Standing in the street, you probably wouldn't notice it.

Yet Dusseldorf, the affluent capital of the western German state of North Rhine Westphalia, is home to more than 730,000 square meters of greenery above people's heads.

That's roughly equivalent to 100 football fields spread over some 1,700 buildings and last year this feat won the city an award from the International Green Roof Association (IGRA).

More than just a rooftop garden, a 'green roof' is a different way of approaching architecture, using vegetation supported by waterproof materials instead of conventional construction materials.

Green roofs live longer

Green roofs have a number of benefits, including improved insulation and energy savings, but for some they also represent an aesthetic statement.

"We wanted to make our car park more attractive, and at the time, replacing the facade and roof was imminent," said Ernst Hagemann, the owner and operator of a green roofed multi-story car park in Dusseldorf's city center.

Although he admits that being a "nature-lover" influenced his decision when it came time to modernize in the 1980s, he says there were obvious practical benefits to covering his roof in earth and moss.

Back then, the City of Dusseldorf subsidized 50 percent of the cost through a green roofing program. While green roofs are more expensive to build than standard ones, Hagemann saw economic savings in the long run.

"From the cost perspective, it was only marginally more expensive with the subsidy that we received from the city," he said. "If you consider that typical roofing felt needs to be repaired every five years or so and that after 12 or 15 years it needs to be completely replaced, then a green roof is definitely cheaper."

The green roof on Hagemann's multi-story car park has not needed any repairs in the 25 years since it was installed. This is because the layer of vegetation that has been planted over a waterproofing membrane protects the actual roof underneath it. And there are other economic advantages too

Green roofs deliver savings through energy efficiency. In the summer months, the roofs help save on air conditioning because of their cooling properties, and in winter they help save on heating because of their insulation.

Services rendered

They also offer a service to cities as a whole, helping moderate temperatures in the concrete jungle – a major reason why Dusseldorf is backing green roof uptake, says Katja Holzmueller from the city's Environment Department.

"The ability of green roofs to reduce thermal heating in cities is our primary motivation to promote green roofing," she said. "Green roofs absorb rain water and so it evaporates more slowly…(they) therefore have a cooling effect."

This cooling effect is especially interesting for areas that get hot in summer, because of cities' tendency to trap heat, which can be deadly – especially for the elderly – during heat waves.

The roofs also provide an opportunity for flora and fauna to find room to survive in cities. According to the Berlin-based IGRA, green roofs especially promote biodiversity, providing shelter and food for bees, butterflies and beetles. Car park owner Ernst Hagemann is pleased with the little creatures his green roof has attracted, despite its location just off a busy Dusseldorf street.

"There are many insects and birds here that were long gone from the city. I've seen a wren, chaffinches, greenfinches – all kinds of birds," he said.

The benefits of green roofs don't stop here though, according to IGRA. They can also enhance the quality of life for city slickers by reducing noise levels, and can reduce flooding and damage to sewer systems because they reduce water run-off by 50-90 percent during storms.

Productivity gains

Green roofs are also catching on in the corporate world a means of improving productivity.

"It's very important that employees have green areas where they can take a break… important studies about this have been conducted in Germany," said Christian Schulze-Ardey from the Bonn-based Landscape Research, Development & Construction Society, which publishes technical guidelines for green roofs and buildings.

"For example, BMW put a lot of green structure in its offices and the result was a very low rate of illness. I think that's a sign of the importance of green structures and green roofing," he said.

Schulze-Ardey also believes German expertise in green roofing is highly prized overseas, because of the country's decades-long experience in engineering the structures.

"We often have demand from people in America to buy our guidelines," he said, adding that green roofs had an even more important role to play in the US where cities are much large than in Germany.

In the meantime, green roof leader Dusseldorf isn't resting on its laurels. The city introduced a new incentive this year – a reduced sewage charge for buildings with green roofs. The city is also conducting research to see just how much of an impact green roofs have in curbing temperatures and cutting run-off.

Author: Laura Schweiger
Editor: Nathan Witkop