Green Roof 

As the recreational land use is not enough in the inner city, most of the building try to promote the roof top garden to provide more leisure space for the building user. They will try to use the roof top for the gardening use and leisure farming use. It can create one more place for the people to rest and enjoy the green cover.

The other benefit for building the green roof to reduce the sunlight to hits the top of the building. It can reduce the temperature of the building because of the less cement cover area. Therefore, the energy can be saved for the cooling facilities.

Although the green roof have some advantage, there are also some barriers to promote it to public. First is the maintenance cost is high, the second is the loading of the building may not support it.  

Reference:

http://www.npihome.com/2015/1888/whats-a-green-roof/