Published by City Farmer, Canada's Office of Urban Agriculture


Green Roofs

By Wayne Roberts

The ancient Vikings set the North American standard for energy-efficient, nature-friendly construction some 800 years ago when they built their pioneer shelters in Newfoundland with sod on the roof.

Avant-garde building engineers caught up with the trend this month when an unusual coalition of grassroots environmentalists, high-end roofing companies and top-down government agencies teamed up to deliver the first science-based and comprehensive evaluations of green roofs, and pronounced them a way cool way to save lives, energy, water, money and space.

Though at risk of losing its place as North America's front runner in implementing green roofs, Toronto City Hall offered up the garden plots on its third floor podium roof, built by companies backing the Green Roofs for Healthy Cities coalition, for a case study by the Institute for Research in Construction, a branch of the Ottawa-based National Research Council.

Each of the eight garden plots on the City Hall roof test a different style of construction and maintenance. Some are "extensive" - with shallow beds, low weight, low cost, but on which only hardy alpine plants thrive. Some are "intensive," more costly but featuring lots of soil and plants with deep roots. Some plots are designed to attract birds and butterflies to an area of the city that rarely harbours habitat for wild creatures. Some are just pretty. Two of the plots test whether food and farmers have any place overlooking the economy of the next century's cities.

The new science shows that the profits of bloom, not the prophets of doom, can carry the day when it comes to honouring the Kyoto agreement and bringing in urban changes that reduce air pollution and global warming. According to the computer models generated from the City Hall study, rooftop flower power can save building owners money on energy bills and maintenance while generating a host of free public health benefits to the city, and keeping an estimated 620,000 tonnes of global warming gases out of the picture.

We now know that green roofs reduce energy use, thereby eliminating the fossil fuels and air pollution that come along with energy production, in two distinct ways.

First and most obvious, the soil and plants add a layer of insulation at the top, helping to keep heat in the building during the winter, and keep it out during the summer. Owners of a typical highrise can expect to see energy savings of about 25 per cent during the winter, and of at least that amount for air conditioning during the summer.

Second and more surprising, the green roofs don't just act as passive insulation to regulate the building's temperature. They become a living machine, a breathing and sweating place that air conditions the entire city.

It's this second phenomenon that has scientists shouting from the rooftops. They see green roofs as an oasis countering what's called the Urban Heat Island Effect, the way city construction acts to turn up the heat on sweltering days of summer.

Cities make their own heat in the summer, often cranking up the thermometer by as much as 8 degrees C, by changing the physics of how sunlight works.

When sunlight hits light surfaces, for instance, much of the energy is reflected back into the atmosphere. That's why white walls create bright rooms, why skiers get sunburned from a double hit of the sun's rays, why smart dressers wear light colors in summer, and why all the white buildings in ancient Athens created a city that was cooler than its neighbouring countryside.

Black, by contrast, the same for black jeans and shirts as with pavement or tarred roofs, absorbs the heat from sunlight. By changing the color scheme of life and giving more power to the dark side of environments, construction methods create a hot time in the old town.

The problem is compounded by the fact - again due to physics -- that standard city building materials, such as cement and asphalt, store that heat. That's why summer nights bring none of the relief from the heat they used to bring in the good old days. And - this due to the duhhhh! factor rather than physics - it's the hot air hovering near roofs, which can be as much as 30 degrees C hotter than the day's real temperature, that has to be cooled by air conditioning units commonly placed atop most highrise offices.

Green roofs counteract the Urban Heat Island Effect by doing what humans do when it's hot - sweating, which cools the body by giving over its heat to the process of evaporation. The transformation from liquid to gas takes the heat from beads of sweat on your body so the heat can be carried away by a breeze.

Plants create the same cooling effect when they give up their water, which is why it's so cool and refreshing to step into a forest and escape the heat.

Studying the positive environmental impact of this process, known as evapo-transpiration, is the speciality of Dr. Brad Bass, a researcher with Environment Canada working to prepare cities for the adaptations they will have to make to heat waves in the coming era of global warming.

Just by countering the Heat Island Effect, green roofs can reduce the demand for summer electricity by 5 to 10 per cent, Bass estimates.

Bass emphasizes that this is only one of the ways green roofs cut energy bills and air pollution.

The plants function as air fresheners, taking in carbon dioxide and pumping out oxygen, the perfect partners to humans, who breathe in oxygen and breathe out carbon dioxide.

Plants also work as unpaid air filters which capture free-floating particles that otherwise go into the air soup known as smog. And the higher-up the plants are in the sky, the more likely they are to capture the particles that create smog, Bass argues.

While not a factor in global warming debates, smog is a major risk to health as well as comfort. Toronto public health officials attribute as many as 1000 premature deaths a year to the city's high smog levels.

Before long, green roofs create their own momentum, replacing the vicious circle of urban heat islands to the virtuous circles of urban green oases. By reducing demand for coal-fired electricity to run air conditioners in Toronto, for instance, green roofs reduce the sulphur dioxide that creates acid rain by 5 per cent. "Just on those grounds alone, green roofs pay for themselves," says Bass.

This by no means completes the list of green roof benefits.

They also extend the life of roofs by protecting the roof's covering from the sun's damaging UV rays and by evening out the temperature fluctuations that put building materials through so much wear, tear and stress.

They store rainfall, keeping it out of the sewage system that gets flooded during moderate rains, forcing beaches to be closed during the summer.

They create a peaceful place to hang out, what city planners call amenity space, very scarce in a bustling downtown.

They also create space for food production in the city. The herbs used on some 6000 meals a day served at Toronto's Royal York Hotel come fresh-picked from the hotel roof. In Vancouver, chefs at the Fairmont Waterfront Hotel help pay for the roofs over their heads by harvesting $30,0000 a year worth of food, including original marsh mallows used for exotic treats.

Unfortunately, while Toronto's been quick to prophesy the benefits of green roofs -they've been touted by the city's Environmental Task Force and Food and Hunger Action Committee as well as the new Official Plan -- it's falling behind the leaders of the pack when it comes to implementation.

In Germany, 80 municipalities provide incentives for green roof conversions, with the result that 13 million square meters of green roofs have been built in the last five years. In Tokyo, the city's eagerness to counter the Urban Heat Island Effect led to laws requiring all large buildings to green at least 20 per cent of their roof. In Portland, Oregon, where green roofs are appreciated for their role in storing rain water and keeping the local river salmon-safe, downtown builders are allowed to increase their density if they provide green roofs. Chicago, which had a traumatic encounter with a death-dealing heat wave during the 1990s, is also making green roofs the law.

Toronto has no such laws or deals to offer.

The problem may come from the inability to recognize a rare occurrence in the economic climate.

We're used to companies that create private fortunes by despoiling public goods in the environment, by polluting waterways with manure, for example.

But we're not tooled up for companies that can create public benefits, such as clear air or clean beaches or butterfly hideaways, through their private expenditure.

The business coalition behind Green Roofs for Healthy Cities stresses the public benefits because they hope to see incentives to help building owners weather the shot-term costs of building or renovating their roofs to bear the extra load of weight from soil and water, and to keep the soil and water from leaking into the building.

At stake is the implementation lead in a multi-billion dollar roofing industry that Toronto's made-for-creatives social climate provided the connections for.

"It's no longer a question of if," says Steve Peck, a former leader of citizen efforts to clean up the Don before becoming director of the business coalition "It's just a question of which city moves first and captures all the benefits of innovation. There are places that could snap this up in a New York minute."

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Revised Thursday, January 23, 2003

Published by City Farmer
Canada's Office of Urban Agriculture