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Green Roofs

Urban Vitality: GREENROOFS

The urban landscape and human development in general are growing at a rapid pace. Already, much of the planet has been developed: farms, roads, dams, towns, sprawls, airports, mines, factories, and more carpet the landscape. The impact of modern human existence on our surroundings is vast.

Let’s not be deceived by the demarcation of visible constructed boundaries. We exert an influence that outstrips this most glaring evidence. Cities in particular have a wide zone of impact. Waste is shipped to a distant out of sight valley, waterways dilute and transport toxins, forests are logged for local construction, smog blankets the region, surrounding wildlife (bears come to mind) is pacified for human safety reasons, forests (commonly designated as parks) receive numerous visitors.

Cities are people magnets. Cultural events, education centres, and jobs draw us to them. A city is a dense and vibrant matrix of human potential. Creativity, imagination, and insight thunder throughout. The urban environment tends to inspire, but that same environment can drain us and harm us as well.

According to the United Nations, over 50% of the world’s population lives in urban centres. That number is ever growing. Meanwhile, the concentration of urbanites is much greater in developed countries.

It’s only common sense to keep your house in good order: to attract, to prosper, to educate, to strengthen, develop, evolve, to keep healthy. The city, those of us who live in one, and those who are affected by one’s decision, are owed nothing short of the best their environment can provide.

So, are we confident that we’re giving it our best: that our cities are whirlwinds of health and potentiality?

Let’s first focus on that which draws many to an urban life: potentiality. This can, among other things, be translated into productivity, business and wealth, creativity and art. It stands to reason that we would strive to maximise this effect. Unsurprisingly, there exist several means to this end, one of which can be termed greenroof technology.

What is a greenroof?

Simply put, a greenroof is a garden integrated into the roof structure of a building. It does not just sit atop the roof but is a part of it. A greenroof, from the bottom up, is composed of a waterproof membrane, a root barrier, insulation (optional), a drainage layer, filter fabric for fine soils, the soil substrate, and plant material. Furthermore, there are two types of green roofs: extensive and intensive.

An extensive green roof is relatively low-impact and lightweight as it contains plants of a smaller nature, requiring a soil substrate that is not as deep. The height of all the components is 5-15cm. Grasses, moss, and sedums are examples of the plant material. Generally, the plants are hardy and self-seeding in order to minimise or eliminate maintenance.

An intensive green roof is high impact and heavy-load. The plant matter on it is much larger: trees are present. The height of all components is 36 20-60cm. Due to the added weight of a roof of this type, additional structural support may be needed.

Vitality

Now that we have a basic understanding of green roofs, we can return to the idea of potentiality. As humans, we have spent the greater part of our specie’s evolutionary track in the presence of nature. The lack of greenspace coupled with the tendency of monotones and basic geometric shapes can cause stress. Numerous psychological studies support the use of greenspace to increase individuals’ quality of life.

Simply imagine staring out of one window and gazing over a grey road hugged by grey sidewalks on which are grey concrete buildings topped by flat gravel sprinkled roofs. Now imagine looking out another window and viewing an urban landscape marked by green grasses, vibrant flowers, highlighted trees, swaying plants, flittering birds, along with the heavy electric construct of a prosperous city. Which would you prefer looking out on from your 12th floor apartment? Which would you prefer to see as a patient in a hospital? Which would flare into fancy and fantasy as a child? Which would inspire and enrich?

Not only is the injection of nature in a hive of focused human energy therapeutic, but it also contributes to the overall pleasure and wealth of a city. It can affect the output and productivity of individuals, and it certainly raises property values. In a slice of land where property can be quite expensive, the roof is mostly unused real-estate that is open to development. The greenroof can be a solution to community gardens (you only need 400mm of soil to grow vegetables), and can act as micro-habitats.

Insulation

It’s time to sink our teeth into more substantiative morsels of news and information. Daytime summer temperatures for a commercial rock/membrane coating roof generally range from 49° C to 52° C in North America and Europe, though temperatures as high as 80° C have been recorded in Europe. A 1998 NASA study found the following average results in the month of May in Atlanta, USA: tree-shaded grass 28° C, tree canopy 21° C, asphalt 50° C. The temperatures were taken during full afternoon sunlight. Recent soil substrate temperature readings taken from experimental greenroofs in Toronto have averaged at 25° C.

This rooftop temperature difference of roughly 25° C has a noticeable effect on the energy requirements of cooling a given building. Every 3-7° C of ambient air temperature affects the resource requirement by 10%. Air conditioners are not cheap. In the far from tropical city of Toronto, Canada, summer air conditioning uses more energy than winter heating. In the summer of 2002, the city failed to meet its own energy requirements and so had to import electricity at much higher cost. Air conditioners were blamed and city officials pleaded for consumer restraint. Of course, simply asking citizens to forgo cool air and accept the heat had little effect. Greenroofs, meanwhile, could be used to decrease a building’s energy expenditure during hot days, resulting in savings for consumers as well decreasing or eliminating the risk of brownouts.

Western Design Consultants estimates that widespread use of greenroofs in Chicago would save 720 megawatts in peak demand. This is equivalent to a small nuclear power plant or several coal-fire generators.

Urban Heat Island

If enough greenroofs are present, the cooling effect will extend to the city’s ambient air temperature. Concrete and asphalt absorb solar energy and later release it as thermal infrared radiation, heating their surroundings. NASA satellite studies displayed significantly higher temperature concentrations in urban areas: a phenomenon termed urban heat island effect.

A rise in air temperature can adversely affect air quality. The urban heat island effect increases the frequency of smog or poor air quality days, limiting the city dwellers as well as posing a serious health risk. It is proven that microclimates are influenced by the quantity and quality of greenspace. In this case, greenroofs could be used to counter the urban heat island effect and thus decrease the number of smog days. Simply put, warm air over greenroofs tends to rise, cooling the area.

Filtration

The presence of pollutants in the air threatens us as we breathe in our favoured urban landscape. The pollutants are often trapped in air pockets throughout the vertical concrete, steel, and stone of our constructed environment, and there accumulate. Here, again, greenroofs can help.

Particles are captured by plant matter: leaves, blades of grass. Gaseous pollutants, such as carbon dioxide, are absorbed during photosynthesis. Tree-lined streets were found to have 10-15% the total dust particles in other similar streets. In addition, studies found that even a single fig tree can purify 10m3 of air per day.

Stormwater Management

Cities tend to have another environmental problem: rain. Due to the common use of impervious material during construction, water is not absorbed but rather washed along this surface down into the sewers, where it is hopefully treated before being flushed out to larger bodies of water. This rainwater picks up a good deal of pollutants from the atmosphere, and the surface of the urban environment before even reaching the sewers. The cost of completing this cycle of purification is nothing to balk at.

During major rainstorms, the sewers tend to reach capacity and fail unless we release excess untreated water in order for there not to be an overload. Despite the release of untreated waters, which is a health hazard, the costs of running the operation are significant during these periods of natural stress.

Meteorologists have observed a pattern of relatively concentrated thunderstorms over cities. It is believed that the higher urban summer temperatures increase instability in the atmosphere, encouraging rainstorms and thunderstorms. Cologne, for example, has 27% more rainfall than its immediate surroundings.

Greenroofs can compensate for the loss of absorbent surface resulting from construction using impervious material. Numerous studies conclude that the average absorption of greenroofs is 50-60%. This works out to be a significant decrease in runoff, saving the city a fair deal and aiding increase the stormwater management capacity.

Hopscotch Habitat

Habitat loss is another negative side-effect of our urban expansion. Greenroofs can minimise this form of damage, though they will in no way reverse it. A comprehensive network of greenroofs can be used as habitat corridors to link ground level nature havens such as parks. Even sporadic implementation will result in island habitats.

This will especially benefit insect and bird life, also aiding those species of flora that depend upon them. It’s been found that butterflies will visit a garden up to 20 stories high while birds will visit one up to 19 stories high.

Implementation

A 2002 opinion poll of the European Economic Community, published in La Libre Belgique, named the environment as people’s number one concern. This may explain the success of greenroofs in Europe, especially in Germany. A 1996 survey of 80 German cities found that half of them offered incentives for greenroofs. 13 of the cities charge only 50-80% of the stormwater management taxes for those buildings that have greenroofs. 29 cities had direct subsidies, and 27 of those in the survey had zone districts that demand greenroofs on all flat roofs. By 1996, Germany had 10 Million m2 of greenroofs.

Chicago has recently implemented an ordinance requiring minimum levels of reflectivity on certain buildings in order to combat the urban heat island effect. Greenroofs have been included in this reflectivity ordinance. The city of Tokyo has pressed its own case. Starting April 2001, any new building over 1000m2 must green at least 20% of its useable roof space. Other cities, such as Toronto and Kobe, are looking into similar measures and incentives.

Think of this as simply a summary of some of the benefits of greenroofs. Sound dampening, and winter insulation properties, to name a couple, are others. Greenroofs can more than pay for themselves, and go a long way to vitalise and enliven our homes.

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Categories: Editorial, Environment
  1. deepali
    March 19, 2007 at 2:30 pm

    Thanks for the primer!

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