 1950s and 1960s flat roofs can possibly take green roofs, but structures like big sheds might not be able to,” she said. “For these we can use a spanning system but I suspect that it will add too much to the cost.”

However, she said, planners should seriously think about the environmental benefits of green roofs such as drainage, heat reduction and biodiversity alongside other environmental concerns.

“So much emphasis has been put under the Code for Sustainable Homes on energy efficiency that water and biodiversity have been forced to take second place. We should push for getting a more balanced reward for developers.”

It will be for time to tell if planners and developers adopt green roofs as an elevated answer to the impact of a changing climate and disappearing habitats, or whether they consider them just pie in the sky. But as weather conditions become more extreme and developers have to take into account situations such as flooding and the effects of urban heat islands, they will be forced to adopt new building techniques. Green roofs are proving popular among environmentalists and some planners and designers, so it may be sooner than we think before we come to rely on the sedum in the sky.

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