Many 'green roof' schemes, which aim to make buildings more eco-friendly by turning rooftops into gardens, are failing because of a lack of basic research on their efficient installation and maintenance.

Green roofs can help to cool buildings and minimize water run-off. Credit: DOE/NREL - Katrin Scholz-Barth

Government subsidies have caused a flowering of green roofs across Europe and North America, as people hope to cash in on their aesthetic, environmental and economic benefits. And research certainly supports the idea that they can reduce the need for air conditioning or heating; cut the amount of rain that runs off the roof, helping to mitigate flooding; and even provide urban habitat for insects and birds. But there has been little work on establishing a basic standard for such roofs, says Stephan Brenneisen, an ecologist at the Zurich University of Applied Sciences in Switzerland.

Brenneisen cites an unpublished report from Green City Zurich, the city department responsible for planning and maintenance of public parks, which found that more than 50% of green roofs in Zurich have less than 5 centimetres of the soil substitute that binds the vegetation to the roof — insufficient to support most plants.

Green roofs have to be done right, and our hope is to help manufacturers understand how to improve their designs. Mark Simmons , University of Texas, Austin

In Switzerland, legislation requiring greening of new and renovated flat roofs has been introduced by city and regional authorities over the past ten years. But regulators have lacked the research to determine the most effective amount of substrate, says Brenneisen, and building companies have cut costs by thinning down this layer. This reduces the water retention and the insulating effect of the roof as well as its ecological benefits, he says.

Green roof face-off

Few countries have rules governing green roofs. Germany's guidelines, developed by the Bonn-based Landscape Research, Development and Construction Society (FLL), are generally regarded as the industry standard across Europe and North America. According to, a non-profit advisory group supported by the UK's London Development Agency, over 30 million square metres of green roofs have been installed in Germany since 2000.

But planning regulations and incentive schemes tend to be implemented on a local scale, which means a lack of nationwide data about the number and types of roof that have been installed. Zurich recently introduced a minimum substrate depth of 10 centimetres for new roofs to counter the problem described by Brenneisen. But a recent study has highlighted the fact that even with the same substrate depth, not all green roofs are created equal.

Scientists at the Lady Bird Johnson Wildflower Center at the University of Texas, Austin, asked six local manufacturers to design a green roof, each with the same 10-centimetre depth of soil substrate, and then compared them to an ordinary roof. There was a significant difference in the amount of water retained by the green roofs, ranging from 26 to 88% in one rainstorm, simply due to the types of substrates and drainage systems used1. Even worse, the difference between the green roofs was often greater than that between a green and a non-vegetated roof.

"Just having a green roof may not mean anything in terms of preventing water from reaching the street level, for instance," says Mark Simmons, an ecologist at the Lady Bird Johnson Centre, and one of the authors of the study. "Green roofs have to be done right, and our hope is to help manufacturers understand how to improve their designs."

Some of the ecological implications of the failure of these roofs are only just starting to come to light. "Ground-nesting birds in Switzerland, such as the northern lapwings, are using the green roofs as breeding grounds," says Brenneisen, "but the chicks are usually dying because there is not enough water and food on the roofs." His Zurich colleagues Nathalie Baumann and Doris Tausendpfund will be presenting this research at the World Green Roof Congress in London on 18 September.

But there is plenty of evidence that a well-designed roof can deliver all the promised benefits. The California Academy of Sciences in Golden Gate Park, San Francisco, has an undulating green roof that has become a popular habitat for insects and birds within a year of installation. "I've seen barn swallows, lesser goldfinches and hummingbirds all visiting, and the bees are really active," says Frank Almeda, a botanist with the academy.