A simple garden might make use of roll outs, which are mats seeded with wild flowers about which there was an article on Wizzley by Blackspanielgallery, the title being A Garden Made Simple. These have the advantage of being ready made and quite light, as they do not contain much soil, and as wild flowers do not use many nutrients [in fact they do not like nutrient-rich soil] they do not require much feeding. One advantage is that these roll outs do not require containers and therefore impose less strain on the roof than containers do.
Kitchen gardens on roofs are possible, though deep rooted vegetables such as potatoes and parsnips are possibly difficult in any bulk. In fact, you don't want bulk plantings, like potatoes, but lighter, specialist ones, for example a herb garden. Herbs like quite light, well drained soils and are not on the whole heavy. I have heard that somewhere in London there is a chef who grows his herbs in a roof garden.
A herb garden on the roof is ideal, There are four main families of herbs. First is the Umbellifers. These include parsley, sweet cicely , coriander, chervil and fennel. This family includes basil, sage and thyme. Then there is the Mint family, with all the rich varieties of mint. We find the liliaceae, the lily family, whose edible members include garlic, chives leeks and onions. The Brassica family include mustard and horse radish. The fifth main herb family is ginger, but that is a tropical tree and so not suitable for roofs or a colder climate.
Yet a roof garden can also provide a rich range of salad vegetables,planted in succession. And one great advantage is that the roof is freer of perennial weeds than the soil is. you will not find bindweed in your pots. The roof is therefore a controlled environment. You might even have a greenhouse, ideally a fabric one to save weight, and some roofs. This would give you tomatoes, cucumbers and melons.
But for many people their roof is a sanctuary, a place to sit above the hustle of the city. They enjoy a rich network of flowers. These will not be densely packed, for they will be laid in light containers separated by paths. Roses are very popular, but there can be a whole range of flowers according to taste. In this sense a roof garden has an advantage over a balcony, as balconies can be shaded. Only recently I was visited by a young lady from Buenos Aires [seeing my youngest son] who told me that she would love to grow something on her balcony, but it was too shaded. My solution: I went to my book shelf and gave her Plants for Shade. I needed it when taking my gardening certificates, but never since then, as shade is not an issue for me. I am promised that she will send me a picture of her balcony.
But roofs can be a safe space for bees. The city tends to be free of the worst pesticides that are decimating bee colonies. It is also replete with flowery gardens. Bee heaven! Yet another advantage is that bees dislike dogs and horses, neither of which are common on roofs. Dogs maybe on occasion, but when did you ever see a horse on a roof? Cats, maybe, but bees do not dislike them as they dislike dogs