Already this summer, London has experienced three heatwaves with temperatures regularly climbing above 30 degrees Celsius.
Like many major cities, it retains heat due to the radiative and thermal properties of the buildings.
The use of green roofs, or rooftop gardens, to help bring down daytime temperatures in the city is becoming increasingly popular.
They can have benefits during winter too, by providing a layer of insulation.
Dr Oscar Brousse, from the Institute of Environmental Design and Engineering at University College London, explains that the green roof “changes how the energy coming from the sun is redistributed within our urban system”.
“Typically vegetative surfaces are cooler than asphalt surfaces,” he explains.
Dr Oscar Brousse says green roofs also capture rainwater
However, London is experiencing more extreme weather in general, with heavy rainfall too.
Due to a combination of factors, including an aging drainage system, the city can sometimes experience flooding.
Green roofs can help in terms of water management.
“When it rains for example, they will capture that rain which will benefit the plants and will avoid water run-off and flooding at the ground level,” Dr Brousse explains.
Wild Gardens
Located in the heart of the City of London, among some of the most iconic skyscrapers, is the investment bank Nomura PLC.
It has gone a step further than many of its neighbours by turning its green roof into a wild garden.
Dr Brousse notes that vegetative surfaces are cooler than asphalt surfaces
The space is home to birds, rare plants and also beehives, allowing for honey to be collected by the staff.
Nina Swallow, Environment and Sustainability Manager at Nomura, explains that the wild garden has turned into a passion project for many of the bank’s staff who have an interest in biodiversity.
“Temperatures are increasing and that’s why we are seeing the rare orchid that resides here on the rooftop because it’s used to Mediterranean temperatures.
“The green roof is about becoming adaptable and resilient to climate change, of which these solutions do help,” she explains.
Beehives allow for honey to be collected by Nomura staff
Cool Roofs
While green roofs have been found to reduce daytime temperatures, they’ve also been found to release heat at night.
It’s led some to argue that “cool roofs”, which are sprayed with reflective properties like white paint, might be even more effective at reducing temperatures.
They’re also lighter than rooftop gardens, which would suit some buildings.
“In our research we do not argue for one solution as being better than another,” Dr Brousse says.
“If you’re concerned about biodiversity or water management then green roofs will also have many benefits which their cool roofs counterpart may not have at all”, he explains.
Cool roofs would also have to be regularly cleaned, to maintain their reflective capacity.
