Green roofs provide many environmental advantages but can this environmental technology overcome vested interests and lack of statutory regulations? Ben Kochan reports
With development in many town and city centre becoming ever denser with vital open space being eaten up, the importance of providing some greenery on the roofs of new buildings is becoming ever greater to improve air quality and managing rainfall. However, the conservatism of architects and the lack of statutory requirements is hindering progress.
Green roofs are becoming more common in the UK on such high profile buildings as parts of Canary Wharf in London’s Docklands, Cannon Street station in the City of London and the Greenwich Millennium Village but interest is still much less compared to other countries.
UK trails rest of Europe
In Germany, the world leaders in roof greening, in 2001 13.5million sq m of green roofs were installed, which is many times more than in the UK, according to the construction research body CIRIA. “The UK is far behind its European neighbours and green roof technology is limited to showcase buildings and ‘green centres.’” It suggests that the need for more green roofs is greater than ever since the move to develop more brownfield sites which will see the loss of essential urban green space.
In some of the provinces in Germany, green roofs are a statutory requirement and grants are made available to act as an incentive and cover the additional costs. Green roofs are increasingly coming on to the agenda in the UK. Many local authorities are considering introducing a requirement in their local plans. The London Plan currently encourages their use, however, amendments are likely to be introduced shortly which will require their use in some situations and other cities such as Birmingham and Brighton are likely to follow suit.
The initial interest in green roofs came from ecologists in the 1970s who saw the benefits in terms of biodiversity and wildlife in urban areas, according to Dustie Gedge, who is a consultant and runs the website livingroofs.org which is supported by the major contractors and provides extensive information on the practicalities of installing them. He says that the driving force is now not only that the roofs can provide homes for a very wide range of animals and plants, removing pollutants from the air and absorbing carbon dioxide. They offer a range of other environmental benefits and public amenities. They attenuate storm water by delaying runoff to drainage systems, so reducing the dangers of flooding. The green roofs can store more than half the rain water and on some buildings it goes as high as 70 per cent. He cites the example of the green roof on the Pottsdamer Platz development in Berlin, which retains 50 per cent of the water and the rest is harvested for the toilets and other uses in the mixed office and shoppingcentre. Gedge says that they can stop rivers flooding in times of heavy rainfall. The level of retention is also related to the thickness of the vege-tative layer.
Heating and cooling
The thermal performance of green roofs is also important in terms of reducing the heating and cooling requirements of the building. The green covering can improve the insulation value of the roof by up to 10 per cent. According to Peter Allnutt, from the roofing contractor Bauder says that one of the green roofs at Canary Wharf saved the occupier £6,000 a year in air conditioning bills which they had not expected. The roofs also reduce the heat island effect by ensuring that the roofs do not absorb heat, which is emitted in the evening when surrounding areas are cooling down.
Green roofs can also be combined with photovoltaic panels making them more efficient. “The reduced heat as a result of the green roofs, makes the PV work more efficiently,” explained Allnutt. He also suggests that the greenery absorbs ultraviolet rays so lengthening the life of the roof covering below it.
Green or brown?
There are two broad categories of living roofs. Extensive green roofs are low growing but potentially colourful carpets of planting. These roofs combine visual attractions and ecological benefits. They are generally planted with sedums – plants with thick fleshy leaves and stems, which make them particularly suitable for growing in the inhospitable conditions found on a roof. Intensive green roofs involve landscaping generally found on the ground offering recreational opportunities as well as ecological benefits but they require considerable maintenance.
There are also brown roofs which are covered with substrate or loose material – brick rubble, crushed concrete, and subsoils. Such roofs may colonise spontaneously with vegetation and can also provide habitat for a range of invertebrates and birds.
The advantage of the extensive green roof is that it is very low maintenance and can be fitted quite easily. “The layer of soil does not have to be very thick so the weight which the structure has to carry is not particularly great,” explained Gedge. They tend to require about eight cm of gravel. Jonathan Hines from the architects Architype, argues that the structure of the green roof should be kept simple requiring little maintenance. The system which he specifies has only four layers involving a waterproofing membrane, a protective fleece, the growing medium and the plants themselves. There is a lot of misinformation about the problems associated with green roofs but many are illfounded – or systems have been developed to address them. Hines has revisited roofs that he has specified 10 years later, which have been thriving, he says.
There are particular concerns about leakage. Gedge does urge that the waterproofing system is tested extensively before the upper layers are installed.
The planting can also go awry. “There are horror stories around of green roofs which go wrong – they look awful – this is often because the wrong kind of planting has been used and horticulturalists have been called in to sort them out.” Architype specifies the green roof, a specialist roofing contractor will put up the roofing structure and a horticulturalist then does the final layers.
New NHBC standards
The National House Building Council published new standards for green roofs in June. Before then, the only recognized standards were German. Peter Crane, the NHBC’s head of Standards said that the problems with green roofs were no different from any other. He emphasized the importance of the roof being supplied and installed by the same company. “The roof needs to be purchased and installed as a package to avoid problems and arguments about responsibility,” he said.
Green roofs can work on most buildings in most locations but Hines suggests that the greatest benefits are in dense urban areas and on buildings with expansive roofs – such as industrial sheds and large shopping centres. The composition of the roof however must vary depending on the location. Additional weight of materials is required if installing a roof on tall building to ensure that it is not blown away.
There have been some quite imaginative green roofs. Peter Allnutt from the contractors, Bauders, highlights the example of the Crew Centre in Brighton which has an ‘S’ shaped curved roof. This helps the building to fit into the surrounding area, which persuaded the local authority to give it planning permission. Another roof designed and installed by the company was for a training centre in Cookham in Berkshire for John Lewis that was in the shape of an aircraft wing with angles up to 35 degrees. This is an intensive green roof and a remote controlled lawnmower installed on the roof cuts the grass. Green roofs need to be at an angle to allow for drainage, particularly during periods of heavy rainfall. If the angle is above 10 degrees then it will require irrigation, explains Allnutt. Steeper roofs and even green walls require more soil and need to be shored up.
It is also possible to retrofit green roofs on to existing structures if they are strong enough and in appropriate locations. Gedge says that timber structures are generally not strong enough but otherwise it is possible. However, if the building is surrounded by tall buildings there are dangers of wind tunnel effects blowing the materials away. The composition of the planting and growth medium must be chosen carefully to fit the location, he urges. The property developer, British Land has developed a sustainable green roof solution formed out of recycled and low embodied energy materials that can be installed on both new build and as a retrofit option. Substrates used in this system include roof shingle, railway ballast and sewage sludge pellets.
The need for maintenance of living roofs is somewhat contentious. Gedge argues that too much attention is given to issues of maintenance and irrigation. “If the greenery dries out in the summer, it doesn’t matter, it will grow back. In other countries, they are not worried if the roof is not very aesthetically pleasing,” he points out. In Germany, they never irrigate their roofs, the sedum goes reddy brown as it dries out. Allnutt suggests that “just like traditional roofs, green roofs need to be inspected twice a year.”
One of the fallacies, which the industry is trying to counter, is about the additional expense. Allnutt suggested that the initial outlay on the roof may be more, but there were savings in other elements. “A less extensive drainage system is required because water is retained on the roof,” he explained. In addition, the green roof covering tends to double the life of the waterproofing. “If all the benefits and other savings are taken into account, the costs are certainly not any greater than a traditional roof and they can pay for themselves in up to nine years.”
Manchester social housing initiative
A green roof has been built in a social housing development in the new Millennium Village scheme in Manchester that was completed earlier this year.
The Manchester Methodist Housing Association decided to incorporate the roof into the design of the 14 new homes because of a shortage of gardens in the area. “It is a dense urban area without much open space, so the green roofs will compensate for that,” explained the association’s assistant director Jim McMillan. The colourful flowers will be visible from the tall buildings which are planned around the houses, he added.
One of the reasons for choosing a green roof was to reduce the run-off, which would drain into a nearby canal being created as part of the development.
The roof is more expensive than traditional designs but as one of the government’s exemplar scheme, the Housing Corporation was prepared to give an additional grant. “With the Government now concentrating on maximizing the number of homes, it may be more difficult for other associations to get this top-up,” he suggested.
The roof chosen is semi-intensive which requires some maintenance, but not too much. The plants are sedum with wildflowers. The association’s maintenance team needed some persuasion that the roof would not place excessive demands on their resources. It will require twice yearly checks to ensure that the drains are clear and for weeding, explained McMillan.