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Green roofs are becoming ever more popular and are increasingly demanded by local biodiversity action plans. At the recent AJ conference, 'Green Roofs for Architects', a sceptical Austin Williams got more information about the costs and benefits of green roofs from a selection of their advocates.

Director, Architype As the head of an architectural practice specialising in social and environmental sustainability, Jonathan Hines has completed a number of green roofs over the years. 'Green roofs, ' he says, 'are beautiful, and the more we show people, the more likely we are to convince people of the importance of this architectural feature.' Hines comes across as a bit of a green-roof zealot. His objective is to spread the word and raise the public's consciousness.

He says: 'Many people will do it [install a green roof] because it looks nice, and I challenge whether that's a valid enough reason.

Really, a green roof should only be done as part of an integrated sustainability strategy.' Posing some general questions that architects are likely to be confronted with when convincing clients of the merits of green roofs, he provides some easy answers:

How do you manage/cut them?

You don't.

Will they look scruffy?

It depends on your definition.

Will they leak?


Founder, Livingroofs. org David Essex meets Johnny Rotten: Dusty Gedge is the co-founder of the first 'independent, not-for-profit, green-roof organisation in the UK'. He still seems to do quite well out of it. He personally wrote the environmental planning constraints for Camden council, for example, and boasts that, as the principal authority on the subject (because he had written it), he 'was then asked to resolve the constraints? and they paid a lot for it'. But far be it for me to cast aspersions.

Gedge is an ornithologist specialising in the Black Redstart - a species of small European bird that has recently colonised these shores. It is protected under Schedule 1 of the Wildlife and Countryside Act, 1981.

By campaigning for better urban habitats for this bird, Gedge has encouraged many local authorities to implement green-roof strategies as a part of a biodiversity development plan. Lewisham, for example, is starting to impose a rule that 30 per cent of all buildings on development sites across the borough will be provided with green roofs to satisfy the needs of the Black Redstart Action Plan.

Citing Switzerland as a model, construction law in Basel now insists that all new flat-roofed buildings should be covered in some form of vegetation to protect endangered beetles and birds.

Gedge is currently writing guidance with CIRIA on the development of a Code of Practice.

Managing director, Irritech Davey heads one of the largest independent irrigation consultancy practices in Europe and presents a non-judgmental view of green roofs notable for its lack of environmental grandstanding. Pointing out the relatively uncontroversial (and incontrovertible) fact that plants die without water, Davey was scolded by Jonathan Hines from Architype. Re-emphasising his maxim that the 'scruffy appearance' of a green roof is a matter of personal definition, Hines suggests green roofs need no maintenance at all - including watering. He promotes the aesthetic merits of letting nature kill off plants. Green roofs, it seems, suffer from human intervention.

Meanwhile, for those who take the view that a large investment in landscaping should be protected, Davey reminds us that, within the past year it has become illegal to use mains water for irrigation purposes unless specific approval has been given by the relevant water provider under the Water Regulations Act 2003.

Direct connection to a mains supply is also illegal due to potential contamination, and an air gap or break tank must be provided.

However, 20m 3 of borehole/groundwater can be drawn, subject to Environment Agency approval, without an extraction licence.

Mains water costs about 90p/m 3 (220 gallons), but for large green-roof areas there are several factors that need to be taken into account:

? Turf and shrubs require 4-6mm of water per day per m 2; medium trees require 10 times that amount. Manually applied irrigation will use 50 per cent more water than an automatic irrigation system (mainly due to spillage and application inaccuracy).

? Hard water can calcify spray heads and can also block drainage membranes.

? Water applied at night stands a better chance of reaching the root zone.

? Computer-controlled irrigation systems, tied into the buildingmanagement system, can conserve water and save money.

Sustainable Drainage Associates Sustainable urban drainage systems (SUDS) 'are intended to mimic natural drainage' say Steve Wilson and Bob Bray who laud the ability of nature to cope, but fail to address the evidence of history that sugggests society prefers to improve on natural systems.

Wilson is a 'chartered engineer and chartered environmentalist' and lead author of CIRIA Report C609 (the only UK document that recognises green roofs as a SUDS component). He cites a number of examples - from Zurich station to Swiss factories - that have used this technology as part of a 'storm-water management train'. From site tests in Brussels, the comparative rainwater run-offs are: a standard (non-green roof) single-ply covered roof allows 81 per cent run-off; a green roof with 50mm substrate (50 per cent); and a green roof with 150mm substrate (40 per cent). Both Wilson and Bray argue that green roofs should be taken into consideration in drainage design calculations, allowing for fewer outlets, narrower downpipes and consequent cost savings - offsetting the cost of the roof itself, perhaps. They are working towards producing guidance in CIRIA RP714 Biodiversity and Buildings.

Head of environmental and green-roof concepts, Bauder Peter Allnutt argues for increased regulation, for green-roof standards and the need for positive legislation.

Product manager, Sarnafil Evaluating the costs of green roofs, Mark Harris uses the results from a recent survey at the Springboard Centre in Bridgewater, comparing various green roofs with an 'ordinary, uncovered roof' to make his case. He defines his terms as follows:

? Intensive green roofs - public-access areas with soil depths of 300-350mm, which add significant additional loads to the structure.

? Extensive green roofs - roofs not intended for public access, with soil depths of 25-125mm.

? Sedum - plug-planted succulents with a fully established root system used for visual impact.

? Bio-diverse roofs - using local plants and alpines with locally sourced growing materials. These are more attractive to wildlife and designed to entice local birds and insects.

A typical sedum roof weighs 90kg/m 2 and a biodiverse roof weighs 100kg/m 2. Taking into consideration a range of factors, from energy management to maintenance and capital costs, the bio-diverse roof was found to be the most efficient, of the green roofs, followed by the sedum, while the 'ordinary' uncovered roof came third.

Contracts director, Willerby Landscapes Involved in the Jubilee Park project in Canary Wharf, London, John Melmore sees it as a 'haven of peace and lushness in the heart of the city? [which uses] a simple yet dynamic palette of materials that coalesce within a framework of technical and logistical constraints'.

Design director, INTEGER Intelligent & Green Craig Anders is an architect of some repute and is perhaps best known for his Millennium House featured in the BBC's Dreamhouse programme, hosted by Carol Vorderman. He issues a plea for architects to remember the simple truths about designing for the local environment. Speaking about the ancient primitive shelters constructed of stone with turf roofs, Anders says that doors used to be set against the prevailing wind to reduce heat loss. Architects today, he suggests, would do well to think about these simple design solutions. But he fails to address the fact that, with the invention of the door - as opposed to the doorway - humans were able to orient their buildings with less concern for natural constraints.

Some of the Integer family of buildings - Dublin House or the Classroom for the Future in Telford - are not very attractive (although as Jonathan Hines suggests, that depends on one's viewpoint). Anders does confirm that architects should write in a green-roof specification clause, or ensure there is a contractual stipulation that it be watered for the first month.

Secretary, Single Ply Roofing Association Dispelling the myth that there is any significant difference between a warranty and a guarantee, Jim Hooker asks whether we should seriously suggest that contractors provide 60-year guarantees on green-roof construction. He believes there is a great deal of misunderstanding, particularly by Housing Association clients, who insist on this timescale and he calls for 're-education on the matter'.

Taking NBS clause J42 as his starting point ('guarantees of 10 years offered by the roof-covering system are preferable to warranties on the individual components of 20 years'), he says a client would be reasonable in asking for a guarantee covering the initial setting-up period, but not effectively into perpetuity. There will undoubtedly be a drive to manufacture and provide 'systems' as opposed to components in the future, he says. Hooker makes the useful observations that:

? Where waterproofing membranes extend beyond and above the soil and vegetation in green roofs - to form upstands, say - problems of maintenance arise and careful detailing is essential.

? Flood testing the roofs prior to laying the soil is not enough, and independent electronic testing is highly recommended.

Hooker also poses the question of whether green roofs should be constructed with the insulation above the waterproof membrane or whether it should be kept as dry as possible by adopting a warm-roof condition, with concomitant pressure on good vapour-control detailing (see details, page 39).

Finally, he points out that, since January this year, the contractor is not allowed to offer insolvency insurance, and the manufacturer cannot offer an insurance-backed guarantee (unless backed up by its own assets).

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