By Liz Morgan

First of a two part article on green roofs …

it makes good economic and environmental sense to examine more cost-effective and environmentally sustainable practices for keeping our cities’ atmospheres healthy and livable

FAVOURITES – 21 April 2009 – “The city is a granite garden, composed of many smaller gardens, set in a garden world….The city is part of nature.”

Award-winning American landscape architect, Anne Whishton Spirn

From a bird’s eye view, Australian cities do indeed look like Spirn’s “granite garden”, albeit interspersed with the shimmering blue of harbour or river waters, and the glaucous greenery of native vegetation dotted between suburban conurbations. Weaving like warp and weft are the thick arteries of multi-lane motorways and railway lines, essential threads of the fabric of our highly urbanised landscape.

With state and federal governments’ planning policies focusing on urban consolidation, and higher-density residential living in particular, Australia’s “granite gardens” are destined to become greyer, denser, hotter if planners, architects and developers carry on with a “business as usual” attitude. Coupled with predicted rises in average temperatures due to a combination of climate change and urban consolidation, the quality of our metropolitan environments will deteriorate further and the cost of maintaining acceptable temperatures in our living and working environments will soar.

The “solution” to changing temperatures to date has been to crank up the air-conditioning (or central heating) when the weather becomes unbearably hot (or cold), but with the advent of higher energy costs, depleting fossil fuel reserves and some form of mandatory carbon credit system or tax on the horizon, it makes good economic and environmental sense to examine more cost-effective and environmentally sustainable practices for keeping our cities’ atmospheres healthy and livable. This is before we even consider the economic, social and environmental benefits that will flow from new, sustainable practices.

This 1990s rooftop garden on 5th Avenue in New York was focused more on design than sustainability. These days the dark gravel and metal edging would most likely be replaced with less heat absorbent materials. Photo: Lynne Blundell

A “granite garden” city is, in essence, an array of dark, heat absorbing surfaces: roads, footpaths, bridges, buildings, car parks, industrial parks and roofs. We need only a few days of the searing temperatures that Adelaide and Melbourne experienced in January-February 2009 to witness what effect rising city temperatures can have on infrastructure and services; on productivity; on human health and well being: electricity blackouts and transport chaos; millions of dollars in lost productivity due to employees’ inability to get to work, or through heat-related illness; economic loss as people stay at home in cool air-conditioning rather than going out for entertainment and shopping.

One practical solution, and a relatively cheap and resource-friendly one, for lowering temperatures in built-up environments is to ‘green’ roof spaces. The concept of cooling roof and wall gardens is not new (the Hanging Gardens of Babylon kept its royalty cool in the Iraqi desert as long ago as 650BC). Neither is it rocket science, but its uptake globally has been exceedingly low, with the notable exception of Germany, where green roofs have been championed by state authorities since World War II. By 2002, one in 10 flat-roofed buildings in Germany was greened, thanks, in large part, to mandatory targets for green roofs on new developments.

In the US, Chicago has been an enthusiastic proponent of the green roof concept and not because the city’s lawmakers are “greener” than other cities’. In 1995 as many as 600 Chicago citizens died during a shocking heat wave that caused a blackout for two days. (It is worth mentioning that the temperature in the Chicago heat wave hit 41C – although readings of 48C and 52C were recorded at the city’s airports. This is a temperature reached in most Australian capital cities on at least one day of every year, if not many more than that).

Soon afterwards Chicago’s city hall roof was chosen as a pilot project for greening the city’s rooftops to mitigate the so-called “urban heat island” effect. This is when metropolitan areas become significantly warmer than their surrounding rural areas, in large part because of the thermal properties of materials like black tar, concrete and asphalt, coupled with the so-called heat-trapping “canyon effect” of high-rise buildings clustered in close proximity. The roof was completed in 2001 and tests have shown that the average temperature on the city hall green roof is 8°C cooler than on the adjacent administration building’s tar roof. Chicago now has more than 250 green roofs, stimulated by a municipal scheme that matches each building owner’s green rooftop investments up to $US100,000.

In London, a densely packed and growing city, the mayor’s Design for London team has initiated a Living Roofs scheme as part of its sustainability plan to make the city a more livable place, especially as its rising population exerts more strain on existing resources. The team’s director, Peter Bishop, said: “In increased high density living and with the growing environmental considerations we need to explore innovative solutions. Living Roofs looks to London’s most under-used asset – the space above our heads – to provide one such answer.”

“The mopoke,” Liz Morgan tells us,” also known as the Southern Boobook, is Australia’s most common owl; a lovely little creature that still thinks our cities and towns are worthwhile places in which to live, and a fantastic indicator that these urban environments can still provide adequate food and shelter for a fairly high-order predator.”