IT'S TIME YOU HIT THE ROOF

Roof Gardens are the height of fashion. As cities get more crowded, people seeking their oasis of outdoor calm look towards the sky. Take any flat surface way above street level, and devoted gardeners can create something that will give pleasure out of all proportion to its often minimal size. In the last dozen years, Londoners have discovered that there is a lot more room at the top than they imagined. I visited four contrasting roofs in the capital that show what it is possible to do with a little imagination and a head for heights.

SOUTH KENSINGTON: Mohammed, museums and mahonia

Visitors to the Victoria & Albert museum may be struck by the contrast between its extravagant architecture and the simple modern building opposite. The Ismaili Centre was opened in 1984 as the British headquarters of the Shia Imami Ismaili Moslem Commun-ity. On its roof is a superbly designed garden with distinct eastern charac- teristics, a haven of spiritual renewal offering unique bird's-eye views of the surrounding museums.

John Stokes, general manager of the centre, is an enthusiastic weekend gardener and keeps a close eye on the beds that surround the spacious courtyard, with a black marble fountain at its centre. The original planting was overseen by the redoubtable Lanning Roper, but John has had to change some of the shrubs, mainly because of the wind. The bay trees did not survive and have been replaced with holly: "It's much hardier and the birds like the berries." When the maples began to look miserable he took them out and planted an unusual weeping pear, Pyrus salicifolia pendula.

The beds - some at ground level and some raised by about 3ft - are designed to be seen both from the courtyard itself and the glass-walled walkway that surrounds it like a medieval cloister. Evergreens predominate, planted to exploit the contrast in the shades of their foliage. The weeping pear has a greyish leaf that sets off the shiny green camellias alongside. Euphorbias, hostas, pieris, mahonia, fern, a fig and a juniper all provide variations on green, and a clump of black grass Ophiogon nigrescens makes for further contrast.

When it comes to flowers, white is the dominant colour. A cistus is planted close to a white rose that climbs up a trellis on one of the walls. In summer, John puts in white nicotiana and marguerites, with pansies in winter and crocuses and daffodils for the spring.

The effect is cool and restrained, blending with the relaxing sound of the water flowing from a raised pond through a rill to the central fountain. It is a fitting environment for contemplating the mysteries of the spirit and it all looks deceptively easy to manage; but the flower beds have to be renewed seasonally and the shrubs demand constant trimming to keep them in shape. "It needs a lot of care and you have to keep at it or it will run away with you," observes John. One man's spiritual renewal is another's hard graft.

HOLLOWAY ROAD: creating a garden haven above the shop

Rustic pleasures are the last thing you expect from the Holloway Road, that long and broad artery leading north from the City, flanked by buildings that have seen better days. Yet that did not daunt Geoff Bec when, three years ago, he became manager of the new Waitrose supermarket on the site of the old Jones Brothers department store.

Outside the top floor staff, or "partners'"dining room, is a flat roof where the builders had placed several planters of varying shapes and filled them skimpily with common subjects such as hebes and castor-oil plants Ricinus communis. When the store opened in October 1993, the first thing green-fingered Geoff did was to fill the planters with daffodils, tulips and crocuses, to cheer everyone up the following spring. "Holloway Road has a dowdy image and I wanted to do some- thing different," he says.

Now the planters are filled with shrubs, perennials and annuals: strawberry trees, cordyline, a few conifers, begonias, nicotiana, sunflowers, geraniums, fuchsias, busy Lizzie and much else. Some are bought, others grown from cuttings from Geoff's own garden. "The aim is to spend as little money as possible, so we improvise."

The supermarket itself is a useful source. It does a good trade in pot plants but there are always one or two that pass their sell-by date and begin to look the worse for wear. Instead of throwing them away, Geoff brings them to the roof and sometimes they recover: he showed me some thriving begonias and chrysanthemums thus revived.

He likes to ring the changes. "For spring I've got buckets and tubs and planted them up with red tulips, then put them in among the shrubs. That will raise the height of the tulips. I'm going to get some wallflowers, too.

"The only thing you really have to do up here is feed and water, because things dry out quickly: there aren't any tall buildings so we get sun all day. It's good to spend just 10 minutes sitting out here, to get away from the stresses and strains of supermarket life."

VAUXHALL: wind power rules in a designer's moveable garden

Dan Pearson is a leading garden designer and presenter of Garden Doctors on Channel 4. Although he got into a glossy magazine's list of London's most fashionable partygoers, he still thinks of himself as a countryman.

So it was a surprise to his friends when, three years ago, he decided to leave Suffolk and rent a couple of floors of a cramped Victorian terraced house near Vauxhall station, south London, with no adjacent garden. His designer's eye had lighted on the flat, empty roof of the back extension, with views of the new MI6 building and the Palace of Westminster. He saw in it possibilities that nobody else had dreamed of.

"I'd been gardening on the ground for such a long time, it appealed to me to garden on a roof," he recalls. As he was a tenant, he created a garden he could take with him when he moved. That meant planting everything in manageable containers; but he had first to be sure that the roof could take the strain.

"I took advice from two structural engineers. One said I couldn't have anything more than one person and a deckchair and the other said I could have us much as I wanted. I took the middle view." He put a wooden deck down to cover the 15ft by 10ft asphalt surface and to spread the weight and placed most of his containers on shelves cantilevered off the walls, reducing the load on the roof itself.

He enclosed the space with troughs, planted mainly with lavender. "I wanted everything to be highly scented so that whenever you came into the garden you had a response from the plant." Among the sweetest-smelling flowers he uses are nicotiana and, just by the entrance, a magnificent datura that he takes indoors for the winter.

Another quality he wanted was movement. Wind is a problem in a roof garden; Dan exploited it by choosing plants that sway elegantly, including Convolvulus cneorum, coyote willow Salix exigua, and tall grasses. A bead curtain, swishing in the breeze, adds a sense of mystery, while a seat backed with woven willow branches provides protection against the wind, and a static topiary bird, shaped in box, imperiously surveys the scene.

Dan chooses shrubs and grasses with grey foliage, to reflect the prevailing colour of the London sky. His custom-made planters pay further homage to the rooftop site - tall and slender aluminium pots resembling chimneys in shape but lighter than the genuine article. For planting on a lower level he bought metallic wastepaper baskets from Ikea, toning in with the taller pots.

He became so hooked on sky-level gardens that he designed one for last year's Chelsea Flower Show. Now he intends to move on. "It's been a fantastic three years but I get frustrated. The garden now feels to me like a collection of ornaments. I desperately need more space and some real soil."

HIGH STREET KENSINGTON: how to win awards upstairs downstairs

The first surprise about Virginia Gray's prize-winning roof garden is that you walk down to it rather than up. She and her husband have an apartment on the second, third and fourth floors of a Kensington mansion block. Their garden is on the roof of the flat below.

"We moved here in 1991 from a flat with no garden," she says. "I'd always hankered after a garden - when I grew up in Acton we had one 120ft long. This roof seemed ideal, although there was nothing here when we arrived except two prunus trees in tubs."

Her first thought was to hire a professional designer but their prices seemed absurd so she decided to do it herself. "I got loads of books from the library and came up with the basic idea. Then I went down to a local timber merchant and bought loads of wood and built 11 planters of all sizes. I did it in the lounge and drove my husband nuts."

In a space only 20ft by 12ft she has built in plenty of variation in height and has introduced lighting and a small water feature - a half- barrel with a plastic tub inside, surrounded with flowers. A pump keeps the water circulating.

The garden is surrounded by railings and trellises painted dark green to set off the shrubs. The pale leaves of a pseudo-acacia are especially striking against the dark background. The centrepiece is a pot planted with heliotrope. Last year her colour scheme was mainly yellow, white and pale purple, with petunias trailing round white and blue lobelias and a display of white lilies.

The effect is surprisingly mature and in no sense does it have an amateur look; it would be quite at home as a show garden at Chelsea. Last year it won her the London Garden Society Award for the best roof garden or patio, and for the last two years she has walked off with a "Brighter Kensington and Chelsea" first prize.

Like other roof gardeners she finds the wind hard to cope with. She has lost a laurel and a eucalyptus; but she is learning what she can and cannot do and the garden is full of joy and incident, a miniature adventure tour. There is even sculpture: a plaque representing autumn and a ceramic tortoise and hed-gehog to symbolise that, one day, she will transfer her skills to a rural setting.

Meanwhile, Virginia and thousands like her delight in pitting their enthusiasm against the adversities of the London skyline. If, pace Dorothy Gurney, you are nearer God's heart in a garden than anywhere else on earth, geographical logic suggests that you are still nearer to it in a garden on a roof. !