COMMENT | Should green roofs be mandatory?

After the long hot summer and flooding of recent years, is it a sign that it’s time for Manchester to invest in green roofs, writes Andrew Watson, associate landscape architect at Optimised Environments.

Green roofs are now part of mainstream building technology and their benefits have been well publicised, including reduced heating or cooling, water management, habitat creation and compensation for loss of green space, absorption of CO² and pollutants, noise protection, increased roof lifespan, and urban farming.

Importantly though, green roofs should not be viewed in isolation, as green bling or environmental tokenism. Just as natural landscapes are a composition of varying habitats and ecosystems, the role of green roofs is to contribute to a wider cityscape of green infrastructure, encompassing trees, parks, gardens, bio-swales and green walls.

Although easy to add to a CGI, it remains frustrating to see how few green roofs make it to reality. With climate change inevitable, cities like Manchester must adapt to become more permeable, more bio-diverse and more green.

The UK green roof industry has grown significantly over recent years, but still over 42% of green roofs are restricted to Greater London. This is partly the result of the London Plan which puts an expectation, although not mandatory, for developments to include green roofs. Other cities, such as Copenhagen and Toronto, have adopted legislation making it a requirement for green roofs to be incorporated into new developments.

Is it time for Greater Manchester to follow suit? A requirement for green roofs through planning policy would be a positive step. Developers would then be able to build this into their appraisals, as a necessity rather than an aspiration, in the knowledge that they are competing on a level playing field.

Incentivising green roofs for new development only tackles part of the problem though. Manchester must take a lead from other cities and press the case for retrofitting to existing buildings.

Amsterdam launched a green roof incentives programme in 2009 offering subsidies to owners to install green roofs. Return on investment have been demonstrated within 2-6 years with the quickest payback being seen from roofs providing a degree of access, despite the higher initial costs.

Similarly, New York passed legislation in 2008 to provide a one-year tax abatement for green roof installation covering at least 50% of the roof.

Although awareness of green roofs has increased, misconceptions remain about their inclusion on existing buildings and the range of options, from extensive sedum plants to intensive green terraces. To highlight this, research following London’s GI Audit process has suggested that between 25-32% the city centre land area could be retrofitted with green roofs without significant change to the existing buildings.

With the exception of a small number of high-profile examples, for instance the Town Hall, there are still few green roofs retrofitted to Manchester’s buildings. As the skyline grows taller, the roofscape is becoming increasingly visible as you look down from on high, further highlighting the city’s green roof deficiency.

From Stockholm to Singapore, world cities are benefiting from investment in green roofs. It’s time for Manchester to start investing too.

• Read more about Open’s green roofing strategy here, in Manchester’s Growing Skyline. The strategy is released in the run-up to Green GB Week – Open will jointly be hosting a launch event with AFL Architects and engineers Hilson Moran on 15 October 2018