Breaking through the grass ceiling

13th February 2012


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With substantial energy and biodiversity benefits, firms all over the UK are investing in green roofs. Becky Allen learns more

On the southern fringes of Sheffield city centre sits South Yorkshire’s newest local nature reserve. At just 2,000m2, it’s also one of the county’s smallest, but what makes the reserve unique is its location – on the roof of Sharrow School. Designated in 2009, Sharrow is the first green roof in England to be afforded nature reserve status, marking a significant milestone in the green-roof movement.

Once seen as the preserve of the environmental fringe, green roofs are joining the mainstream. London’s Canary Wharf estate now has the highest concentration of green roofs in the UK, and Barclays Bank, Waitrose, London Zoo and the Museum of London have all added a touch of grass to the tops of their buildings.

Sheffield leads the way

Away from London, Sheffield is staking its claim as the UK’s green-roof capital, thanks to the university’s Green Roof Centre. Its director, Dr Nigel Dunnett, confirms: “There are 60 or 70 major green roofs in Sheffield – more than any city outside London.” Until recession hit the construction industry, green roofs were springing up across the city. “The reason for this growth was that green roofs started to become part of planning policy in Sheffield,” Dunnett explains.

For the past two years buildings in Sheffield of over 10,000m2 must have a green roof, as must residential developments with more than 10 flat-roofed dwellings. As a result, more than 25,000m2 of the city’s roofs have gone green. “Only when green roofs are included in policy do they start to take off. That’s why it began in Germany, because it was part of planning requirements.”

Although they owe their resurgence over the past 35 years to developments in Germany, the roots of green roofs stretch much further back. They have been constructed for thousands of years, from the Hanging Gardens of Babylon to the more humble sod-topped dwellings of rural Scandinavia.

Germany’s green-roof trend began in the 1960s and by the 1980s had become the focus of a significant amount of interest and research. By 2001, 13.5 million square metres of German roof space had gone green.

It was travelling in Germany more than a decade ago that planted the seeds of Dunnett’s interest in green roofs. “There, they are often called an ecological protection layer and in many ways it’s a factory product, a very technical approach to creating a green roof.” But as an ecologist, he admits: “I found them very monotonous and it occurred to me that we could do different things in the UK with our different climate.”

Intensive benefits

So, while German firms majored in lightweight, plastic-based roofs with sedum, the UK ploughed its own furrow, developing a greater diversity of green roofs. These two approaches are usually described as extensive and intensive green roofs. While both are laid over a waterproof membrane and root barrier, intensive roofs have a much deeper substrate, supporting a greater variety of vegetation.

They are often built to be accessible to people as well as wildlife, and require greater input and more maintenance compared with their extensive counterparts, which are designed with minimal depth of substrate and are often planted with varieties of sedum, which can not only tolerate heat, cold, wind and drought, but have a height and growth habit that means they need almost no maintenance.

The benefits of intensive systems, say their proponents, are legion, but green roofs are most often installed to boost biodiversity. Conducted at Royal Holloway London for a PhD thesis, the first long-term study of green roofs and biodiversity in the UK discovered that they provide valuable habitats for invertebrates, increasing their populations tenfold over conventional roofs.

According to researcher Dr Gyongyver Kadas: “The most remarkable fact about green-roof habitats is that they host a high percentage of species of interest. On both the green and biodiverse roofs studies, on average 20% of the spiders and 15% of beetles found had either a local or national importance, including species listed as threatened by the International Union for Conservation of Nature and Natural Resources.”

In cities, where space is limited and development is removing the brownfield sites on which much urban wildlife depends, using roofs to provide space for nature seems logical. Roofs make up 16% of Greater London’s area, and green roofs cover 10 times as much land as Richmond Park, the capital’s largest open space.

But biodiversity is only one of the benefits of green roofs, as Dunnett points out. “They bring life to otherwise sterile surfaces and introduce nature back into cities, but they also help solve the problems of surface water runoff, the urban heat island effect and social issues. Addressing these is hard because there’s not much space in cities, so we need to be radical and start to look at roofs.”

In an era of climate change, cities need to find ways of mitigating its effects. According to the London Climate Change Partnership, by 2050 our summers will be 1.5–3.5oC hotter, and in central London the heat island effect currently adds 5–6oC to summer night-time temperatures and will intensify in the future.

The green roofs of London

As well as biodiversity, hydrology is another hot topic for green-roof research. In the coming decades, the UK’s rainfall is expected to arrive in more intense storms, increasing peak rainfall rates by up to 40%. These rainstorms will exacerbate the risk of surface flooding, and to mitigate that risk we need first to better understand it.

In the heart of the City, the Museum of London is one of several test beds for Drain London – a project looking at flooding from surface water in the capital. As part of a major £20 million refurbishment that involved refitting galleries, exhibition spaces and adding two new cafes, the museum is also retrofitting several green roofs.

“We had a failing existing roof covering and over the past 18 months we’ve renewed 4,200m2, and we’ve installed a number of different types of green roof,” explains the museum’s projects manager, Gavin McCourt. “For two projects, the Rotunda Garden and roof garden, we decided we’d try planting sedum to give us a particular colour to go with the garden.”

The complexity of the roof layout, with its different levels, aspects and degrees of shade generating a variety of microclimates, has also allowed the museum to be adventurous, incorporating wildflower plug plants and a wildflower blanket. A rooftop planter running round the edge of the building has been relined and planted with wildflowers and hedges, and bee-friendly plants feed the museum’s hive, which this year yielded 15kg of honey for staff and visitors.

With funding from Drain London, one of the museum’s roofs has been half covered with green roof and half with just a cap sheet. Flow meters installed by the University of East London will measure their relative impact on storm-water attenuation. The museum now acts as a centre of excellence, allowing building managers and architects to see different green roofs in action, fitting in well with its educational remit and netting the museum several awards in the process. The green roofs also help deliver the museum’s agenda for corporate social responsibility through biodiversity benefits and CO2 savings, but McCourt says that making a solid business case for the green roofs was crucial.

“The project generates enthusiasm but you still need to have a very good business case to sell it to the finance director ... We’re saving 10% a year on our energy costs as a result of the green roof and will be able to reduce the size of our new heating and cooling plant as a result,” he explains.

One of the Museum of London’s many fans is climate change and sustainability manager Aylin McNamara. She works at London Zoo, which installed its first green roof in 1992 and has since added a new one every five years, most recently above the Galapagos Tortoise House.

In 2008/09, the zoo added a so-called brown roof – where the growing medium comes from local spoil – over its Komodo Dragon House. “It’s good for biodiversity and is more sustainable because it used reclaimed building materials,” explains McNamara. “The base is builders’ rubble and it’s sown with a wildflower mix. We wanted to mitigate the environmental impact of the build and enhance biodiversity on-site, which is important to us as a conservation organisation in London, where space for nature is limited.”

Upkeep

London Zoo’s long-term experience has helped dispel some of the myths surrounding green roofs, not least their cost and maintenance. McNamara admits their green roofs cost 25%–50% more than traditional roofs to build but, she stresses, the greater initial spend must be viewed in the round.

“The added benefits of low maintenance and biodiversity mean it’s worth it,” she says. “Although there’s a risk that any problems would be more costly to repair, ours have done very well and we’ve had no problems. And they’ve been relatively maintenance-free – less than traditional roofs – which is a real plus ... Another benefit is their longevity, because they last for the lifespan of the building.”

Dunnett too is keen to address preconceptions preventing greater uptake of green roofs in the UK: “One survey we did found that a major concern was the amount of maintenance, but in reality it’s not a major burden so that’s just a misconception. The same is true for leakage and flat roofs, but that’s to do with waterproofing, not green roofs per se.

“They work, not just environmentally but economically. Buildings with green roofs are more attractive environments for investment and for people to live and work in,” he concludes. All of which suggests that as far as our cities’ roofs are concerned, the future’s bright – the future’s green.

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