Developers putting green roofs on their buildings may have to think again after one of the world鈥檚 largest insurers declared they were a fire risk.

Swiss insurer Zurich, the third largest insurer of commercial property in the UK, said the roofs could dry out and become flammable. Green roofs have been promoted as one of the best ways to green new buildings and, last week, London mayor Boris Johnson put them at the heart of the capital鈥檚 climate change mitigation strategy.

Stuart Blackie, risk management consultant at Zurich, said he was concerned the roofs 鈥渨ould become a hazard in a period of drought, particularly on school buildings鈥�. Many green roofs contain sedum, which is a succulent plant, but Blackie said other plants tend to dry out in the summer months.

He pointed to the dangers of arson. 鈥淚f roof access is easy, it could be quite an easy way to set fire to a school,鈥� he said.

Blackie said he recommended a maintenance regime or irrigation procedures to be put in place as well as fire breaks and limited access to the roof. He added that the insurer should be part of the design process.

鈥淭his concept of construction is often sold on its environmental benefits,鈥� Blackie wrote in a 2006 report. 鈥淭he issue of fire spread, combustibility and indeed fire safety are often overlooked.鈥� Zurich was unable to immediately point to any instances of fire.

In a statement, Zurich said it would not refuse to insure buildings with green roofs as long as 鈥渁ppropriate guidelines have been followed鈥�. These include a continuing maintenance regime and risk assessment.

But Dusty Gedge, co-founder of LivingRoofs.org, said he thought that insurance firms were not properly researching the matter.

He said: 鈥淭hey are responding to changes in technology by being reactive, as opposed to thinking about whether this is an issue in countries with more mature green roof markets. In Germany, where there are 35 million m2 of green roofs, you get a reduction on fire insurance if you鈥檝e got one.鈥� Many insurance agencies assess buildings on a case by case basis. Others have not identified green roofs as a fire risk.

Alan Gairns, technical manager at insurance group RSA, said: 鈥淲e are conducting research into the area and are looking to learn from countries where green roofs are more prevalent.鈥�

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