Before starting a garden on the roof of your flat, or the roof of an extension, check the local building regulations about the construction of roof gardens. If you have any doubt, consult the local authority's building regulations officer. If you rent your home, make sure you consult your landlord before doing anything.
Lay a thick sheet of plastic on the roof to protect it from seepage and plant roots. Then construct a box frame of 4 x 2in/100 x 50 mm wood and set it on top of the plastic. Reinforce the corners with angle irons.
Fill the frame with well-rotted garden compost, if you can get hold of some. Perlite can be added to the mixture to improve its structure. (Beware of peat moss, which becomes very heavy when wet.)
Water thoroughly and add 5lb/2.3kg of fertiliser per 100sq ft/9.3m2. Water again lightly, then set out the plants and sow the seeds. (Herbs, vegetables such as onions, radishes, lettuce, green beans and peas, and strawberries and dwarf varieties of tomatoes thrive in roof gardens.)
Keep the surface of the 'soil' evenly moist until germination, then increase watering daily. Feed with a soluble fertiliser every fourth or fifth watering.
