Balconies vary considerably in their construction and appearance, ranging from older, wrought-iron types to functional, modern brick ones. Roofs might not vary so much, but, especially if you are gardening on a flat-roofed extension, you do still need to be sure it is safe.
Before you begin designing the garden, check the balcony or roof structurally, to make sure that it will bear the additional weight of the plants and containers you intend to bring in. A medium-sized tub, when planted up and watered, can weigh as much as 9kg (201b), and you are likely to want a good many more than one. This is more likely to be a problem on an older building, where the wear and tear of the years may be taking their toll on the brickwork and mortar. The extra weight is difficult to judge, because it will not be constant, as the containers will weigh more when they have just been watered than as they dry out. This means that they may weigh more in winter than in summer, although this will also depend on local conditions and whether the plants have shed their leaves or not.
Consider the weight problem at the planning stage. How much extra load your roof or balcony can bear will depend on its construction and ‘age; it is a wise precaution to explain the plans to a qualified builder or architect, and be guided by their opinion. Careful consideration of the materials to be used, however, can keep the extra weight to a minimum. Timber decking or tiles are lighter alternatives to paving slabs and require less effort to handle – a definite advantage for a roof garden, where everything has to be brought up from ground level. Screens of wattle, bamboo and plastic are lighter than timber fencing. Wooden trellis comes in a variety of weights, all of which are heavier than plastic.
Containers themselves also van’ considerably in weight; plastic and wood weigh much less than reconstituted stone or concrete, or even terracotta. Even the compost (soil mix) can make a difference, because soil-based composts weigh a great deal more than soilless ones, even when these are wet. The addition of perlite or vermiculite to the compost reduces the weight still further, although it will also speed up drainage, so you may have to water more often. (Mixing water-retaining gel into the planting mixture when you plant up the containers helps it to retain water.) Every little helps: consider using broken polystyrene in the base of the container, rather than the traditional pieces of broken crockery.