Copenhagen Green Roof Policy

Copenhagen, the capital of Denmark, adopted a green roof policy in 2010, making it one of the first European capitals to require vegetated roofs. Under the city's guidelines, new buildings with flat or low-slope roofs, defined as roofs with a slope of less than 30 degrees, must have green roofs.

Purpose

Copenhagen's policy is tied to climate adaptation, particularly managing intense rainfall. Green roofs absorb rainwater and slow its flow into the sewer system, reducing the risk of overloaded drains and flooding. The city has also promoted green roofs for their cooling, biodiversity and recreational benefits.

Climate adaptation context

Green roofs form one part of the city's wider effort to adapt to heavier rainfall by retaining and delaying stormwater before it reaches sewers.

Comparison with other cities

• Toronto (2009) sets coverage percentages by building size.
• Basel (2002) requires greening of flat roofs with native soils and plants.
• Copenhagen (2010) uses a roof slope threshold of 30 degrees to define which new roofs must be green.

Building considerations

Roofs pitched at up to 30 degrees can be greened, but steeper green roofs need measures to stop substrate slipping, such as edge restraints and stabilising grids. The added weight must be considered in structural design, and waterproofing and root barriers must be suitable for long-term vegetation cover.

Expected stormwater benefit

Research summarized by the US EPA indicates that extensive green roofs can reduce runoff by about 60 percent, with the best performance in small and moderate storms, and that green roof surfaces stay far cooler than conventional roofs in summer. For a city managing frequent heavy rain, such reductions in roof runoff ease the load on drainage networks.