Some experts have opposed Beijing's green roof project, for which the government has allocated millions of yuan a year. They argue that the cost of greening rooftops in Beijing outweighs the benefits, but there are enough reasons to believe that it's worth the effort.

A worker maintains the green rooftop of a building at the Beijing University of Chinese Medicine in Beijing, capital of China, July 12, 2013. [Photo/Xinhua]

Air pollution is Beijing's worst environmental problem, with smog posing a threat to people's health. According to Tan Tianying, president of Beijing Roof Garden Association, if all the rooftops in Beijing were covered with vegetation, they would absorb carbon dioxide at the rate of 4,835 tons a day, minimizing the concentration of PM2.5 in the air. PM2.5 refers to particulate matter with a diameter of 2.5 micrometer or less, which can enter the human respiratory system and cause health problems.

Reducing the rate of heat absorption through rooftop plants can also alleviate the "heat island" effect and help maintain a bearable temperature of about 30C on rooftops in summer. The insulation provided by green roofs significantly reduces the costs of heating and air conditioning, and more importantly, saves a great amount of energy.

In addition, green roofs can play an important role in storm water management. Since Beijing's urban drainage system is quite vulnerable during storms, promoting green roofs can save the money that would be needed to improve the drainage system and enlarge the area of natural sinks to prevent rainwater from running off.

Some people are worried that a green roof is an invitation to water leakage. But the roof-greening technique is so advanced today that as long as the waterproof membrane is intact there won't be any leaks, says an official of the landscape and forestry bureau. What's more, the structure of a building will be assessed before the roof-greening process begins in order to ensure that nothing goes wrong.

Although not all rooftops are suitable for the greening project, reportedly the potential green roof area in Beijing is about 100 million square meters. Unfortunately, only 2 percent of Beijing's rooftop areas have been covered after eight years of greening. Compared with the percentage of coverage in Western cities like Toronto, Beijing has a long way to go in greening its rootops.

Jingyi Zhang, via e-mail

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(China Daily 10/22/2013 page9)