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- Basel is considered a pioneer in green roofing. How did this development come about, and what sets the city apart from others?
On 25 March 1976, the Grand Council of Basel introduced a mandatory green roofing requirement for flat roofs on the then widespread single-storey commercial buildings. Systematic green roofing in Basel began in 1995, inspired by the Year of Nature. Research initiatives by the NLU (Institute for Nature and Landscape) and Pro Natura followed. A pilot green roof was installed on the NLU building in collaboration with the Office of Building Construction – Basel’s first green roof was born! Insights from this pilot project were incorporated in 1998 into the “Better Flat Roof” incentive programme, aiming to establish green roofs across various commercial sectors. By the end of 1998, 135 roofs had been greened, covering an area equivalent to eight football fields. From this initial success, close collaboration between research, authorities, and businesses developed and continues to this day.In 1999, flat roof greening was formalised in the Building and Planning Act (BPG § 72): “Unused flat roofs must be covered with a layer of vegetation.” According to permitting practice, this applies to flat roofs with a slope of up to 10° (equivalent to an 18% gradient) and a minimum area of 10 m².
- Building permits set requirements for substrate type and thickness, seeding, and structural elements, such as small-scale habitat features like deadwood.What concrete benefits do green roofs provide for the urban climate, biodiversity, and water management?
The original idea was to replace rare and increasingly disappearing ruderal habitats* on rooftops. Functioning ruderal habitats exhibit high biodiversity and are therefore ecologically valuable. Green roofs create a mosaic of new habitats that act as stepping stones for ecological connectivity. With rising urban temperatures, the functions of green roofs have evolved: in addition to promoting biodiversity, they help mitigate heat through evapotranspiration and retain rainwater. Green flat roofs can retain up to 70% of rainfall, whereas non-green roofs can discharge up to 95% of precipitation. Benefits thus include climate regulation, local cooling, water retention, enhanced quality of life, increased urban greenery, improved building insulation, and protection of the building envelope.
- What can architectural practices do, beyond proactive awareness-raising, to bring the topic of biodiversity to the forefront?
The surrounding environment should be fully assessed, and greening should be integrated into the planning from the outset so that it becomes a natural part of the building. Urban greenery not only supports biodiversity but also promotes human well-being and health (dust binding, cooling, etc.).Which types of green roofs are distinguished, and what are the differences (e.g., extensive vs. intensive)?
- Which plant species are particularly suitable?Generally, native plant species are recommended. For extensive roofs, ruderal and drought-tolerant species are suitable.
- What advice would you give to owners and architects considering green roofs? Where should they start, and what should they consider?
A non-binding preliminary inquiry to the city’s gardening department is recommended. Structural integrity must be checked first, and the roof membrane must be intact.
- Are there funding programmes? Is greening economically worthwhile, e.g., through energy savings or increased property value?
Co-financing for green roofs is possible through the Value-Added Tax Fund and the City Climate Concept incentive programme. Green roofs are certainly worthwhile. Beyond the ecological benefits, they optimise buildings, enhancing insulation and protecting the building envelope.
- Are there legal requirements or bureaucratic hurdles that make constructing green roofs in Basel easier or more difficult?
According to the Building and Planning Act (BPG § 72), unused flat roofs must be greened. Planning challenges arise when competing uses, such as photovoltaic systems or rooftop terraces, occupy roof space. The aforementioned funding programmes, however, help facilitate implementation.
- What is your vision for the future – how can cities like Basel better harness the potential of green roofs?
Urban greenery should not be seen as an obligation, but as a matter of course.
*Ruderal habitats are areas shaped by human activity, characterised by pioneer vegetation. They typically develop on nutrient-rich, often open soils found in places such as rubble sites, road edges, industrial wastelands, or construction sites.
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