A study by Environment Canada found that a green roof results in a 26-percent reduction in summer cooling needs for air conditioning and a 26-percent reduction in winter heat losses.

Minhang research showed that indoor temperatures of a building with a green roof are 3-5 degrees Celsius lower than those of a building with an exposed roof.

This means less greenhouse gas is discharged and there's reduced need for air conditioning. A small way to fight global warming.

Extensive use of green roofs also help with storm-water runoff management.

Heavy storms can overload the wastewater system and cause flooding, dumping raw sewage into the local waterways.

Green roofs can reduce the amount of runoff from the roof.

Studies show that roofs can retain up to 75 percent of the rain water, gradually releasing it back into the atmosphere through condensation and transpiration, while retaining pollutants in their soil.

Green roofs also can provide habitats for insects and birds. Even gardens 19 stories high can attract birds, bees and butterflies.

Clearly, gardens are heavy and not every roof is suitable for retrofitting and special water proofing.

As the soil is not very deep, roof gardens require vegetation with spreading transverse roots, not deep roots.

Trees and vegetation need to be sturdy and capable of withstanding wind.

Yulan magnolia, sweet osmanthus, small conifers and box trees are widely used in Minhang.

Early roof gardens

The earliest elevated gardens can be traced back to 2000 BC in Ur in the lower reaches of the Euphrates River (today's Iraq).

Traces of vegetation were found on the third level of a ziggurat (terraced pyramid), not on the top.

The first real roof garden appeared in Babylon in about 600 BC, the famed Hanging Gardens of Babylon, or the Hanging Gardens of Semiramis, one of the original seven wonders of the ancient world. The site is near present-day Al Hillah, Babil, in Iraq.

The gardens, on multiple levels, were as high as 23 meters. They were built by Nebuchadnezzar II to please his ailing wife who longed for the fragrant trees and plants of her native Persia.

The gardens were destroyed by several earthquakes after the 2nd century BC.

Roof gardens in China

The first basic roof gardens appeared in China in the 1960s. Vegetable and fruit gardens were planted on the roofs of factories, offices and office buildings in Chengdu, capital city of Sichuan Province, and Chongqing.

The first proper rooftop garden was built on the roof of the Guangzhou Dongfang Hotel in Guangdong Province in the 1970s. The first big outdoor roof garden was built in 1983 on the roof of the Great Wall Hotel in Beijing.

(Shanghai Daily April 14, 2009)