Room at the top

The only way is up when looking for new habitats for urban wildlife, says Peter Marren. Roof-top sanctuaries are cheap, fashionable, and effective

If all goes to plan, London Zoo's remaining Komodo dragon will soon share its home with rare bumblebees and spiders. The Komodo's building is the latest to include an "eco-roof", more commonly known as a "green roof", which combines indoor comfort and outdoor habitat. The idea is that the city roof-scape need not be a barren space visited only by Mary Poppins and the odd sleepy pigeon. With a modest outlay and a certain amount of luck, roofs can be converted into wild gardens, greening the city skies and sheltering the sorts of plants and insects now confined mainly to nature reserves.

If all goes to plan, London Zoo's remaining Komodo dragon will soon share its home with rare bumblebees and spiders. The Komodo's building is the latest to include an "eco-roof", more commonly known as a "green roof", which combines indoor comfort and outdoor habitat. The idea is that the city roof-scape need not be a barren space visited only by Mary Poppins and the odd sleepy pigeon. With a modest outlay and a certain amount of luck, roofs can be converted into wild gardens, greening the city skies and sheltering the sorts of plants and insects now confined mainly to nature reserves.

Green roofs are a new fashion in nature conservation, promoted enthusiastically by the wildlife campaigner Dusty Gedge, of Livingroofs.org. It all began with the black redstart. This attractive, little, black-and-orange songbird nests mainly in city centres and has become the signature species of urban conservation. It has friends in high places. The Government chose it as a species deserving as much protection and encouragement as possible, not only for its own sake but for ours.

A city with black redstarts trilling cheerfully from the rooftops is, in its small way, a nicer place than a sterile metropolis of concrete and traffic. Gedge is the bird's official "co-coordinator", his job to persuade developers to help themselves by helping the black redstart.

Black redstarts have little difficulty finding nesting space in development sites around the capital. One pair even nested on the Millennium Dome. But they find it harder to get enough protein-rich insects and spiders to feed their chicks. To encourage black redstarts, you need to find ways of bringing more insects into the city. This is where green roofs come in.

The technique of developing such "wildlife gardens in the sky" was first developed in Switzerland. The project, which has been promoted very successfully by the Swiss embassy in London, led Dusty Gedge to team up with his opposite number, Stephan Brenneisen, a lecturer in landscape at Wadenswil University. Together they took 12 prominent architects and journalists on a tour of Swiss green roofs.

The variety of possible roof-scape habitats surprised most of the visitors. They varied from simple sedum stonecrop mats to relatively lush pastures with wild flowers, sandy banks and even the occasional shallow pond. Zurich's railway station is custom-designed to conserve the wall lizard, with a vista of artfully-placed shingle banks. None compromise the building's main purpose, and, once set up, the maintenance is minimal. Nor do the roofs attract the wrong sort of wildlife. Lizards and bumblebees love them; rats and social wasps aren't interested. More, their promoters claim, green roofs help to prevent flash floods and conserve energy.

The essential difference behind green roofs and conventional roof gardens, says Dusty Gedge, is that wild plants don't need topsoil. The roof-top environment is, effectively, a semi-desert - hot, dry and windy. The plants that grow best are those that colonise the low nutrient conditions of brownfield sites, species like willowherb, wild carrot, kidney vetch and viper's bugloss. Meadow grasses prefer soil to hold a little moisture, while insects like bees need not only nectar sources but sunny banks of compressed sand to burrow into.

Further diversity can be created using larger stones, railway sleepers or mounds of compacted dune-sand and crushed brick. Where architects need persuading, says Gedge, is in regarding these gardens as a process, not a product. Allowing flowers, grasses and mosses to colonise takes a little time. The instinct of most designers is to buy something ready-made, like a sedum mat, a "lawn" plugged into a membrane which you simply unroll onto the roof like a carpet. Not only is this the most expensive option, it is often the least effective. And a lawn of stonecrops is only slightly better for wildlife than bare concrete.

The costs are relatively low, vanishingly so in terms of inner-city development. For the green roof at the Laban Dance Centre in London, the first to be designed specifically for biodiversity and black redstarts, it came out as £26.89 per square metre, or £10,756 in total, of which half was for the hire of a mobile crane. Savings were made by sourcing the aggregate material used from the original site, and by deciding not to use seed or young plants. English Nature's estimate of the cost for more typical green roofs is £98 per square metre.

With an estimated 24,000 hectares of roof space in London alone - an area 28 times the size of Richmond Park - the potential of green roofs is clear. As brownfield sites are developed, the only direction left for wildlife in many areas is up. But can life, apart from black redstarts, really thrive up there in the windy spaces among the chimney pots and masts?

It seems so. A recent investigation of eight green roofs - by the entomologist Richard Jones - revealed that 136 species of invertebrates had found their way there, including several rare ones. Among them were Oxypoda lurida, a rove beetle normally found on coastal shingle, Pardosa agrestis, a wolf spider of mudflats and chalk-pits, and Olibrus flavicornis, a flower beetle associated with brownfield sites.

These developing communities of invertebrates are of a kind not found in the wild, drawing hardy species from different habitats, from grassland and banks to gravel pits and coastal dunes. A new word has been coined to describe the incomers - "tecticolous", from the Latin tectum, meaning roof. According to Jones, their new habitat could become much more important, given greater variety and better design.

Given its novelty and recent origin, the idea of green roofs has taken off surprisingly well. Dusty Gedge can point to several ambitious and imaginative examples in London and the South East. One is to be on the new headquarters of Barclay's Bank in Canary Wharf. Another, on the spacious roof of the Rolls Royce factory in Chichester, mimics a gravel pit, and has already attracted a pair of little ringed plovers. Yet another will be on the halls of residence at Royal Holloway College.

Prestige is clearly one reason why architects are taking green roofs seriously. Another reason is local authority policy. They have been made a planning condition for new buildings in the area of Deptford Creek, where the black redstart first raised its tail more than 50 years ago. In the Greenwich Gateway, for example, 50 per cent of buildings are to have green roofs.

Rooftop habitats are by their nature artificial, and their success relies largely on how well they can imitate nature. In London, the ideal might be a roof design that can successfully act as a substitute for the very insect-rich brownfield sites of the Thames estuary. Further afield, Dusty Gedge suggests local geology should play a part. In County Durham, a green roof based on the local magnesian limestone might play a part in mitigating the loss of natural magnesian grassland to mining and agricultural improvement. Perhaps Swindon could have chalk roofs, and Aberdeen rocks and cliffs of grey, glistening granite.

Indeed, wildlife-friendly building need not be confined to the roof. On Canvey Island there are plans to build offices that are compatible with the exceptionally rich insect habitat nearby. A modern office, it has been pointed out, occupies only a small proportion of its proprietary space. The rest is normally given over to car parks, pavements, lawns and the ubiquitous cotoneaster. But with careful, ecologically-informed design, these spaces could become havens for bumblebees, solitary wasps and ground beetles. Architects of the near future might regard insects and wild flowers as clients, perhaps incorporating specially-built walls into their designs, or using the buildings to enclose warm, sheltered spaces with flower banks and ponds. A glittering prize will await the designer of the most wildlife-efficient hanging basket.

As for the Komodo dragon house, perhaps the most high-profile green roof so far, little persuasion was needed. Both the link with biodiversity and the cutting-edge design principles appealed to the zoo. With Dusty Gedge's help, an experimental roof-scape was devised to test a variety of substrates, including clay, sand and a mixture of crushed brick and soil. Part has been sown with wild flower seeds similar to those used on dry agricultural headlands. With so many scientists on its doorstep to monitor the results, the Komodo dragon house may become a template for green roof design in general.

One quiet hope you are unlikely to find in the formal plan is that the native common lizard will colonise the roof. Green roofs, with their inbuilt basking facilities and abundance of fast food, are highly attractive to lizards. Perhaps, in a few years' time, careful observers will spot the world's largest lizard bedding down with one of the smallest. What began as a comparatively simple dragon's lair is being transformed into an urban designer's vision of nature.

For more information on green roofs, see www.livingroofs.org

More on black redstarts and green roofs at www.blackredstarts.org.uk