Intensive green roofs have deep soil that can support trees, shrubs, and lots of visitors
Building a green rooftop involves installing a waterproof barrier over the roof, then overlaying one to six inches of soil. There are two systems of green roof design: extensive and intensive. Both systems include rainwater catchments that send the filtered runoff into barrels for landscape irrigation, either feeding back to the roof garden during droughts or to surrounding yards and preventing runoff that typically pollutes waterways and groundwater.
Extensive roof design is the most lightweight with the least amount of soil. It supports water-retentive plants such as succulents, which need little maintenance once established.
Intensive design uses a deeper soil level to support vegetables and even trees for a productive garden.
"An intensive green roof is usually specified when the green roof goals are aesthetic, versus simply economic, in nature whilst an extensive green roof is primarily implemented for environmental and economic purposes," said Mehta.
The layer of soil helps moderate the temperature within the buildings underneath the rooftop garden, and the landscape maintains a lower temperature than a typical roof – as much as 25 to 80 degrees lower. A study conducted by Environment Canada found that a typical one-story building with a grass roof and 3.9 inches of growing medium reduces summer cooling needs by 25 percent. A six-inch extensive green roof can reduce heat gains by 95 percent and heat losses by 26 percent. Chicago City Hall has recorded roof temperatures 100 degrees cooler than a neighboring building with traditional roofing.