How Green Roofs can Improve our Cities
We all love a room with a view, but when it comes to planning for the future of a building we tend to forget about the world beyond its walls. We home in on the structure itself – its foundations and floors, cavities and cracks – isolating it from its natural surroundings. But the performance of a building depends very much on conditions outside. The smartest designs are an active part of local ecosystems: they harness heat from the sun, facilitate the flow of fresh air, or take advantage of trees and hillsides for shelter. And they give back, too: habitats for wildlife, drainage for stormwater, greenery to keep a dense city block cool.
The value that local ecosystems offer urban areas is just beginning to be recognised. A recent study in New York City found its trees to be worth $122 million thanks to their part in reducing pollution, improving aesthetics, and keeping inner city temperatures comfortable [see 'What's a tree really worth?']. But these ‘services’ are rarely factored in when planning teams get to grips with a retrofit project. Their rigorous assessments may cover a whole range of technologies, assessing their potential energy savings, costs and returns, but they may not take into account the benefits a green roof could offer. They try to minimise the cost to the planet through carbon emissions, but may not account for the cost of a newly sealed loft to any bats hanging out in the ’hood.
It’s a missed opportunity. When we do look at the big picture, we realise that many measures make sense for both the built environment and its natural surroundings. Take green roofs, for example. Not only do they insulate a building like a duvet but they control storm water runoff, help keep built-up urban areas cool, and support precious pollinators and other wildlife. Vegetation can also prolong the service life of a roof, by reducing strain on the materials typically caused by erosion and weathering.
The downside is that green roofs are expensive to install, in some cases twice as much as a conventional roof. But the upfront cost can be paid back through energy savings. Some projects in the US have reduced air-conditioning costs by as much as one-third, according to Paul Mankiewicz, Executive Director of the Gaia Institute, a New York-based environmental institution. Similarly, 6,000m2 of greenery installed in Canary Wharf, London, has resulted in massive savings on heating bills. According to the building manager of 10 South Colonnade, home to Barclays Capital, the new roof cut the need to heat or cool the top floor of the building completely, “saving us £4,000 to £5,000 a year”. And in Singapore, the Changi General Hospital has found that hydroponically grown vegetables on the rooftops not only provide food for patients but absorb heat from the roof and cool the wards facing it. Savings on utility bills are channelled towards patient care.
Energy savings aren’t the only economic benefit of green roofs. They have also been linked to heightened productivity and reduced turnover among people working in urban offices – a phenomenon known as ‘biophilia’.
Eight storeys above the din of New York’s Avenue of the Americas, a small wilderness of clover, grasses and flowers shows the passage of the seasons. This roof terrace is the home and handy work of architects Cook + Fox. Through its windows, employees watch dragonflies and monarch butterflies flit over pink and yellow perennials, sprouting on what was formerly a desert of black tar. This new carpet of colour grows out of black nylon bags called Green Paks. Filled with a mixture of rock and compost, these offer a lighter infrastructure than some green roofs, whose compost and filtration layers require structural reinforcement. Moreover, they come at half the cost: $10 per square foot, instead of $18-$20. The firm’s partners maintain that the installation, completed in 2006, is one of the best decisions they ever made. It may be that the tenants on the seventh floor get the most benefit from the roof’s cooling qualities, but Rick Cook claims his firm’s profits are fatter thanks to the view.
The potential of a biodiverse built environment to boost profits has been spotted by others, too. British Land, the UK’s largest developer, has planned a “green necklace” around a shopping centre in Teeside – all part of its £26,000 refurbishment – to include an otter holt, ponds, hedges and bird boxes. “It’s about people feeling more connected to nature and enjoying the places they work in”, says Sarah Cary of British Land. But she admits that it’s difficult to factor this investment into the accounts. “Sadly, the [perceived] value is more social”, she says.
Rafael Marks of architects Penoyre & Prasad agrees: “The way the commissioning and tendering process works means biodiversity is mostly a poor cousin. It all depends on the desires of the client.”
Currently, Marks is working on the refurbishment of a dilapidated youth centre, housed in an old electrical substation in a north London park. Its new function will be a state-of-the-art educational ecology centre, so it’s a great opportunity to make the building a better fit for its surroundings.
One solution is external lighting with a hooded ‘eyelid’ design, limiting light pollution which plays havoc with local bats. Bats come out to feed when the sun goes down, but increased levels of artificial light in urban areas mean they simply cannot judge when dusk has come. “The lights will be as low as possible without making it unsafe to walk in the park”, says Marks. The site will also feature green roofs, greywater recycling, and maximum use of daylight within the buildings.
But back to those bats. Numbers have been badly hit as we convert our lofts and seal up our houses. The Bat Conservation Trust recommends leaving a 10cm gap at the edge of a loft: just enough to allow an entrance for bats, and an important means of ventilation. You can also avoid entombing bats in cavity walls by insulating from the bottom up, giving them a chance to rouse themselves and make an exit.
Another retrofit project had to plan around a parliament of owls living in an 18th century barn. “When we planned the conversion of a barn near Cambridge we built an owl house at each gable end of the roof”, says Katie Thornburrow of Granta Architects who specialise in sustainable design. Her client, Chris Bristow, feels “honoured [that] these beautiful creatures live in our house. The cost [of the owl house] was in the hundreds of pounds.”
It sounds cheap enough, but such efforts could well remain a niche concern, without any financial incentives to prompt planners.
“Let’s be honest”, prompts Stuart Wykes, Managing Director – Aggregates at Lafarge A&C UK. “Quarrying and construction activities are, by their very nature, intrusive in the landscape. But they also give an opportunity to create a landscape and habitat: to improve on what you started with. The way we see it, the day that you start stripping the material off is the day that you start your restoration scheme.”
Who wouldn’t agree? The question is, will conservationists and natural capitalists spot the opportunity retrofits present to bring our built environment into harmony with valuable ecosystems? Or will our efforts to cut carbon be at the cost of local life?
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