Waitakere's indigenous green roof - Rooflink Magazine

Plans for Waitakere City Council’s new civic centre were well advanced when it was decided the eco city would develop a vegetative green roof on a flat section of the $38.5 million development in Henderson.

The roof is believed to be the first extensive green roof in New Zealand featuring indigenous plants and covers an area of 500 square metres.

With no existing commercial sites in New Zealand with green roofs, the Waitakere project could pave the way for wider application of roof systems that lower the impact of stormwater run off, increase the energy efficiency of buildings and contribute to native bio diversity in urban environments.

Renee Lambert, Service Manager, Parks Planning for Waitakere City Council says when they embarked on this project there were no credible guidelines or information to govern the design and application of a green roof system in New Zealand.

Green roofs have been used in Germany and Norway for many years and are increasingly being installed in the US and England, but little is known about their performance on commercial roofs in New Zealand.

The light weight of the Waitakere green roof, which features coastal and dry river bed locations, is quite unlike the conventional roof gardens which are generally heavier and require both irrigation systems and regular maintenance.



Waterproofing
RANZ member Nuplex Industries Ltd undertook the waterproofing and drainage systems, installing a torch-on membrane to the concrete podium deck, the concrete having been insulated with extruded polystyrene.

Nuplex supplied the Suprema Eco Garden System, manufactured in France, incorporating layers of Soprema Flam 180 and Soprema Jardin anti root torch on membranes. Following installation of Nuplex roof vents, the roof was firstly coated with Nuplex membrane primer prior to application of Soprema Flam 180 over all external edges and internal corners and around all penetrations and drains. This was followed by another layer of Soprema Flam 180 with 80mm wide side laps and 150mm wide end laps and finally a layer of Soprema Jardine over the entire surface including flashings, upstands and other detail work. Over the entire membrane surface, an Atlantis recycled plastic drainage cell layer has been installed followed by a Geotextile cloth, protecting the membrane from any root damage and allowing drainage to the roof outlets.

Substrate
Weight being a critical element of the green roof concept, much research into the most suitable substrate has taken place, including field trials on a range of mixes – pumice, vermiculite and composts – on the Waitakere roof. Mixes were required to weigh less than 230kg per square metre at a depth of 150mm to meet the structure loading requirements of the building, and allowing for plants and other materials and maintenance staff accessing the roof. Wind resistant substrates that avoid windborne debris were also a consideration. The final mix selected from these trials was expanded clay, pumice and garden mix.

Waitakere's green roof is expected to reduce the total roof run-off, with the run-off from the installed green roof to be intensively measured to evaluate quantity and quality of the stormwater and other data to demonstrate how the roof performs and the extent to which the plants are able to reduce moisture use and loss due to the spong like impact of the soil substrate selected.

Roof water is pumped to a roof tank to be used for flushing toilets in the buildings while rain from the civic square surface is being collected in water features and pumped back up to a main tank for reuse in these features. The main water supply for the water features come from the roof of the Civic Centre building. There are a series of rain gardens throughout the site collecting rainwater, with overflow water being collected in a main drain leading to a sand filter.

Plants
The ten species of native planted on the roof were trialled over four months starting last November by scientists and ecologists from Landcare Research, the criteria being that plants needed to be drought and wind tolerant and with shallow root systems. The growth rates and health of the plants was monitored until February, when the trial was removed to allow flood testing of the roof membranes.

The most successful of the plants undergoing trials were the New Zealand ice plant, iris and sand-dune coprosma, all of these included in the mix of plants being trialled from the planting, which took place in June. There are 25 plants in every 2 square metres on the roof to provide a variety of heights (50 – 100mm), textures and colours on a background of red clay and pumice. The roof will only be watered in exceptionally dry conditions over the three summers the plants are being trialled. The green roof is expected to form a dense, weed resistant cover which will provide habitat or food in the form of nectar or fruit for native insects and birds.

Public viewing
Although most green roofs are not seen by the general public. Waitakere has provided a viewing room of its newly planted roof so members of the public, on request, will be able to view the seasonal changes that will take place.

Environmentally conscience Waitakere City Council have taken a unique approach, the green roof expected to reduce the total roof run-off during storms by 50 per cent and clean the water run off from the roof.

The plants
New Zealand native plants trialled on Waitakere's green roof are:

• Liberita peregrinans (NZ iris)
• Festuca coxii (native tussock)
• Acaena microphylla (NZ bidibid)
• Pimelea prostrate (NZ daphne)
• Selliera radicans
• Disphyma astrale (NZ ice plant)
• Coprosma acerosa (sand coprosma)
• Leptostigma stulosa
• Dichondra repens "piha" (Mercury bay weed)
• Calystegia soldanella (sand convolvulus)
• Muehlenbeckia complexa
• Muehlenbeckia axillaries
• Muehlenbeckia ephendroides

Article kindly supplied by Rooflink Magazine and the Roofing Association of New Zealand (RANZ)