Green roofs in Nairobi save energy, water

These gardens in the sky sometimes boast trees, as well as grass and other plants, which cool the building while sucking up carbon-dioxide pollution.

Kenyan architects are designing buildings with green roofs covered in vegetation to cool their interiors, conserve energy and water, and help curb greenhouse-gas emissions.

The capital, Nairobi, is experiencing growth in green-roofed construction, according to architects who specialize in the climate-friendly technology.

Some of these gardens in the sky – which require a flat roof and replace the vegetation destroyed when ground is cleared for construction – boast trees, as well as grass and other plants.

With urban trees and nearby forests being cut down for firewood and new development, green roofs are increasingly seen as a means of restoring city environments, while their plants suck up planet-warming carbon dioxide.

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The East African Coca Cola Company, headquarters on the outskirts of downtown Nairobi, has led the way. Completed in 2008, and located in a smart suburb hosting several embassies, it is one of the most expensive buildings in Africa.

Its green roof garden serves as a recreation area for employees and ensures there is minimal heat gain through the roof, according to Triad Architects, which worked on the project.

"The green roof garden keeps the building cool,” said Bob Okello, communications manager at Coca Cola in Nairobi.

Green roofs have a modest cooling effect on building interiors, which cuts energy consumption by air conditioning, experts say.

They can also recycle water used inside the building, if mini water-treatment plants are installed on the roofs, according to architectural consultant Francis Gichuhi.

“Water from showers, sinks, and baths is treated and re-used within the building. Green roofs can save on water bills by up to 30 percent,” he said.

The roofs also absorb up to 95 percent of rain that falls on them, which provides instant irrigation and reduces storm water runoff. And they create an unusual but welcome habitat for animals and birds, Gichuhi added.

At the Coca Cola building, waste water is recycled and distributed to various parts of the building from tanks on the roof and below ground, including the gym and the toilets, Okello said.

“The green roof has helped the company reduce on costs associated with water as well as electricity, because we do not need constant air-conditioning in hot weather," he said.

A well-designed green roof costs between $15 and $20 per square foot (0.093 square meters), but investing in one can increase a building’s value by up to 25 percent, Gichuhi said.

Green roofs also tend to last longer, because they protect roof membranes from ultraviolet radiation and temperature extremes, according to researchers at Michigan State University.

Germany is widely regarded as the world leader in green roof research, technology, and usage, with the sector growing 10 to 15 percent per year in the European nation, the university’s website says.

In North America, the green roof industry grew by 28.5 percent in 2010, up from 16 percent in 2009, according to an annual survey.

The global market is expected to continue expanding, thanks to consumer interest in adopting green practices and pressure to minimize energy consumption by buildings in an effort to curb carbon emissions.

Hopes are also high for the green construction technique in some developing markets like Kenya, where government officials are trying to push it up the political agenda.

A draft law on green roofing has been forwarded to Kenya’s parliament by the environment ministry, and is set to be debated in the current session, which ends in May or June. It proposes that all large new urban buildings, such as apartment blocks, should have green roofs.

There are several other major construction projects in the pipeline that also feature green roofing. They include Tattoo City, a housing and commercial building scheme that will create a whole new Nairobi suburb, offering housing, hospitals, hotels, and restaurants – many of them sporting rooftop vegetation.

Another new urban settlement due to be built in the coming years is Konza City, which will be located near Machakos, a small town around 45 minutes drive from Nairobi.

Land has already been allocated for the construction of green-roofed residential, office, and commercial buildings, in what is designed to become a “silicon savanna” once completed in 2020.

Wealthy individuals are following the trend too. John Mutisya, a Nairobi businessman, is halfway through building his own green-roofed house.

“I love the beauty but also the comfort that comes with such a house,” he said. “And I know one spends less on electricity and air-conditioning to keep it cool during humid months.”

• Gitonga Njeru is a science journalist based in Nairobi. This article originally appeared at AlertNet, the Thomson Reuters Foundation humanitarian news service.

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