Forget traditional slates and tiles. Why not cover your roof with living, sprouting flowers instead? The idea may have been around since ancient times — think of the hanging gardens of Babylon — but green roofs are finally becoming mainstream in Britain, the latest manifestation of our growing interest in sustainable, ecological building.
Extensive green roofs — the most common type on both residential and commercial buildings in Britain — consist of a waterproof underlay, covered with soil or crushed stones, then coated with sedum mats or wild flowers. More wild meadow than croquet lawn, they aim to recreate the original footprint of the building beneath by using local soils and varieties of flowers. In summer they come to life, with colour-strewn patches interrupting the urban monotony of asphalt roofs and air conditioning units — although in winter the flowers do die back, revealing the soil underneath.
They also require very little maintenance: you may need to check for rubbish or unsightly weeds once a year, but you certainly won’t be hoiking a lawn mower up on to the roof in order to give it a trim. Alex Michaelis, an architect who created a sedum roof on his house in Ladbroke Grove, west London, in 2004, claims his requires “no maintenance whatsoever”.
On environmental performance, green roofs outstrip any other roofing material by a mile. They replace land lost through building and encourage biodiversity: Michaelis’s roof, for example, has become a haven for urban foxes. Such roofs also absorb water run-off: a 100mm-deep moss and sedum layer over a 50mm-deep gravel bed retains about 71% of the water that collects on it — an essential tool against flash-flooding.
Green roofs also provide insulation, keeping homes warm in the winter and cool in the summer, so shaving money off electricity bills. They can block out noise, too — a bonus in our traffic-riddled cities — and absorb air pollutants, dust and greenhouse gases.
A green roof can be installed on any building, so long as the structure can take the weight and the waterproofing layer is secure. The whole roofing process could take just a few days.
This ease of construction, coupled with the aesthetic and environmental benefits, has caused a dramatic increase in demand for domestic green roofs — albeit from a low base — over the past two years. There are now thought to be thousands in the UK. Greenfix, a leading company in the field (0870 600 0216, www.greenfix.co.uk), completes on average four large projects and several more modest ones each month. And Michaelis is currently installing green roofs on three of his new projects.
Green Roofs and Facades by Gary Grant, a recent book sponsored by the Building Research Establishment (£22.50, from www.constructionbooksdirect.com), sets out the advantages of the technique, and the Construction Industry Research Information Association (Ciria) will be publishing its own guidance on the topic, Building Greener, later this year.
Whereas in some cases the cost of a green roof is comparable to more traditional cladding materials, such as slate, prices range from £25 to about £60 per square metre depending on the size of the roof and its pitch.
“Green roofs are great on flat-roofed garages, extensions, or on new-builds, but when the pitch of the roof exceeds 40 degrees, they become unfeasibly expensive,” says Dusty Gedge, founder of Livingroofs.org, a website that campaigns for green roofs. “I have been campaigning for green roofs for 10 years, but I don’t have one. For many people, installing photovoltaic panels is a more pragmatic and environmentally effective option.”
Many local authorities, including the Greater London Authority, look favourably on domestic green roofs. Installing one is likely to help, rather than hinder, a planning application. Unfortunately, despite murmurs of approval from Yvette Cooper, the housing minister, there are no financial incentives or obligatory requirements winging their way through parliament just yet.
This is in stark contrast to our continental neighbours. Cities such as Berlin, Basel and Vienna have strict policies in place. In Germany, for example, residents are taxed €6 (£4) per square metre annually on any hard roof surface that allows water to run off directly into the drainage system.
Yet plans are afoot to bring the World Green Roof Conference to London towards the end of this year, and pressure is mounting on London mayor Ken Livingstone to make it compulsory for all new developments in the capital to install green roofs.
“Once the acres of flat roofs — the supermarkets, industrial sheds and tower blocks, as well as all feasible residential buildings — are covered in green, we can really make a dent in climate change,” says Gedge. If you already have a flat roof or are planning to build an extension or a new home, going green is worth considering. As Michaelis says: “Just go for it.”
