This roof garden has been around for a while now. It has been planted with a whole range of plants e.g. Tradescantia discolor (synonym: Rhoeo discolor; common name: Boat Lily, Oyster Plant; Family: Commelinaceae), Ananas bracteatus (common name: Red Pineapple; Family: Bromeliaceae), Agave and Sansevieria trifasciata (common name: Mothers-in-law Tongue; Family: Ruscaceae/Agavaceae), that are drought tolerant.
What is interesting is they all have almost the same form. Some links on the plants are shown here: