A roof garden needs careful thought, especially if you plan one from scratch, and you may need to take specialist advice before you start work.

One advantage of roof gardens is that they invariably have wide views - it's a bit like gardening on top of a hill; however, this also means that the garden will be windier than at ground level, and will face greater extremes of temperature.

Planning

Check planning or conservation area restraints do not apply to your proposed new roof garden. These relate to structures visible from the ground or those that alter the roofline of a building. The structural strength of your roof must be surveyed; it must be capable of supporting the additional weight of your garden. The bulk of extra weight in roof gardens should be concentrated around the sides, where it is supported by load-bearing walls.

Access

Remember when planning your garden that access will be restricted. Everything in the rooftop garden must be brought up through your living area and out into the garden.

Safety

Safety is an important consideration. Every feature must be securely and safely fitted. Winds are often more extreme, so baffling screens to make wind shelters are a good idea. So too are structures for shade as temperatures are more extreme, and heat will be radiated back from the roof surface. These should all be well designed and secured in the garden. Containers must be stable, and not placed where there is danger of them being blown away.

Materials

Consider your garden's layout and the materials you'll use. You will probably need some form of screening on boundaries to make shelter and perhaps to hide some eyesores, such as cooling fans and other roof top services.

When choosing materials, always try to consider reducing the weight as much as possible. Hard landscaping materials include:

• Timber and plastic are both useful for screens, arbours and other structures.
• Canvas awnings, tightly secured, provide good instant shaded spaces.
• Decking is a good, useful choice for seating areas.
• Fibreglass artificial rocks look realistic and are much lighter than the real thing for making natural features.
• Galvanised metal has become popular for containers and raised beds.

Beds and containers should be lined and given adequate base drainage before filling with a lightweight planting medium.

Climate

When planting, remember that temperatures will be approximately 5°C higher than the air temperature at ground level. Wind funnelling can be a problem, with wind speed factors doubled on average, which can cause a high rate of water loss from plants.

Make areas of shelter to widen planting options. Roof garden plants need to be tolerant of high levels of sun, wind and drought, and by making areas of shelter; you will prolong their growing season. Alternatively, Mediterranean plants are a good choice, as they thrive in similar conditions.