Green roofs, sown with grasses, wild flowers, herbs or alpines, are becoming as much a part of eco-friendly development as solar heating and rainwater recycling. They have long been popular in Switzerland and Germany — where flat roofs over a certain size have to be “greened” by law — and are now catching on in this country.

There are already some serious examples of green-roof projects in Britain — for example, Nos 20-30 South Colonnade, Canary Wharf, in east London; the new Rolls-Royce factory at Goodwood; BedZED, a prize-winning sustainable housing project in Sutton, south London, and the Greenfields community housing project in Maidenhead, Berkshire. Of course, the traditional crofts of the Highlands and Islands have long sported green roofs made from turf, but these were built more from economic necessity than a desire to save the environment.

Unlike a roof garden or “intensive” green roofs, these “extensive” roofs are not really designed to take more than occasional maintenance traffic. They do demand a high degree of support, however, as well as good waterproofing. They can be applied only to flat roofs or those with a pitch under 38 degrees, or plant material will slide to the bottom of the slope and all the soil nutrients will wash away.

Although wildflowers can grow in impoverished soil, sedums are the most practical green roof-covering material in Britain. They need little watering and only a thin, lightweight layer of substrate on which to survive. They look attractive all year round, changing colour from green to pink, yellow or purple without going through the scruffy phase that grasses and herbaceous plants suffer from. And they don’t need cutting back.

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Diarmuid Gavin plays it safe at this year's Chelsea Flower Show

Horticultural revolutionary Diarmuid Gavin is nothing if not controversial. But for this year’s Chelsea Flower Show, he seems to be hedging his bets

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Blackdown Horticultural Consultants, the leading supplier of green roofs in Britain, provides plants, mulches and even mats of sedum that can be rolled out to provide instant greenery. The cost of a new roof, including the underlying structure above waterproofing, is about £100 per square metre.

But why would anybody want to have sedums on their roof? Jon Broome, an architect specialising in sustainable design and construction who has incorporated several green areas on his own roof in south London, says he wants to increase biodiversity in urban spaces. As more and more brownfield sites are developed, their habitat is lost to birds, flora and fauna. A green roof can replace something of that.

Chris Wilford, of Bill Dunster Architects, the firm that designed BedZED, agrees: “We chose sedum matting as it provides a natural habitat — a green space that is not walked on or accessible to humans.”

Jonathan Hines, of Architype, an eco-sensitive architecture firm based in London and Gloucestershire, says: “Plants absorb carbon dioxide and breathe out oxygen, so they clean up the air around them. You could create an area the size of a national park on the roofs of London.”

There are drawbacks, however, not least aesthetic. “You are never going to get a roof like a perfect English lawn, with nice stripes,” says Hines. “It is going to be shaggy and untidy.”

Harry Charrington, an architect who lived in eco-friendly Finland for many years, says green roofs, though to some a bit olde-worlde, are “one of the most visible things you can do, and they shout out your green credentials”. From the rooftop.

Blackdown Horticultural Consultants, 01460 234 582, www.greenroof.co.uk; Erisco Bauder, 01473 257 671 (green roof supplier); Bill Dunster Architects, 020 8404 1380, www.zedfactory.com; Architype, 01594 825 775; Jon Broome, 020 8291 9167; Harry Charrington, 0117 328 3267

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