In any case, if the water drains away through pipes, which are part of the building, it is essential to make sure that solid material from the plants and soil does not accumulate and clog the drainpipes. Sun exposure is essential to the growth of your plants. Rooftops are the ideal location for maximum sun exposure although they also have the highest potential for overexposure in the summer months. At minimum, you plants need about five hours per day of direct sunlight to thrive. Observe you location over a period of time to determine if there is an appropriate amount of sunlight.
Wind is much greater on the roof of a building than on the street level.
Rooftops often are so windy that one might need to water the plants more than once per day. If the garden is not a full time endeavor, then this may be an insurmountable setback. Each location will have different wind considerations just as sunlight varies from place to place. Fortunately, many of the objects in the list of planters are also excellent wind guards and sunshades if placed strategically. But you should also make sure that all plants and garden supplies are secure and will not be blown off a ledge, wall or other high structure.