It is possible too much available organic substances i.e. Waste part of vegetables, fruit.. but good quality of water is scarcity.

@ Sakir, I am still not satisfied. We should encourage city people for rooftop farming by hydrophonic technique using specially designed greenhouse. Green roots reduce the heat flux through the roof, and less energy for cooling or heating can lead to significant cost savings in addition to the produce. In summer the green roofs protects the building from direct solar heat. In winter the green roofs minimises heat loss through added insulation on the roof.

Above idea is good but how it possible good quality of water supply. We already know that water scarcity is a major problem.

@ Chandrabhan, good question. I think one of the possibility is to harvest rain water, stored and re-use it on site rather than allowing to run-off.

there are advantages and disadvantages when applied in cities

Still too little activities regarding this matter. Still much to do. May be subsidizing the components of small hydroponic systems can encourage the expansion of such activities. Regarding the scarcity of water, rain harvesting is very hard in the cities and also rare rain fall make it even harder. Maybe harvesting water from air conditioners of each building can be a practical method for cultivating the roof of each building

Green roofs or agriculture in the cities are still limited in our countries. at least for the time being, it is used only to grow some leafy vegetables for family use. the broader concept of the activities has not been fully implemented in most of developing countries.