Do you think having rooftops gardens will be effective against certain natural disasters such as floods?
Yes! A green roof helps a building's owner and residents, as well as the community at large. Here's how: Ordinary rooftops shed rainwater through storm drains into the sewer system. Many cities like New York have aged systems that mix this water with untreated sewage. When such sewers overflow, they flush raw waste into nearby waterways. Green roofs absorb a significant amount of the rainfall that hits them—as much as 70 to 90 percent in warm weather, 25 to 40 percent in winter—decreasing runoff and reducing the amount of debris and pollutants washed into lakes, streams, and rivers.
Green roofs also save energy by helping to insulate buildings, and they reduce the workload for HVAC (heating, ventilation, and air conditioning) equipment. They also serve as wildlife habitats, providing food and protection for birds and insects and act as "stepping-stones" for species that migrate through urban areas.
Hope this helps.