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Green roof myths

By Ruth Slavid
There are still misapprehensions about green roofs, as the doyen of their development, Dusty Gedge, has discovered.

We all know about green roofs now, don’t we? They are a generally good thing, they are enshrined in policy in London and several other cities, and we can all recite the advantages – biodiversity, reduction in temperature variation, ameliorating the urban heat island. Where once pioneers trumpeted the advantages of green roofs, now we all know what there is to know don’t we? Well, no we don’t. At least not according to the doyen of green roofs (and I hope he is chuckling at that description) Dusty Gedge.

Currently president of the European Federation of Green Roof Associations (‘from punk to president, what a rise!’ he says), Dusty came to his enthusiasm for green roofs through bird watching and a desire to recreate the rubble habitats that were lost when derelict sites were redeveloped. As a pioneer he was leading tours to Switzerland well over a decade ago to enable architects to see how enlightened planning (and impressive injections of cash) could result in a proliferation of successful, biodiverse and well-loved green roofs.

Myth one: green roofs and PV
When I went on one of these trips in 2004, one of the high points as far as Dusty was concerned was to demonstrate a roof that combined planting and PV generation. There is evidence closer to hand as well that it can work. In the London Borough of Lewisham, for example, which has been one of the leading proponents of green roofs, some 20% of these roofs house solar panels. Yet there is still, he argues, a myth that the two are incompatible. It is a myth that he is keen to dispel.

As a result of these concerns, Dusty has been the driving force behind the formation of the Biosolar Roof project, a consortium of organisations in several European countries with the aim of building accredited training programmes that will develop workforce skills to install and maintain a green roof that will hold solar panels as well as stimulate pollinators.

Dusty explains why this is important. ‘You have constructors who understand green roofs and constructors who understand solar panels,’ he says, ‘but there is a need for constructors to understand both simuitaneously.’ This is not just a question of technology but also of thinking about biodiversity. ‘How could a solar panel make a green roof better for pollinators, for solitary bees and for butterflies?’ he asks. ‘The solar panel creates topography, it creates shade, and it creates structure on a roof which can affect what type of vegetation can grow, in many ways in a positive way.’

In order to get these advantages, he says, it is important to get the balance right between energy generation, biodiversity and the other benefits of green roofs. This means, he says, ‘that you can’t pack a roof with PV.’ A typical distance should be around 1m between panels, and also around 1m between rows. Rainwater will drip off the edges of the panels, providing extra moisture to the plants that are near these edges and enhancing biodiversity. And too often, Dusty says, we think about PV and ignore the benefits of solar thermal. ‘Solar thermal,’ he says, ‘can allow rain through and provide shade in times of extended sun – thus allowing for different planting schedules.’

Because there is a complex inter-relationship between the different elements, both in terms of biodiversity and of fixing, it is important that one person takes responsibility. And since this is a biodiverse roof that also generates energy, rather than a solar installation that happens to have some planting beneath it, the right person to do this, believes Dusty, is the landscape architect. ‘In many ways,’ he says, ‘the landscape architect needs to take control of the green roof element and the features that the solar panels sit on to ensure that the landscape and solar integration works. Supplying the panels themselves and taking control of the wiring should be the limit of the engineer’s responsibility.’

Myth two: brown roofs
If the incompatibility of PV and green roofs is a myth that Dusty has always been ready to dispel, his other myth is more surprising – not least because it is one that he believed in himself for some time. Speaking at an event at the start of this year organised by SWIG (the Sustainable Water Industry Group) he said that brown roofs don’t really work, at least not in the way that they are conceived in this country. Yet, he admits, ‘I was one of the five people who came up with the brown roof idea’.

When you understand where his belief in green roofs comes from it is easy to see why the brown roof was, superficially, so attractive. While certain green roofs – especially intensive roofs with garden-like planting – can provide fantastic amenity, this does not necessarily equate with biodiversity. And it is the biodiversity that is Dusty’s driver, starting in particular from his lifelong interest in birdwatching, and in particular in the fate of the black redstart, the bird that so successfully colonised London’s bombsites.

Green roof enthusiasts are keen on recreating those habitats as closely as possible. Sedum mats, while simple to apply and creating a reasonable appearance rapidly, are not popular with ecologists either. Instead, why not just put some rubble up on the roof, and let nature take its course? The problem is that this approach, so good in theory, just doesn’t work. What starts as an unappealing-looking collection of brown rubble too often ends up as an unappealing looking collection of brown rubble. Either not much grows, or the wrong things – for example colonisation by that rampaging thug buddleia davidii.

And yet it worked so well in Switzerland, that country whose green roof policy all enthusiasts are eager to emulate. Why couldn’t it work here? The reason is geology. Most brown roofs (or rubble roofs) are created by scraping up material from the floor of the construction site and putting it on the roof. And depending on where you are, that can be very different. In Switzerland, especially in cities like Basel, which pioneered much of the green roof development, the ground consists of alluvial gravels. Use these for your brown roof and it has a good chance of prospering. But in London, for instance, you are likely to get a mixture of London clay and old concrete and, experience shows, it doesn’t work. One of the reasons is that it just doesn’t have the water-holding properties that are needed. ‘Substrates and growing mediums need to be able to hold water,’ Dusty says, ‘not only for successful vegetation but also to provide sustainable drainage control.’

The underlying reason behind this is that roofs need to be about replication, not reproduction. You cannot simply reproduce what happened on the ground by lifting off the top surface and putting it on the roof, because it is just not the same. And that is for a reason that is easily understood once you think about it. The soil has a whole structure lying beneath it, of subsoil and underlying rock, which influences the way that the surface behaves in terms of holding nutrients and water. Put that same surface on a roof and instead what it has beneath it is a roofing membrane, some insulation and, ultimately, a building. So to recreate the habitat that you want, you have to do it by different means. Fortunately, Dusty says, there are good substrates available commercially. ‘A good green roof substrate is in keeping the with the original green roof idea – generally brick-based and provides the right balance to create the type of dry grasslands associated with many brownfield sites.’

By seeding these roofs, you create the biodiversity that you want almost immediately. The roofs look better, and you keep out those brutish invaders, or at least make life more difficult for them. As so often, the ‘natural effect’ is best created by intelligently artificial means.

What these two myths reveal is that, while the principles of green roofs are now generally appreciated, there is still a lot to be learnt about the details of their design and execution. And that is scarcely surprising. As anybody who has experienced a roof failure can attest, we have had conventional roofs for millennia, and we are still learning how to make them better.

The Biosolar Roof Project

The primary aim of this project is to build accredited training programmes that will develop workforce skills to install and maintain a green roof that will hold solar panels as well as stimulate pollinators.

It is supported by a consortium that is a mixture of small to large, public and private organisations united in developing an innovative curriculum around sustainable rooftop construction. It offers higher education research and vets providers allied with private construction businesses successfully selling this service in the European marketplace.

The intended outputs are to produce a programme of training for contractors on green roofs that is;
• designed to facilitate solar panels
• designed to stimulate pollinators
• designed to do both the above.

For more information see
The European Biosolar Roof Conference will take place in London on 28 September. See

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