There is a common perception that green roofs have to as verdantly green as grass. Certainly from a nature conservation and biodiversity perspective, grass is a bit of thug. Many invertebrates like bare and sparsely vegetated habitat. These xeric species will soon be pushed from a green roof once it is has become dominated by grass.
The 'green' roofs at the Hauptbahnhof Zurich couldn't be further from the general perception of green roofs. They are true living roofs, designed specifically for a number of important species that like the dry stony habitat associated railway lines. Above the platforms at the southern end of the station are a serious of 'vegetated shingle 'green' roofs. These roofs have been designed for the most northern population of Wall Lizards (Podarcus muralis) in Switzerland that favour the dry stony habitat in the railway lands. They have also been designed for a rare wasp associated with dry glacial riverbeds found in Central Europe.
What is particularly interesting about this ecological approach to green roofs is how the structural beams and rafters have been used in the over all ecological design. Above the rafters and pillars deeper mounds of growing medium, with high biomass have been created, to increase both the flora diversity and the diversity of the structure of the vegetation. This is good ecological design. As the Wall Lizard likes it pretty sparsely vegetated, but also some requires refuge, the roof fulfils both functions. Also as generally the roofs of railway platforms are relatively lightweight the designers have worked with these limitations to create an interesting ecological landscape at roof level using both ecological and structural constraints/needs.
At the ends of the roofs are gabion pillars to allow free passage for the lizards between the landscape at roof level and that at ground level. Thus the roofs are contiguous. There are even information boards on the gabion pillars to inform the commuters of the ecological activity that is going on above their weary heads.
Interestingly the roofs are part of a 'zero discharge' zone; whereby no rainwater is allowed to leave the site. As the green roofs have relatively low storage capacity there are areas of swales between the railway tracks and a parking area to the south has been constructed of permeable paving. This is a relatively common design consideration in Germany and Switzerland, that is becoming more prevalent in North America. Slowly the UK is starting to recognised the value of an integrated approach to sustainable urban drainage!
Although visually 'challenging' to many people who have visited, this excellent green roof design integrates an excellent response to the ecological needs, whilst working with both the structural and civil engineering limitations and needs.
Vegetated shingle is a very important habitat in many parts of the world, whether it be coastal or along rivers. In the UK our largest SSSI is actually Dungeness, comprising of variety of different vegetated shingle habitats. Along the south coast due to development and tourism vegetated shingle is under pressure. The green roofs at the Zurich Mainline station offer a potential solution.
