Roof top gardens have multiple purposes; they are not only a source of beauty but may also work as a way to fight heat in hot regions by providing insulation against heat to the roofs. The best way is to have the plants in planters as that way they do not put much weight over the roof. They may also be arranged in a manner that the more sensitive plants are always protected by more sun and dry wind resistant species-- you can use certain species of creepers for this purpose. Roof top garden is destroyed by sun and wind as plants need an atmosphere where there is moderate temperature with lots of humidity. Roof gardens do not survive a harsh winter unless you make a green house. Fiber mesh can be used to protect the garden from excess sunlight while sprinklers can be used to provide moisture. It would be better to have some jute or other mesh around the planters that stays wet and provide moisture to the atmosphere. Always put the plants close to each other to preserve moisture/humidity as plants flourish in company. And don't forget to makes sure that the roof is properly waterproofed before a garden is arranged over it.