What to watch out for with green roofs?

I wrote this in 2010 for Building Products News. It was one of the first articles about green roofs in a building publication and the general principles still apply. Although nowhere near the number in Germany, Switzerland, New York, Singapore and Tokyo, green roofs in Australia are now on commercial, educational, and larger-scale residential developments especially ones aiming for sustainability ratings and reputation.  The increasing popularity of vertical walls joins the growth in horizontal greenery.

What to watch out for with green roofs?

By Deborah Singerman

There is more to a green roof than meets the eye. Whether a heavier “intensive” roof, irrigated and with grass, shrubs and plants, or the lighter “extensive” variety, with thinner covering and lower maintenance, the fundamentals are the same – the roofs are a build-up of layers of products and material and installation know-how.

The hardware, the “black arts”, comprises structural support, then the waterproofing membrane, filter fabric protection and the drainage system. This is followed by the software, the “green arts”, the substrate growing media (“more than 300 mm for intensive and less than 300 mm for extensive”, according to Elmich, which designs and manufacturers landscape engineering products) and then the plants themselves.

“You have to understand the physical properties of the media on top before you can fully select the media under it,” says Alistair Scott-Young, director of Junglefy, a company that designs, installs and maintains green roofs and walls.

While the basic principles and build-up is not complicated, there is no room for error, says landscape designer and horticulturalist Sidonie Carpenter, and president of Green Roofs Australia, the collective voice for Australia’s young but growing industry. “In Australia we have very different building conditions and different species,” Carpenter says.

First, you have to work out “why the client wants to install a green roof”, says Jeff Sorrill at the research and demonstration Green Roof Centre in Sheffield (www.thegreenroof centre.co.uk) in the north of England. “It all depends on what they want to achieve.”

The benefits of green roofs include reducing pollution (plants can trap particulates), stormwater retention, alleviating floods, reducing energy use, providing insulation, aesthetics, biodiversity (depending on media depth) and reducing the urban heat island effect. Painting the roof a light colour to reflect heat and keep the building cooler is not enough, especially as it deflects heat back into the city, says Groof Consultants director, Ben Nicholson, who is also vice-president of Green Roofs Australia.

In contrast, green roofs absorb heat from the sun and lead to “evaporative cooling as plants transpire moisture into the atmosphere”, Nicholson wrote in his Churchill Fellowship paper after studying green roofs overseas. Scale is important too. An influential Toronto study found that for a city’s temperature to drop by two degrees you would need about 10 per cent green roof coverage.

Promoting corporate social responsibility is another benefit, though Carpenter warns that while green commercial projects in Australia are growing rapidly there is a danger of using green roofs (and walls) as a marketing tool and of “people wanting to be seen to be green”.

Both she and Nicholson agree on the importance of integrating the supply chain and coordinating professionals and trades on projects, from architects, engineers and waterproofing contractors to green roof installers, horticulturalists, planting labour and maintenance. These are also some of the areas where a lot more research – and work – is needed in Australia before reaching this ideal.

With water for green roofs, a grey water system (carefully monitored to avoid toxic cleaning products, for instance) could become part of the whole, filtering and pumping water back on to the roof. “As soon as you have water in the system you can significantly reduce substrate depth because you don’t need to get the substrate to act as a water holding element,” Nicholson says. “Hence you can start to increase the plant species able to live up there.”

Another barrier is weight. “Most of our roofs are engineered to 25 kg/sq m and even with a simple lightweight green roof you are looking at 100 kg/sq m fully saturated,” Nicholson says.

Building code stipulations, engineering solutions and cost will need to be considered especially for identifying what building stock can be retrofitted. There is no Australian standard for green roofs, an absence especially noted for substrates and plants.

Australian products include Fytogreen’s Hydrocell RG30 lightweight water retentive substrate, but it uses a different soil structure from the normal, Carpenter says. More typical is Elmich’s VersiCell, a lightweight, high strength polypropylene modular drainage cell, or VersiDrain 25P, an interlocking plastic water retention and drainage tray.

Atlantis has come up with Go-Wall vertical garden system modules to be fixed to structural walls. Australia does not produce full modular roof systems, Carpenter says, such as ones like LiveRoof, which promises “an instant green roof which arrives to the job site with full-grown plants”.

Zinco, a long-standing, holistic green roof system, is negotiating with an Australian company for distribution here. The German company is backed by years of project experience, technical engineering expertise, and simulation methods to test parameters such as climate, budget, media depth and water availability.

Junglefy worked with Zinco’s filter fabric and drainage storage layers (only part of the full system) on the Venny children’s community centre in Kensington, Melbourne. This is an extensive roof collaboration between the City of Melbourne (Ralph Webster is senior architect and urban designer) and the University of Melbourne’s School of Land and the Environment (John Rayner is urban horticultural lecturer at the Burnley campus). The project is jointly funded by City of Melbourne and Melbourne Water.

Plant selection at different depths and data collection on water, weather, substrate moisture and so on will be ongoing. Burnley (including with Nicholas Williams) is also looking at rooftop plants in drought which so far suggests only succulents can survive long periods of water stress. Melbourne University and Monash University will also be monitoring stormwater discharges among other things at a civic centre roof garden and Bent Architecture’s green roof at 131 Queen Street, which won a design competition run by the Committee for Melbourne’s Future Focus Group, with a grassy hillock and circular path, is still under construction.

Another lesson, one of several from the Venny project, is that without good coordination contracts may overrun and necessary planting seasons be missed. Scott-Young speaks for many when he says, “Trades need to understand the responsibilities they have to each other.”