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Green roofs offer the potential of keeping buildings cooler and reducing rain run-off – but not all green roofs are the same, according to new research carried out at the The Lady Bird Johnson Wildflower Center, University of Texas at Austin.

In temperate climates, research has shown that green roofs can offer benefits to both buildings and the environment. But in subtropical climates where weather is more extreme – with higher temperature and frequent flash flooding – green roofs could be even more beneficial.

Until now, though, there has been little or no evidence to support the intrinsic benefits of using green roofs in more extreme environments. So researchers from the Wildflower Center in Texas set out to compare six different green roofs to see how they cool down the interior of a building and capture rain during a downpour.

Using simulated roof conditions, the researchers found that on average maximum green roof temperatures are 38°C cooler at the surface and 18°C cooler inside than conventional tar-based black-top roofs.

That means using a green roof could translate into savings of 21% on air conditioning bills compared to a conventional roof, say the researchers.

However, when it comes to run-off retention, there was a big vairation in the performance of green roofs from different manufacturers and some showed very little retention of rainwater at all.

The variation seems to depend upon the type of plant used in the roof. Native plants appear to help green roofs capture more rainwater than sedums alone, the type of succulent traditionally used on most generic green roofs.

The researchers can’t give a simple recommendation of the best green roof, they say, because there is too much variation between products and their performance under different conditions.

“Green roofs have to be done right, and our hope is to help manufacturers understand how to improve their designs,” says lead researcher Mark Simmons.

For the consumer, says Simmons, it is imperative that you tell the manufacturer what kind of plants and other features you want. “It’s up to them to then tailor the green roof to your needs,” he says.

For further information:
Simmons, M. T., Gardiner, B., Windhager, S., and Tinsley, J., Green roofs are not created equal: the hydrologic and thermal performance of six different extensive green roofs and reflective and non-reflective roofs in a sub-tropical climate. Urban Ecosystems (2008), doi: 10.1007/s11252-008-0069-4
www.wildflower.org