The main drawbacks of roof gardens and balconies are the strong winds and sunshine they are generally subjected to. Take advantage of any existing protection, such as walls or fences.

If erecting trellis or screens ensure that any fixings are well supported. Balconies also often have the additional problem of an overhang, which prevents rainfall from reaching the containerised plants. Hosepipe access on balconies is also usually limited.

Before designing any roof garden it is essential to check with an architect or structural engineer how much weight the roof can take, whether planning permission is needed and also to check whether or not the roof is waterproof. Ideally place heavy containers near load-bearing walls, or over a load-bearing beam or joist.

Compost and containers

Because containers will dry out rapidly in the excess heat and wind, attempt to provide quite a deep container.

If using small containers use ones made of non-porous materials such as plastic, metal or fibreglass to reduce potential moisture loss. The container should also be lightweight, and drainage should be optimum.

Peat-based or peat substitute composts are more lightweight than loam-based materials, and can be lightened further still if required by adding material such as perlite. Bear in mind however that adding additional drainage materials will increase many plants’ water demands. Large plants should be well anchored to stop them from blowing over.

Watering and maintenance

Containerised plants will need watering all year round, except in freezing conditions. In summer plants may need watering twice a day if not more. Runoff of excess irrigation water or rainfall requires an outlet to a drainpipe. This must be easily accessible for cleaning purposes. An automated irrigation system saves labour, but may only be cost effective for larger areas.

It is essential to check planters periodically to ensure plant roots remain contained. Standing pots on pot feet deters root escape.

Plants

Windbreak plants such as Pinus mugo and phormiums tolerate both sun and wind and can act as a shield for less robust plants. Dense evergreens such as Viburnum tinus and junipers will create shelter if the existing structure does not. Low-growing plants (including most alpines) can avoid the worst of the winds, or are sturdy enough to withstand them. Dwarf spring-flowering bulbs can also withstand exposed sites.