Roof greening in Beijing

Due to the rapid growth and the boosting population, a large portion of the recent reports on Beijing was related to the air pollution problem. However, large area of impenetrable surfaces, e.g. roads, car parks and roof tops, leads to a new challenge for the city.

Because of the increment of the impenetrable layers, massive amount of rain water enters the storm water system and overwhelms the designed capacity. There were a number of inner city flooding events in the recent years. The Urban heat island (UHI) effect is also enhanced because of the loss of vegetation zones, and the temperature difference can be up to 6.5C°.

Roof-top greening was then proposed under this circumstance. There are about 93 million square metres roof areas in Beijing are suitable for roof-top greening. The estimated benefits include reducing the air pollution by 880,000 tonnes, prevent up to 3.6 million cube metre storm water from entering the storm water system during heavy rains, and lower the average temperature by 0.32C° during the summer. However, 29 billion Yuan (AU$5.8 billion) plus the annual maintenance cost are required for such project.

In fact, the Beijing government already started to subsidy the roof-top greening since 2008. The city greening area was growing at a rate of more than 100,000 sqm per year since 2005.

The roof-top greening may have positive effect to Beijing because many government buildings occupies large area of lands and are mostly flat roofs. However, besides the huge cost required for this plan, a big concern is how the greening would affect the roof structure and maintenance.

Similar to how the designed capacity of storm water system is overwhelmed, existing roofs are not designed to bear soil and plants. If 360,000 tonnes of water are expected to be saved from entering the storm water system, that means 360,000 tonnes of water will remain on the roofs on top of the soil and plants. Possible corrosion to the surface of the roofs from fertiliser and irrigation is also a serious problem, especially because the inspection on the roof are limited by the greening.

In my opinion, there are a lot of potential problems with the roof-top greening whilst the benefits are neglect. If there is budget as big as 30 billion Yuan, it should be used in better projects.

https://www.chinadialogue.net/article/show/single/ch/5625-Beijing-needs-a-green-roof-revolution-