Using rooftops is an ideal way for urbanites and city dwellers to turn sterile, unused areas into productive vegetable growing spaces. In fact, if you look out of your window right now, you’ll probably see a suitable one you can use – on a shed, a garage or a cottage.

One of the simplest ways of creating a rooftop garden is to place a container or two on a roof. Many apartment blocks also have a flat top or a level section over the parking area that would be ideal. However,  before you start piling soil or placing boxes on your roof, there are some practical aspects to investigate. Apart from the usual  elements that go into any well planned vegetable garden, you  need to consider the following:

Weight bearing and safety

“Before you start, consult a structural engineer to assess the  roof,” recommends Marc Nel of Evagroup, an experienced rooftop landscaper, who has designed and installed numerous  rooftop gardens in Durban. “You might find certain areas can  carry higher loads than others, or you might have to create a  reinforcing framework. Once the loading capacity has been  measured, a kilogram per square metre rate is given.  Remember to calculate your garden in a wet state as water is heavy. Choosing light materials will help keep the weight down. “Working on a rooftop brings other challenges, such as safety.

A balustrade is generally the cheapest way to ensure that there are no accidents. This must be installed before work  commences.”

Drainage, water flow and waterproofing

Understanding water flow is important. It must be able to drain off quickly as stationary water increases the risk of damp and  adds to the weight. Although flat roofs look flat, they’re designed for water to flow off them. The structural engineer will give you the percentage fall and anything greater than one per cent is good. Ascertain where the lowest point of the roof is and aim the run-off in that direction.

Ideally, install pathways to assist water in exiting the garden. Marc uses recycled decking for pathways, which not only provide access to the beds but also creates channels where water can flow freely towards the drains.

“A waterproofing system must be applied to a flat roof before you begin construction,” advises Marc. “Using a torch-on heat- used UV-stable membrane is the best method, and if the roof already has one, ensure there are no cracks.”

Wind, water and sun

Wind can blow a garden to smithereens on a rooftop – especially in a coastal city. To prevent this Marc suggests sticking to  low-growing plants. “Vegetables like tomatoes should be staked to prevent them being blown over and having their roots  exposed.”

When creating permanent structures like seating and pergolas for shade and support, design them to be wind permeable, otherwise they can create wind tunnels or be blown over.

Rooftops tend to dry out more quickly than a normal garden and a reliable water supply is important. Water-wise methods such as mulching, intensive planting and grouping all  help a garden to survive the hot sun. For a larger garden, drip irrigation is ideal. Bare rooftops can be very hot, and simply  planting a garden creates a cooler environment. “One of the beauties of rooftop gardens is the benefits they produce below,”  says Marc. “At one of our gardens the ambient temperature inside the building below it has dropped by an average of 3°C. This  has reduced their air conditioner use by 40% during summer.”





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