IT'S a sign of the times that roof gardens made their debut at this year's Chelsea Flower Show, as increasingly people are searching for much-needed green space in urban areas.

If you live in the city and think you may have a sustainable roof space, there are three Ws to consider, says Andrew Duff, director of the garden design faculty at the Inchbald School of Design in London.

"The first W' is weight. Make sure you can walk on the roof. You may not be able to put heavy pots on it, so before you can consider a roof garden you should consult a structural engineer, or if you are in the process of buying the property you can involve that in the survey," he says.

If you can't put much weight on the roof, sedum matting is a good way to green it up, he adds. It's evergreen, stores water in the leaves and is succulent, and should only need watering during drought.

The second W' is water, as any roof is exposed to the elements.

Andrew says: "You need to think about a water system because the roof is so exposed and water will just evaporate. Consider what plants will survive in dry conditions. Ornamental grasses such as stipa arundinacea (pheasant's tail grass) do well."

The third W' is wind.

Andrew says: "Wind is a huge issue on roofs. You need to find plants which will grow in really exposed sites. Small varieties of pampas grass are ideal. If you want to lay anything more tender, think about putting up a windbreak. Silver birch grow very well on rooftops. They are flexible and don't mind getting battered by the wind. But again, you need to plan ahead for weight of growth. Is a fully-grown plant going to be too heavy?"

If you can put pots on your roof, choose plants that will withstand a lot of exposure. Look for those with small, tough, leathery leaves or grey foliage. Good examples include lavenders, sea buckthorn, griselinia and santolina (cotton lavender).