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Growing planes

Theme: roofingGreen roofs may not be new to the UK but after much trial and success in Switzerland the latest versions are being touted as the perfect way to maintain biodiversity

A rather dull-looking bird called the black redstart could have a major influence on the design of London roofs - at least, if campaigners to save its habitat continue to be as successful as they have been already.

Originally a resident of alpine screes, the bird's population rocketed in London following the Blitz and the bomb sites that resulted.

Now, redevelopment is squeezing the bird out again but conservationists have come up with their preferred solution: recreate the environment on the roofs of new buildings.

Planted roofs are not a new idea, even in the conservative UK - think of Edward Cullinan's RMC headquarters in the late 1980s, or of the garden on Stirling and Wilford's No 1 Poultry. But the latest idea, imported largely from Switzerland - streets ahead in both research and application - is to create biodiverse roofs that support wildlife. This approach is already being imported to the UK on projects such as the Komodo dragon house at London Zoo, designed by Wharmby Kozdon and with a roof provided by Sarna. If the campaigners have their way, we will see many more.

In some ways you can't lose with a green roof - it is interesting and (usually) attractive to look at, it is well insulated and so improves thermal behaviour, it reduces rainwater runoff and the load on the drainage system, and it protects the roof membrane. In addition, water evaporation from the roof should have a cooling effect on the urban environment.

Trouble in paradise But there are potential disadvantages. One of these is weight, since soil or soil substitutes are heavy, and therefore green roofs may require a more beefed-up structure.

The other is the need to get it right. If a green roof springs a leak, it may be very difficult to trace the source of the problem, and involve removing tonnes of wet soil and vegetation.

That is perhaps a peculiarly British reservation, born out of years of mistrust of our standards of workmanship. The Swiss, despite not always building as punctiliously as one might imagine, seem unperturbed by this. But then they have such a great track record.

For instance, at Wollishofen, on the edge of Zurich, there is a water filtration plant that is 90 years old. Its 30,000m 2 of roofs are covered with vegetation, set on a ceiling of slab beams 8cm thick, finished with a layer of 2cm of mastic asphalt. The impression, standing on one of the huge roofs in midsummer, is that one is in a meadow rather than on a rooftop, with grass growing above waist height.

The protective value of the vegetation has been proved by the lifespan of the roof, and its biodiversity by the appearance of many rare and endangered plants, including orchis morio, a species that is otherwise extinct in the surroundings of Zurich. So impressive is the habitat that the canton of Zurich is considering taking special measures to protect it.

Maintenance is also fairly simple, merely requiring cutting once a year. But it could be reduced to nothing if the depth of soil were just 5cm less, argues Stephan Brenneisen, Switzerland's expert on biodiverse roofs. Based at the University of Applied Sciences in Wõdenswil, Brenneisen treats many of the country's green roofs - particularly in Basel, which uses a carrot-and-stick approach to encourage their use - as his own research laboratory.

The issue of the depth of substrate is being studied on the roof of Zurich tram station. Trial patches have been planted and seeded with native species and subsequently received no planting or watering. Different substrates were used at different depths and growth-monitored. It is apparent that the main influence on growth is the quantity of nutrients available. Both total biomass and diversity increase as the quantity of nutrients rises, although of course at very high nutrient levels some species will be squeezed out by more successful rivals. The trick is to get the balance exactly right. This is theoretically feasible on a roof, which means it should be possible to achieve a no-maintenance surface - just a pipe dream at ground level.

Drainage is also important, as demonstrated in an accidental comparison at the distribution centre for clothing company Ackermann at Entlebuch. The building has two roofs, planted predominantly, but not exclusively, with sedums. They are identical in every way except that one is completely flat whereas the other has a slight fall and hence improved drainage. Growth on the better-drained roof is more luxuriant, whereas the flat roof has boggy patches and sparser growth.

However, to the biodiverse roof purist this is not all bad news. They welcome a range of habitats, and are not always looking for the prettiest solution. Indeed, think about the lost habitats they are trying to recreate, and you will realise that these are a world away from 'tidy' English gardening. Since most green roofs are not intended to be accessible or overlooked, this should not be a problem.

The predominant impression in Switzerland, which has a vast number of green roofs, is of numerous building with shaggy tops.

But unplanned ponding is not clever on a roof, as it does not bode well for its durability.

Instead, Brenneisen is keen to create a range of habitats that could support different wildlife.

One way to do this, in addition to wet areas, is to create mounds that will have more nutrients and a different ecosystem. This has been done on a number of projects in Basel including Herzog & de Meuron's Rosetti building at the Cantonal Hospital. Again, success of the projects is assessed visually, through counting vegetation and collecting insects in bee traps (yellow buckets containing water and acid).

The proponents of biodiverse roofs are very much opposed to the use of sedum mats.

If sedums are to be used, they prefer them to be individual plants. They are also in favour of the use of natural soil rather than artificial growing mediums.

Switzerland's collection of green roofs is impressive. It has been brought about through a strong environmental movement, planning requirements for flat roofs to be planted, and incentives such as Basel making 1 million Swiss francs (ú432,000) available in 1996 for the creation of green roofs at a rate of 20 Swiss francs (ú8.60) subsidy per m 2. But it could never happen in the UK - or could it? Another reason such progress has been made in Switzerland is because of the passion of Brenneisen, and London has its very own Brenneisen. He is actor-turned-ornithologist Dusty Gedge, who has masterminded the campaign to preserve the black redstart.

Through his organisation Living Roofs he has been persuading developers to install biodiverse roofs and affecting Greater London Authority policy. Projects include SOM's largely residential New Providence Wharf on the Isle of Dogs, and Nigel Upchurch Associates' Mast Quay development in Woolwich.

With the advent of landfill tax, Gedge is also able to convince developers of the benefits of crushing demolition waste for use on the roof.

Technology, legislation and sheer enthusiasm seem to be combining to ensure a prosperous future for the black redstart.

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