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For those who live in city apartments, a roof garden, however small, can be an invaluable retreat, especially when it is well grown enough to screen off the city views around it.
Some roof gardens, especially those caring for co-operatively by the inhabitants of a block of flats, may include lawns, trees, pools, and fountains, with raised beds behind masonry retaining walls. However, large amounts of topsoil are needed for these grander gardens - trees will need a depth of a least 450 mm to prevent them from being uprooted in the wind.
So, unless you have a large area of a roof that will bear the weight of wet soil, masonry and flooring, the plants should be housed in lighter containers; the lighter they are the more you can have, so it is worth using plastic and fibreglass pots. Check the weight loads which your roof can sustain and plan; accordingly, you will probably need the landlords or agent's permission, as well as the neighbour's agreement before you start on the garden.
You should also make sure that both the waterproofing and drainage of the roof are adequate to prevent water from seeping through the ceiling below and to give it a good tunnel for water to go into the drain below.
A solid layer of ash felt is usually laid on top of the roof to waterproof it; on top of this, a floor of light tilting or wooden decking can be laid with suitable gulley or pipes for drainage. A combination of paving materials will provide a variety in the roof surface which can sometimes look to flat and uniform.
Roof gardens are usually very exposed, bringing problems of excessive wind and sun. Plants are as much in danger as people, for cold winds can kill young shoots and damage even sturdy plants, while long periods of sun bring problems of watering. A screen or pergola is the best solution for both these hazards. Screens can be made of hardy planting, sheets of plastic or glass, or possibly bamboo or reed if the winds are not too strong. They should be securely anchored especially if the roof is several stories high. Again, you may need permission to put up a solid structure which impinges on the neighbours' view.
Since most roofs are entirely open to the sun, deep and thorough watering is essential in the summer, especially for plants in containers; containers dry out faster than large beds. When laying the flooring it is often a good idea to install a fixed irrigation system and a water pump as water on the roof in indispensable and watering by hand is very laborious.
You may also find a small fountain a worth will addition as the sound and movement of plain water is very soothing in the summer. When you water the plants, add liquid fertilizer to replace the richness in the soil which has leached away.
A layer of broken crocks or other damaged material will be necessary at the bottom of the containers. Cover this highly quality planting fibre or with a mixture of soil or vermiculite, an exceptionally lightweight planting medium which contains no nutrients but makes less soil necessary. Timber containers should be on blocks raised off the ground to prevent them from rooting from the bottom.
A combination of evergreen or flowering creepers witch some hardy annuals to provide colour is ideal to the roof garden. Exploit the available height by training plants or espalier trees up the walls on a trellis, all securely fixed to withstand the wind.
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