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Zürich’s green roof programme
Since 1991, the city of Zürich has made it mandatory to have all flat roofs which are not used as roof terraces to be ‘greened’ when constructing new housing developments or renovating older ones. The main reason for this policy is to increase the biodiversity. Of course, these green roofs also play a role in the water cycle as well as reducing heat stress.
In Zürich, effects already have been quantified and insight has already been gained regarding total costs in relation to installation, maintenance and profit, energy savings and role within the water cycle.
Greened roof surfaces in an urban agglomeration are of major importance. As there is little disturbance, green roofs provide a living space to numerous wild plant and animal species, creating a compensating green area which partially compensates for the loss of other green areas through building activities.
The roof biotopes are stepping stones which, together with the earthbound green areas and the seeds distributed by wind and birds, make an important contribution to the urban green infrastructure.
In Zürich four types of roof vegetation are recommended. These have been further developed as examples on the website on the Service for Employment and Living.
Individuals and companies who request a building permit can ask for advice how to install a green roof at the Urban Green Service.
The following roof types can be distinguished:
- Ruderal vegetation
- Sedum/herbs/grass vegetation
- Sedum/moss/herbs vegetation
The reasons for the city of Zürich to choose such an approach are the following:
- Offer living space to plants and animals;
- Retain rainwater and moderate the discharge;
- Improve the urban and interior climate by tempering temperature extremes through the improvement of the radiation balance and the evaporation of moisture;
- Dampen sound;
- Bind particulate matter;
- Protect the roof membrane;
- Can improve the quality of housing and work spaces by creating extra green areas for use;
- Improvement of the view from higher buildings or mountains; and
- Cut long-term costs (lengthening of the roofs’ life span, saving on energy costs through heating, cooling and the reduction of drainage).
On the Zürich website all data regarding design, return and green roof examples can be found. [Stadt Zürich, 2011]
The regulation also includes factual minimum requirements for a green roof. For instance, the substrate layer must be at least 10 cm thick with a minimal retention capacity of 45 l/m2 (4.5 cm) and only indigenous seeds and plants from typical local biotopes may be used.