Discovering Basel’s green roof scene

For the past 15 years, as part of Basel’s biodiversity strategy, green roofs have been mandatory on all new and retrofit buildings with flat roofs and since becoming compulsory more than 1 million square metres of green roofs have been constructed.

Enter Dr Stephan Brenneisen and his team have been written into the planning process – they provide guidance to Swiss developers and contractors on green roof designs that will enable the creation of durable, ecologically-significant green roofs, and evaluate the installations at intervals after handover. The city has, effectively, mandated high-performing green roofs that last.

The aim of our trip was to familiarise the research group with some of the original lightweight roofs that have been specified by Dr Brenneisen and his team in the Basel and Zurich area, to strengthen the research programme we’re undertaking with the KTP Centre at University of Brighton into the commercial application of lightweight natural green roofs.

Despite our visit taking place in the middle of a long Alpine summer heatwave, which had noticeably caused many of the green roofs we saw to have ‘browned-off’, close inspection with our Swiss colleagues revealed that in fact the roofs were teeming with evidence of good plant growth and invertebrate colonies. As we were taken around it became very clear that although the roofs may not have been ‘beautiful’ in an ornamental sense, they were performing very highly against their design criteria: provide build-ups analagous to natural settings to enable rare and endangered plants creatures to flourish within the urban area.

Duncan Baker-Brown and Dr Anja Rott from University of Brighton joined Lee Evans, Simon Giddings and Candice Knights from Organic Roofs on a field trip to find out more about the Swiss approach to green roofs. Our hosts were Magdalena Gerner (Nee Mioduszewska) and Chiara Catalano, both Research Assistants of the Green Roof Competence Centre, Zurich University of Applied Sciences ZHAW as well as world-renowned expert Dr Stephan Brenneisen, Head of the Green Roof Competence Centre, Zurich University of Applied Sciences ZHAW.

Felix-Platter Spital Roof

The first green roof we visited was at Felix-Platter Spital where Dr Brenneisen joined our party. Felix-Platter Spital is a leading university centre for inpatient and outpatient geriatric medicine in Switzerland, based in Basel. A timber framed building with low load bearing capacity, constructed in 2008, the Spital was one of the first commercial buildings to use a straw build up pioneered in the mountains outside Basel (of which more later). The whole roof was laid with straw and half of it covered with a reduced substrate of crushed brick and aggregate gravel. The other half was overlaid with 50-100mm leaf compost.

The entire roof was then covered with cuttings from a meadow and standard seed mixture. Grass, herb and flower growth is successful on the leaf compost side and there is less cracking, the other half has a great deal of cracking, less grass, herb and flower coverage. Taking a soil sample on our visit it was apparent that the straw has now almost entirely decomposed.

Tram Depot BVB

The second green roof we visited was at the main Tram Depot serving the Basel urban area. Constructed in 2010 an initially with an even build-up, the wave profile has varied the substrate levels in the green roof: as the water passes through the substrate the fines naturally flow with the water into the valleys, meaning that i the depression and on the flat surfaces there is more material in which the plants can grow. No maintenance (cutting or watering) has been carried out since the first vegetation season watering in 2011, which was carried out to support seeding -a really important lesson of this approach that we hope to carry forward to our consultations with stakeholders in the design, construction and management of commercial buildings in the UK.

The changed topology has led to variation in the vegetation cover between the hills and the valleys, and this creates different microhabitats which favour different species of invertebrate. ZHAW currently have 20 pitfall traps to capture insects and research the species composition. Species discovered so far include rare ‘Red listed’ grasshoppers and Mantis.

University Hospital Klinikum 2 Basel

The third green roof visited was on the University Hospital Klinikum 2 Basel – a great viewpoint overlooking Basel. The original green roof was installed in 1978 as part of a holistic construction strategy to provide green spaces for patients to look out onto instead of grey roofs to provide a higher quality of convalescence for patients.  The Swiss team reconstructed the roof in 2003, and is home to the first of Stephan’s trials with three different depths of substrate and areas to provide natural habitats for insects to attract birds to create a ‘bird paradise’. The evidence is clear – as is well known in green roofing – that the deeper the substrate the more growth as there is more room for the roots and water.

The substrate used was sandy-loamy gravel, from a riverbank, as well as top soil. Undulating levels, areas with grasses, wooden beds and sand areas were installed to create natural habitats where different species could colonise and create their homes. There has been no maintenance on the roof and the plants are analysed once a year in May. A first for all of us including Lee, was the discovery of solitary bee holes in the sand areas!

This roof was a great example of how good design can do more than just promote plant diversity. The longer grasses that grow in the deeper substrate allows spiders refuge, which in turn then provide food for the the birds which are drawn to the more open, coarse areas, while the addition of ‘land art’ (wood cuttings) also provides refugia and adds modest nutrient to the soil around.

Summary

Magda, our endlessly helpful host – a leading green research scientist, feels there is a battle between ecology and architecture but the design of this roof demonstrates that there doesn’t have to be. If anything, they show that with good design they can work well together.

The key criteria for evaluating green roofs have tended in the UK to be cost, weight and maintenance. This has overwhelmingly ended up with a decision to go with sedum on a thin layer of substrate. Little thought is given to what the green roof might be capable of doing in terms of ecology OR overall building performance. It seemed clear from the first day of our visit that a lightweight alternative to sedum roofs which enabled plant growth of the kind that supported a food chain was doing a lot more than sedum roofs, and was remarkably resilient in the face of extended extreme drought conditions. Moreover there had been no maintenance whatsoever on most of the roofs that we saw.

Excited by the promise of what these examples could do for our research project, we were all looking forward to day 2 and our visit to ZHAW research facility before finishing on some signature green roofs in the Zurich area.

Inspired? Get in touch

This 1 year knowledge exchange partnership, supported by the Regional Growth Fund, is providing rigorous evidence of the applicability of this Swiss approach to the UK retrofit and newbuild markets. If you have a project with a green roof in London or the south-east that would benefit from species-rich, lightweight design please contact us on 01273 421864.