Green roofs, covered with growing vegetation, help conserve energy and water by cooling temperatures inside buildings in hot weather and containing storm water runoff.

New research shows striking differences in the ability of different types of green roof to retain water. Researchers compared six different green roof designs with non-reflective black roofs and reflective white roofs.

The experimental roofs were placed on top of insulated metal boxes and were planted with native, subtropical vegetation. Although the main construction of the green roofs was the same, there were variations in the rest of the structure and materials used.

Under a range of weather conditions, the temperature was taken at various points throughout the structures and rainwater runoff was collected from the roofs. Measurements showed that, compared with the ambient air temperature, lower temperatures were found throughout the structures.

In addition, temperatures below the roof membrane and inside the test platform were significantly lower than the temperatures found in equivalent places in conventional roofs. For example, when the outside temperature was 33ºC, black and white roof membrane temperatures peaked at 68ºC and 42ºC respectively, while temperatures measured at the membranes on the various green roofs ranged from 31-38ºC. Interior temperatures reached 54ºC under black roofs, 50ºC under white roofs and between 36-38º C under green roofs.

When, however, the ambient temperature was much cooler, below 5ºC, the temperature at the membranes in the black and white roofs were 2-5ºC lower than in the green roofs. Monitoring the effect of rainfall runoff showed that all green roof types retained larger quantities of water than either the black or white roofs, although the amount depended on the intensity and duration of the rainfall and the design of the green roof.

For example, if the quantity of rainfall was less than 10mm, all green roofs completely absorbed the water. With heavier rainfall, the green roofs differed considerably in their ability to retain the water: varying from 88 per cent to 26 per cent, when the rainfall was 12mm and from 44 per cent to 13 per cent, when the rainfall was 49mm. In some cases, the total runoff from some types of green roofs was not substantially different from the black or white roofs. The researchers suggest that the structures of the green roofs mainly accounted for the different retention rates, particularly the substrate and drainage layers.

This research suggests that there is no simple, one roof fits all scenario. Instead, planners and consumers should ensure that the design of green roofs meets their specific functional and climatic requirements, before installation.

Source: Simmons. M.T., Gardiner, B. Windhager, S. and Tinsley, J. (2008). 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. DOI 10.1007/s11252-008-0069-4. Contact: