Do green roofs really have any environmental benefits in Hong Kong’s urban jungle?
Oren Tatcher says after the collapse at City University, now is a good time to question the value of green roofs. Instead of decorating roofs and walls with plants, the city should focus on providing real green spaces for people
Hong Kong debate should switch from green roof plan submissions to aim of such projects, surveyor says
And we may do well to use this opportunity to ask whether planted roofs are the sustainable features they claim to be, and whether they are appropriate for Hong Kong.
There is no denying green roofs are prettier than conventional ones, if you happened to look down on them from the windows of a taller building or a plane. But, beyond beauty, their tangible sustainability value appears to be at least questionable.
How a trend to put vegetation on Hong Kong’s roofs ended in a tangled mess
There have been many studies around the world, including in Hong Kong, of the environmental aspects of green roofs. The commonly cited benefits are reduced cooling requirements, better storm water run-off management, reduced urban heat island effect, and increased biodiversity. While some studies are thorough and occasionally critical, many simply deal with the technical aspects of designing and installing green roofs, taking their benefits for granted; indeed, there seems to be an astounding degree of uncritical “accepted wisdom” on green roofs in both public discourse and government regulations, here and around the world.
As it stands, green roofs have become a sustainability fig leaf
True, when talking about sustainability, it is easy to get confused by complex and sometimes contradictory arguments, but that does not mean we should simply accept the “green roofs are good” orthodoxy. Let’s try to take a simple, common-sense, Hong Kong-centric view of the alleged benefits of green roofs.
Reduced cooling requirements? Obviously, that applies only to the floor just below the roof, which benefits from the additional insulation of the planted layer. In a dense city of skyscrapers, it goes without saying that this feature would benefit a tiny fraction of our built floor area. Just as importantly, a white roof – literally, a “normal” roof with a reflective, typically white, roof membrane – achieves that goal much more effectively and sustainably, without the added energy, materials and cost required for building and maintaining planted roofs. Such roofs are now mandatory in new buildings in New York, which has set a goal of turning a million square feet of old roofs into white roofs every year.
Storm water run-off? With extensive rural and urban drainage systems, our city of mountains and skyscrapers drains reasonably well during typhoons and black rain events. Even if green roof coverage was extensive enough to be a factor, the relatively thin soil layer of most green roofs has very limited absorption capacity when it comes to torrential downpours. Think of an overwatered house plant: once the soil is soaked, the water simply flows out of the planter. This is what happens with green roofs during a heavy rainstorm.
