Tech Blog

Not all green roofs created equal, research suggests

The Wildflower Center monitors temperatures inside insulated metal boxes topped with three different roofs – green, white, and blacktop. Visit the Wildlife Center to see what the temperatures are now.

 

Native plants improve performance

Green roofs can keep buildings cooler in the summer and reduce rain runoff into streets and storm drains, but a great deal of variation exists among the types of green roofs available for installation today, according to researchers at The Lady Bird Johnson Wildflower Center at the University of Texas at Austin.

The researchers installed green roofs from 24 different manufacturers atop insulated metal boxes, an experiment designed to simulate today’s popular, if ugly and spiritually demoralizing, corrugated metal buildings. They found that a green roof could reduce a structure’s air conditioning bills about 21 percent compared to traditional tar-based, blacktop roofs. But they also discovered a great deal of variation among the different types of green roofs tested – a variation they attribute largely to the types of plants used.

During one 91-degree day, a blacktopped box without air conditioning reached 129°F inside while the green roof boxes reached only 97° to 100°. “That’s a huge difference to have a 20-or-so degree temperature drop,” says Dr. Mark Simmons, the lead investigator on the study.

An even greater temperature difference was found on the roofs’ surfaces. Blacktop roofs reached 154 degrees on that 91-degree day while the soil on green roofs was between 88 and 100 degrees.

Runoff

The greatest variation was found in the amount of rainfall absorbed by the roofs. Some captured all the water during a 1/2" rainfall and a little less than half the water when two inches of rain fell. Other roofs, however, retained only about a quarter of the water in a 1/2" shower and as little as eight percent during deluges.

In both cases, researches attribute the difference in performance to the types of plants used on the roofs. Native plants outperformed sedums, a type of succulent traditionally used on most green roofs, as they were able to take in more water and release more of it into the atmosphere.