Düsseldorf's Kö-Bogen is a feast for the eyes, and not just because of its elegant shops. The employees in the offices above also appreciate the green courtyards in the two buildings, so-called green roofs with an amazing variety of plants, beetles, insects and birds. The green roofs were constructed on a highly pressure-resistant and dimensionally stable thermal insulation of the underlying reinforced concrete ceiling. Mineral insulation boards from Multipor were used, which could be optimally adapted to the amorphous building geometry. More and more building owners and architects are creating additional added value for people and nature in the context of new buildings or roof renovations. The French parliament recently passed a new law to this effect. According to it, all new commercial buildings must in future be at least partially equipped with green roofs or solar collectors.

Green roofs bring back biodiversity

Whether as a small flat roof with extensive greenery or as a garden full of perennials and shrubs: compared to the classic flat roof, green roofs have many advantages. The first one can be seen and heard. Here it hums and buzzes. Microorganisms, bees, butterflies and beetles nest here. They don't even need much space to do so.

Scientists from the University of Zurich were therefore amazed during a study in May 2022. On behalf of the city of Hamburg, they examined seven green roofs in the Hanseatic city. Already there, they registered 235 different species of beetles, many wasps and wild bees. They even found beetles that are on the Red List of endangered species. As food, these insects and beetles in turn attract birds. So the scientists also discovered breeding gulls, oyster catchers and ducks on the only seven roofs.

Green roofs as a natural air conditioner

But people also benefit from the greenery high above them. Not only because the plant layer absorbs sound and binds fine dust: It also acts like a natural air conditioner. In summer, it protects the roof from overheating and the water evaporating between the plants cools the building envelope. This is becoming increasingly important in view of the United Nations' forecast that almost 70 percent of the world's population will live in cities in 2050. This is because green islands on city roofs reduce the so-called "urban heat island effect": the temperature in the world's inner cities is on average five to seven degrees higher than in the rural regions around them at the same time. According to the Tyndall Centre for Climate Change, just ten percent more plants in cities would significantly mitigate climate change there.

Green roofs constructed with Multipor

The quality of the roof structure is crucial for this. Martin Morgenroth, architect and expert for green roofs at Xella Germany, describes it this way: "First of all, a substrate is important that, like our premium insulation material Multipor, does not compress, shrink or creep. Only in this way can it provide the necessary stability for the heavy construction. It also optimizes the prescribed fire protection." Security against water penetration is provided by a multi-layer waterproofing system in the form of a high-quality root protection film or fleece. Only then is a drainage and storage layer of bulk material such as brick lava substrate, clay and pumice applied, which stores or drains rainwater. The necessary substrates can be designed as a single-layer system or as a multi-layer system. A filter fleece protects the drainage layer from fine particles from the substrate above, in which plants form their roots. Depending on the statics of the house and the roof, even hedges and trees can flourish here.

Green roofs act like a sponge

One thing is close to architect Morgenroth's heart about green roofs, beyond their benefits for people and animals. "They offer us another enormous advantage in times of increasing heavy rain," says Morgenroth. "These roofs act like a sponge. They absorb rainwater and release it in a throttled and time-delayed manner into the usually already overloaded urban sewage system."

Morgenroth is an expert in this area of construction in particular, with great conviction. "Architects and builders can do a lot of good here with simple means - for their quality of life, for biodiversity and for the water management of cities."