Green roofs: what are they and how did they start?

Green roof

When we follow the daily news, we hear news about the construction of new bridges and new urban communities, all at the expense of green spaces. This, of course, is a big problem for a densely populated city like Cairo. With the increase in the urban area, the vision becomes clearer regarding the fact that buildings change the ecological reality, and it seems that the only solution is to work on the external surfaces of the buildings. One of those solutions is beside green walls, green roofs, or rooftop gardens.


What are rooftop gardens and when did they start?

Rooftop gardens, the old version of green roofs known in our modern world, have ancient roots.

The oldest roof gardens known to the ancient world were the Hanging Gardens of Semiramis, which are now in Syria,

considered one of the Seven Wonders of the World. In today’s world, green roof projects are designed for high-end hotels,

business centers and homes. These green roofs, known for their deep pedestals and variety of plantings as old roof gardens,

have the appearance of traditional gardens at ground level. Rooftop gardens usually require significant plant care costs.

Furthermore, they generally contain very shallow soils.

On the contrary, green roofs require less maintenance and effort,

and have an infrastructure to meet different climatic conditions to manage them sustainably.

Green roofs began to appear in their modern form at the beginning of the twentieth century in Germany,

where vegetation was installed on the roofs to mitigate the harmful effects of solar radiation on the roof structure. Green roof technology has spread rapidly due to its wide-ranging environmental benefits.


Environmental benefits of green roofs

The benefits of green roofs fall into three main categories: dealing with rainwater, conserving energy, maintaining green spaces, and tackling air pollution.

1- Dealing with rainwater

Urban areas are dominated by hard, non-porous surfaces that contribute to infrastructure infiltration of rainwater, which can overburden stormwater management facilities and cause sewage to overflow into lakes and rivers. In addition to exacerbating flooding, erosion and sedimentation, urban runoff is also high in pollutants such as petroleum residues, which harm wildlife habitats and pollute the drinking supply.

Traditional rainwater management methods include storage tanks, ponds, built wetlands, and sand filters; However, these surface-area-intensive technologies may be difficult to implement in dense urban centres. Therefore, green roofs are ideal for urban rainwater management because they take advantage of the surface area and prevent water run-off before it leaves the area.

According to a research paper published in 2005, green roofs can reduce the total annual runoff of buildings by as much as 60% to 79%, and estimates based on 10% of green roof coverage suggest that they can reduce the overall runoff over a period. The whole territory level is about 2.7%.


2- Extend the life of the surface

Waterproofing films on conventional roofs deteriorate rapidly in UV light, resulting in membrane brittleness. Thus, these films are more easily damaged due to expansion and contraction caused by fluctuating roof temperatures over the long term. But by physically protecting against UV rays and reducing temperature fluctuations, green roofs extend the life of the roof’s waterproofing membrane and improve the building’s energy conservation potential. Stabilizing the temperature of waterproofing membranes by covering green roofs can extend their useful life by more than 20 years; Also in a study by Erich Burch and Manfred Koehler in 2003, they discovered that some green roofs had persisted in Berlin for 90 years without the need for repairs.

In Ottawa, Canada, another study showed that the unplanted roof reached temperatures above 70°C (°C) in summer, while only the green roof surface temperature reached 30°C.


3- Cooling the building heat in the summer

During warm weather, green roofs reduce the amount of heat that passes through the roof, thus reducing the energy requirements of the building’s cooling system. A study conducted in Madrid showed that a green roof reduced the cooling load in an eight-storey apartment building by 6% during the summer. The cooling load has also been reduced by 10% for the entire building and by 25%, 9%, 2% and 1% for the four floors just below the green roof.

Green roofs will have the greatest impact on the energy consumption of buildings with relatively high roof-to-wall area ratios.

In summer, green roofs reduce heat flow through the roof by enhancing evaporation, effectively shading the roof, and increasing insulation and thermal mass.


4- Maintaining green spaces and a comfortable aesthetic view for the eye

When humans look at green plants and landscapes, they have beneficial health effects, such as reducing stress, lowering blood pressure, relieving muscle spasms, and increasing positive emotions. And of course these benefits ultimately lead to improved health and worker productivity. Employees who had views of nature, such as trees and flowers, were less stressed, had greater job satisfaction, and reported fewer headaches and other illnesses than those without exposure. Also, patients experience a faster recovery from surgery when they are positioned in front of a landscape. Not only are the green surroundings aesthetically pleasing, but landlords can often increase rental rates and hotels can charge more for a “room with a view” than for other arid landscapes.


5- Combating air pollution

Plants can filter out particles and pollutants in the air. Eventually the particles will be washed away in the soil by the movement of rainwater, and some of the pollutants will be absorbed into the plant tissues. Yuk Tan and Sia in their paper published in 2005 found 37% and 21% reductions in sulfur dioxide and nitrous acid, respectively, over a newly installed green surface. Others have estimated that green roofs can remove 0.2 kg of dust particles per year per square meter of a lawn roof.

The adverse health effects of particulate air pollution include increased respiratory problems, decreased lung function, and increased hospitalization and other healthcare visits for respiratory and cardiovascular disease. Increased rates of respiratory morbidity, as measured by absenteeism from work, school, or other restrictions on activity, and increased mortality from heart and lung disease are also caused by air pollution.

For a city like Cairo, Cairo is the most polluted city in the world, according to Eco Experts report. The World Health Organization’s Global Air Quality Database, which focuses on air quality metrics, ranks Cairo second for PM10, or particulates with a diameter of 10 micrometers or less. When judged by PM2.5, or particles with a diameter of 2.5 micrometers or less. Of course Cairo isn’t too bad for air pollution, but it’s pretty close to the bottom.

6- Noise Reduction

Hard roofs in urban areas are more likely to reflect sound, while green roofs absorb sound waves due to the nature of vegetation. At Frankfurt Airport in Germany, for example, Dunnett and Kingsbury show in their paper that a 10-cm-deep green roof reduced noise levels by 5 decibels. Other research shows that 12 cm of green roof substrate alone can reduce noise by 40 decibels.



The benefits of green roofs are not limited to environmental benefits only, but also have economic benefits represented in reducing the level of energy and raising the market value of properties overlooking green views. But here we are about to add a new reality to Arab cities, especially Cairo, which suffers from high levels of noise and air pollution. Are you ready to work with us?

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