Green roofs boost Garden City

17:51, Nov 03 2013

Green roofs can be retrofitted on to existing buildings and houses, the organiser of a demonstration project hopes to show.

The roofs - sown with grasses and other plants - have been hard to apply to existing buildings because of their weight, Sustainable Habitat Challenge (Shac) national co-ordinator Tim Bishop said. But buildings with tile roofs and roofs of other heavy materials can go green if done properly.

To establish the point, Bishop installed a demo project at 18 Cranmer Square, Christchurch, last week. "We want to learn how to do it cheaply in New Zealand," he said. The demo is not a retrofit, but the materials and construction system could be applied in a retrofit, he said.

Green roofs are attractive and cool, they support biodiversity and they clean water that falls on them, Bishop said. "Christchurch wants to be a garden city and this is one way to get there," he said.

The demo is also testing whether clam shells make a good soil aggregate, and what plants will grow best in the Canterbury environment. "Not everything we do here we'll want to do again, but it's a start."

Christchurch landscape architect Di Lucas installed a green roof on her home 15 years ago and said it was "fantastic". Her roof includes dripper irrigation, which is sometimes needed in Christchurch summers to keep plants alive. "It's romantic not to have [irrigation]," she said.


University of Canterbury researchers Dr Aisling O'Sullivan and Dr Tonny de Vries reported last week that they hadn't watered a green roof on campus for 18 months and the plants were thriving. "This coming summer will be exciting as it will be the second year of a summer without irrigation to test the systems."

"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 and different soil depths to help guide new designs for Christchurch," O'Sullivan said.

The retrofitting aspect of Bishop's demo was the most interesting, Equus Southern sales and technical consultant Richard Jack said. Equus is in the green-roof business.

The Shac demo was launched during the Festival of Transitional Architecture and got funding from city council. It's also part of a wider Greening the Rubble project on the Cranmer Square section. Other green roofs will arrive soon.

The Press