Research into the benefits of green planted roofs

The University of Canterbury (UC) is investigating the benefits of green planted roofs for the New Zealand built environment.

Initial findings into a year-long UC ecological pilot study have found more than half the stormwater run-off from a planted roof is reduced compared to an unvegetated roof as it evapo-transpires back to the atmosphere.

The research has been led by natural resources engineering researchers Dr Aisling O’Sullivan and Dr Tonny de Vries, who will present a paper on the topic at the South Pacific Stormwater Conference in Auckland on 10 May.

Dr Aisling O’Sullivan.

“The performance and sustainability of green planted roof systems is determined by the amount of stormwater they reduce and range of plants they can support,” Dr O’Sullivan said.

“Christchurch’s rebuilding plans strongly encourage low-impact designs including green roof systems and UC has established a green roofed building with different types of plants on campus.

“After a year’s trial here we can demonstrate green roof systems respond well under Christchurch’s climate.

“The native grasses and succulent plants have been extremely resilient throughout all seasons, even when covered in snow in June last year and after a remarkably long summer.

“For the long-term sustainability of such living systems, the grasses will likely require some irrigation in periods of extended drought to maintain healthy living communities.

“This could be obtained by rainfall harvesting from adjacent roofs that are not vegetated during rain events. While observations from this study are only preliminary, they do indicate that green roofs are a feasible option for use in the rebuild of Christchurch and are an efficient stormwater management option.”

Engineered green roofs are becoming common in New Zealand as they offer multiple benefits including stormwater control. They have been installed on Waitakere’s Civic Building in Auckland, at the Remarkables Primary School in Queenstown, the NZI Centre in Auckland, Pipitea Plaza in Wellington and many other sites in New Zealand.

The roofs reduce energy consumption and improve local air quality by trapping air particles and smog but are most commonly used for stormwater management by reducing peak flow rates and volumes.

Funding for the UC research has been provided by Stormwater 360, Environment Canterbury, BayerBoost and the New Zealand Hydrological Society.