Dornbirn promotes green roofs to cool the city

Green roofs can store rainwater, release moisture and reduce the heat absorbed by buildings, functioning like natural air conditioning, ORF reported. The city of Dornbirn in Vorarlberg is holding workshops to show residents how they can plant their own roofs.

In the course, participants learn to lay out a fleece, spread soil across the middle of the roof and cover the edges with gravel to keep drains clear before sowing seeds. Stones or deadwood can also be added to create more habitat for insects.

Almost any roof can be greened, ecologist Karin Löning said, whether it is sloped or flat. However, the roof must be watertight and strong enough to support the additional weight. Experts should check these requirements. Roofs already covered with gravel can serve as an indication that they are suitable for greening.

Green roofs need little maintenance

Green roofs are generally considered easy to maintain. Andreas Dür, a gardener with the city of Dornbirn, said they usually need to be cleared once a year, depending on how much they grow. Unwanted shrubs and other plants must also be removed. In most cases, watering is not necessary.

Heat and drought have nevertheless affected green roofs this year. Dornbirn sowed about 1,000 square metres at the CampusVäre in spring, but only a few plants have emerged. It was simply too hot and dry, Dür said.

Wild plants that produce seeds can return on their own next year, Löning said. Plants with established root systems can also recover, while some annuals may not return or may have died completely. Adding more substrate could help the plants grow again.

Green roofs could reduce urban temperatures

Plants on roofs store rainwater and release moisture, reducing the heat absorbed by surfaces. According to Löning, green roofs can theoretically lower the perceived temperature in their surroundings by up to 13 degrees Celsius and make a significant contribution to cooling cities.

Only around 5% to 6% of roofs with the potential for greening are currently planted across Vorarlberg, Löning said. Greening more flat roofs in cities would make a substantial difference, she said, while also stressing the importance of reducing sealed surfaces at ground level.

The city of Dornbirn is using green roofs not only for new buildings but also when renovating existing properties.