Green roofs are getting increased attention as the world considers how to adapt to a changing climate, reducing the energy required to heat and cool the building and mitigating the “heat island” effect that Australian cities particularly suffer from.
Basel in Switzerland – not renowned for being overly hot – has led the way with the greenest roofs in the world, over 5.71m2 per inhabitant. The city initially achieved this by subsidising green roof installation and retrofitting, with the money raised from a tax on customers’ energy bills; it later moved to a green roof mandate on all new and renovated flat roofs.
France then achieved headlines worldwide with the passing of a law in May 2022 mandating that all new commercial buildings have roofs covered with plants or solar panels.
Meanwhile, here in Australia, scientists at the University of Technology Sydney, funded by the City of Sydney Council, have found that surrounding rooftop solar panels with green plants makes them more efficient. Two adjacent buildings in Barangaroo, Sydney, were compared, and the greener version was found to be as much as 20 per cent more efficient at peak times.
In a country that seems to be in a near-permanent state of either flood or drought, it’s no surprise that the top two concerns of insurers around the green roof are water ingress and fire.
The increasing intensity of storms can challenge drainage capacity. Both organic matter and hail can quickly clog drains, leading to a build-up of water that may overflow through doors, windows and roof lights and even breach load limits. When engineering these roofs, many companies will flood-test the “failsafe” situation of blocked drains to ensure escape via designated overflows.
Consideration must be given to drought and the potential for plants and soil to dry out, creating a combustible base across the roof that may serve as fuel for a fire or the means for existing fire to spread more quickly. Choice of plants, substrate, configuration, irrigation, maintenance, and the imposition of fire breaks around any penetrations into the building are all relevant in mitigating these risks, as are a blanket smoking ban and the fitment of fire detection systems and security cameras.
Good design and maintenance can mitigate many of these risks, ensuring that green roofs provide all their manifold benefits and none of their downsides.
Image courtesy of Junglefy