Do roof gardens collectively make a valuable wildlife corridor? Has Boris researched it? Certainly they can be good for a few people, although the question remains: would roof garden grants be better spent on public parks?
Certainly roof gardens may offer wonderful views. If not overlooked, they also offer total privacy when stretched out among even low greenery; just you and the Good Lord. The style of planting is flexible. A garden of rural natives is just as possible as stainless steel and palms or vegetables; just be sure that your lease and your landlord are willing.

Wind, weight and water are the main issues. Install great raised beds of heavy compost, and the building may fail; composts mixed with polystyrene chips keep the weight down. But tall plants in light containers easily blow over, tripods of runner beans would provide shade and a green screen around windows, but they are best chained for safety. Does your insurance cover you against spearing passers-by below?

Roof gardens seen all year from a window or glass doors will benefit from a considerable evergreen element, which means there may be some winter watering, but a roof garden entered through a hatch, mole-fashion, can be planted afresh every year, purely for summer colour, and entirely forgotten for six cold months.

Flat roofs that are looked down upon, but not walked on, can be covered with a carpet of low-growing succulents planted into an artificial substrate to make a green roof. But isn’t the point of a roof garden to get onto it? Then think about the surface: not grass, but maybe decking with a rug thrown over, or fibre tiles? Just beware of blocking off drains.

Stephen Anderton is the Times Gardener