Do green roofs really have any environmental benefits in Hong Kong’s urban jungle?

The flurry of reports and commentary following the truly catastrophic collapse of a new green roof at City University focused, appropriately, on the apparent failure of the building approval system, which is supposed to protect us from man-made disasters. In all fairness, it has done very well in a city full of spindly towers, steep retaining walls and precariously suspended signage subjected to typhoon winds and torrential downpours. How green roofs became a “blind spot” – not just for our Buildings Department, but also for professionals close to the project – is something which will hopefully be investigated, exposed and, most importantly, learned from.

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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.

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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.

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.

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Urban heat island effect? Roofs are a tiny fraction of the hard, heat-absorbing surfaces in our dense urban areas: building facades and street pavements are far greater contributors to the urban heat build-up. And here, too, white roofs would do a much better job reflecting solar radiation and reducing the heat island effect than green roofs.

Increased biodiversity? Yes, green roofs, in theory, can serve as little natural “stepping stones” in the middle of an urban jungle, where plants, insects and birds can enjoy a respite. There is a debate as to how much biodiversity is supported by the hardy, non-native grasses typically used for green roofs. But in a city where heavily vegetated mountains and wide expanses of water are never very far from even the most densely built areas, little pockets of urban greenery presumably play a very minor role in the bigger ecosystem.

Green roofs, like their vertical twins – green walls – are often more about look and image than real environmental benefits. What our city needs is not pretty green surfaces for birds and insects but real green spaces for the use of human beings, with grass people can sit and play on and trees under which they can find shade. Instead of decorating roofs and walls with plants – which, ironically, often manage to look artificial anyway – let’s truly green our steaming streets, our tiled sitting-out areas and our barren podium tops. As it stands, green roofs have become a sustainability fig leaf, projecting a green image while barely hiding the ugly reality of the hostile environment underneath.

Oren Tatcher is an architect and a member of the Hong Kong Institute of Urban Design