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Sun 11 Jan 2009 04:00 AM

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Sky gardens

Green roofs may currently be something of a rarity in the UAE, but following Dubai Municipality's recent announcement backing their promotion in Dubai, signs are they are set to become much more common. Commercial Outdoor Design investigates.

Green roofs may currently be something of a rarity in the UAE, but following Dubai Municipality's recent announcement backing their promotion in Dubai, signs are they are set to become much more common. Commercial Outdoor Design investigates.

The American Society of Landscape Architects has one on its Washington headquarters, Dubai Municipality is trialling one on its buildings and London mayor Boris Johnson is calling for them to be mandatory in certain areas of the UK capital.

World over, green roofs are the latest trend in outdoor design, and they look set to be big news in the Middle East.

The origins of green roofs date back thousands of years to the fabled Hanging Gardens of Babylon, considered one of the Seven Wonders of the World, where they were built by Nebuchadnezzar II reportedly to please his wife who missed the trees and plants of her homeland Persia.

Today, green roofs are commonly seen in parts of Europe, particularly Germany where an estimated 10% of all roofs are greened, and are also increasing in popularity in the US.

In the Middle East, however, they are still a relatively new concept, although this looks set to change following Dubai Municipality's announcement last month that it was beginning a new initiative promoting the installation of green roofs across the emirate.

The Municipality said it was trialling a green roof on its own premises in Dubai, and that it would push for green roofs to be applied on the roofs of all buildings.

In the long term, there is the possibility of green roofs becoming mandatory in the region, a representative of Dubai Municipality told Commercial Outdoor Design. "For the time being, it is a recommendation. It will be tested and if it is approved, it will be transformed to a regulation," he said.

And it's not just in Dubai that green roofs are gaining favour. Abu Dhabi's Urban Planning Council is also including a recommendation for green roofs in its new community guidelines under its Estidama sustainability programme.

Benefits

The benefits of green roofs are numerous. Perhaps the most prized advantage is the reduction of the heat island effect. While traditional building materials absorb the radiation from the sun and then re-emit the heat, green roofs cool temperatures. It is estimated that the under construction 10,000m2 green roof on top of the California Academy of Sciences in San Francisco, for example, will be 40 degrees Fahrenheit cooler than a standard roof.

Johnson, who recently called for more green roofs in London, described green roofs as ‘the simplest method of managing the urban heat island [effect]."

Other advantages of green roofs include reduced sewer cost absorbing rain water - admittedly less of a problem in a region where rain fall is infrequent - and creation of extra leisure space, particularly valuable on residential tower blocks.

Green roofs are also valued from a sustainability standpoint, earning points under the (US) LEED sustainable rating system in categories including reduced site disturbance, water efficient landscaping, and storm water management.

Regional roofs

Given the various perceived benefits, it is surprising then that more green roofs are not already in existence in the region. People often like the idea of green roofs but are reluctant to go ahead with them, comments Karl-Heinz Braun, export manager of ZinCo, a designer and manufacturer of green roofs. "Many projects are planned with a green roof, but it was later cancelled," he says."The green roof is one of the last items on a building, and if the construction has taken the entire budget, it is very easy to cancel or postpone the green roof."

Braun believes there is no reason why green roofs shouldn't work in the Middle East despite the arid climate. "Yes, they can work in the Middle East...provided that there is a technically perfect green roof build up," he says.

"The Middle East as a mostly arid zone does not have many green areas. Green roofs can help a lot to improve and develop more green spaces and reduce the loss of water into the subsoil."

Green roofs typically consist of a series of layers and are generally divided into one of three types - an extensive green roof, semi-intensive green roof, and intensive green roof.

The extensive green roof which is typically covered in low growing vegetation is the lightest form of roof, according to information supplied by ZinCo, and the intensive green roof the heaviest with the semi-intensive green roof somewhere in between. The intensive green roof is typically covered with large plants, shrubs and trees and may include elements intended for human activity such as walkways, benches, and fountains etc.

Considerations

There are, of course, a number of considerations that need to be taken into account in the planning and building of green roofs in the Middle East. A major one is irrigation, says Steven Peck, founder and president of US green roof infrastructure and industry association Green Roofs for Healthy Cities.

"A key design decision on green roof planning is whether or not to include a high efficient irrigation system or not," he says. "If not, the plant selection would be significantly constrained and a good place to start is with native plants that have adapted to local conditions."

Technical informationHow to build green roofs

The specifications for a landscaped roof's construction are manifold. Optimal growth conditions must be ensured for the plants selected and protection of the roof's waterproofing, and the drainage system must be guaranteed to last for decades.

At the same time, the building structure often has a limit on the weight of the installation. For this reason, most green roofs imitate the natural ground and use functional layers, each with an optimised light weight.

A green roof typically has five layers. Starting from the ground up, the first level is the root barrier, a layer that prevents roots from damaging the water proofing, unless a root resistant barrier is used.

The next layer is the moisture retention/protection mat; this provides mechanical protection of the waterproofing and retains moisture and nutrients. Next is the drainage layer, through which excess water is drained off. This is followed by the filter sheet, which prevents fine particles from being washed out of the substrate soil. The top layer is the vegetation layer, a roof planting soil that is adapted to the plant level.

Source:Zinco

"If you irrigate, then the plant selection is of course much broader. The ability of plants to survive also depends on the depth of the growing media which in turn relies on their being sufficient loading capacity in the structure.

Generally speaking, intensive green roofs, those with more than six inches of growing media are almost always irrigated and always accessible to the building occupants. Extensive green roofs with less than six inches are often not accessible, nor are they irrigated."

In a statement on its green roofs initiative, Dubai Municipality suggested that irrigation for plants on green roofs in Dubai could be obtained from water dropped out of air conditioners.

Another consideration is choosing appropriate plant material for green roofs. Rafi Ohanian, general manager of plant supplier Exotica Emirates, which provides green walls, comments that anything with evasive roots needs to be avoided and that the most suitable plants for green roofs are ones that are heat tolerant and draught tolerant.

Dubai Municipality said that it had collaborated with the Public Parks and Horticulture Department in establishing a suitable plant palette.

For the time being, green roofs are still a novelty in the region and it is likely to be their visual appeal rather than their functional benefits that drives their growth. But with the region fast expanding, and the corresponding heat levels rapidly rising, it may not be too long before green roofs become more of a permanent fixture in the UAE.

Case studyLocated in Chicago, US, the green roof on Chicago City Hall is one of the most famous green roofs in the US. David Yocca, director of landscape architecture and planning of the design firm behind the project, Conservation Design Forum (CDF), gives COD an insight into its creation.

"CDF led the design process to convert the Chicago City Hall rooftop into a demonstration rooftop garden employing the latest green roof technology. CDF's scope included layout, specification of the design of the green roof system, grading and drainage design, and plant selection.

Over 100 species of native, cultivated and non-native plants were specified initially to help better understand the application of green roof technology in Chicago's climate. The design has a strong focus on sustainable practices including storm water attenuation and water conservation, and includes rainwater harvesting and recycling for irrigation."

"The project also has an energy focus and was funded in part through a national Urban Heat Island Initiative sponsored by the US. The roof surface has been measured to be over 50 degrees cooled on a hot summer day, and the City has realized over US$60,000 savings per year in energy costs."

"Visible from 36-surrounding structures, the garden provides welcome relief and greening in a dense urban setting, and has become one of the most visible and celebrated rooftops in the city.

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