Roof gardens: how city-dwelling horticulturists are reaching new heights "Whatever you choose to plant, you should definitely be thinking more Russian steppe than tropical island," he declares. Conditions on a rooftop can be harsh and exposed; a plant with large, succulent leaves will be under permanent pressure, because a big leaf acts like a sail getting blown around in the wind, and will quickly desiccate. Far better to go for smaller, silver-coloured grasses; things like lavender work well, too, as does Russian sage (Perovskia). It's even more important to make sure that the roof is strong enough. Not just for people, but for plants. We underestimate their weight, plus the weight of the pots they come in. The solution is to hire a structural engineer, or architect, to test the roof's load-bearing properties. If all your building has had pattering over it in the past is the odd pigeon, it is likely to need extra supports, especially if you plan to launch your new garden with a cocktail party. "It can be a big operation," says Barry Burrows, the managing director of Bartholomew Landscaping. We did a job on a small house recently, which involved punching a hole right through the existing roof, and lifting in RSJs [rolled steel joists] with a crane. This is when things start getting expensive, says Sarah Bevin, from specialists Urban Roof Gardens. "Our lowest quote just for designing a roof garden would be around £10-£20,000," she says. But when the job involves structural work, waterproofing and applying for planning permission, that figure can go up to £100,000. It sounds a lot of money, but a lot of home owners are going down the route of improving rather than moving. A roof garden adds a whole new dimension to a house. Plus a fair bit to the value, according to property buying agent Robert Bailey. "A roof garden can add anything from 10 to 15 per cent onto the asking price," he says. "It is high on my clients" list of desires." So much so that upmarket developers increasingly provide roof gardens as standard. "With some of our penthouses, the gardens are as big as the living space, if not bigger," says Ian Dobie, the managing director of St George West London. "A lot of our buyers are moving out of houses with large gardens, now that their children have left home," he says. The loss of their garden is something they feel keenly. If they are going to move into an apartment, they want something more than a balcony and a window box. This is exactly how Bermudan-born artist and painter Lynn Parotti felt about moving out of her old house and into a new apartment beside Kew Bridge. Not only does she have views up and down the Thames, and across to Kew Gardens, but she has her own 2,000sq ft (186sq m) roof garden overlooking the river. "I need to have the natural world around me," explains Lynn, well known for her exuberant flower paintings. I was so concerned about leaving my old garden that I dug up my favourite trees and plants and took them to my partner's mother's house in Cornwall till the new place was ready. As a result, I have my wisteria, my lovely Japanese acer and my Australian tree ferns, as well as the plants the developers give you: lavender, box and sage. I don't think the builders were too keen when I rocked up with a van-load of trees and plants that needed carrying up here. But I'm pleased I've been able to bring my old garden with me. Lynn Parotti's spacious roof garden by Kew Bridge in London. The only thing she is missing is real grass. But that is par for the course. Water sprinkled on roof gardens can all too often leak into the living area. An exception to this rule is west London's Kensington Roof Gardens, built in the Thirties, on top of what used to be the Derry and Toms department store on Kensington High Street. Not only are the lawns here made of real grass, but the trees are actually rooted in the roof itself, rather than pots and planters. "We've got 70-year-old oaks in just 18 inches of soil," says head gardener David Lewis. They were planted small, and so have accommodated themselves to the conditions in which they find themselves. That said, no one should contemplate spreading bagloads of soil direct onto a roof and starting to plant. First, it's imperative to lay down a waterproof skin, or membrane, as well as building in a slope to encourage rain to drain off the roof and into a gutter. Fail to do this and you risk gaining an impromptu water feature in your bedroom. The priorities were somewhat different for flower photographer Suzie Gibbons when she decided to create a roof garden on top of her Victorian-built house in Crystal Palace, south London. "The good thing was that the roof was flat, at least 12in (31cm) thick, and made of steel and reinforced concrete slabs," she remembers. The not-so-good thing was that there was just this bleak, empty space, with orange and yellow tiles, bitumen everywhere, and a terrifyingly low perimeter wall, given that we had a small child. Her first step was to put up a hedge of Portuguese laurel, both to stop garden-users falling overboard, and to prevent other people from looking in. Next, she built a small conservatory, and covered the ugly stairwell with sedum. She then put timber decking down over the concrete roof, and deployed hardy grasses in the exposed areas (birch trees are now underplanted with carex grasses and erigeron daisies). Lusher specimens were planted in more sheltered parts of the roof (star jasmine and orange blossom for scent, magnolia and Euphorbia mellifera for flowers and foliage). All of this required planning permission, as does any roof garden which seeks to add to a property's height (and most do). At the same time, Suzie and her partner Guy Chadwick set about creating wall sculptures and plant boxes that mirror the rusting chimney stacks and church spires of the south London skyline. "That's the great thing about having a roof garden," says Suzie. It's all very magical and Mary Poppins-like. What's more, there's nothing nicer than coming up here first thing with your cuppa, in your PJs, to take the morning air. Top tips for roof gardens: Call in a structural engineer to advise on access, load-bearing capacity, waterproofing and safety. Consider the degree of exposure - plants will be scorched by wind and sun, as will humans. Use tough, drought-loving plants from exposed, windy habitats such as steppes or beaches. For human comfort, build in wind-permeable screens for shelter and to give privacy. Think "fitted kitchen" in terms of making the best use of the space. The ideal design will incorporate planting, seating, dining, storage and screening. If you have a spectacular skyline, frame the best views. Focus on enclosure and privacy if there's little to look at. Roof gardens are essentially container gardens, albeit on a larger-than-usual scale. Install an irrigation system to cut down on labour (no one wants to schlep watering cans up stairs) and use nematodes to combat vine weevil - once they get a hold of confined plants the results can be devastating.