Urban environmental benefits of green roofs

Different locations of a city experience varying degrees of heat on account of segregated urban development. The temperature of urban areas is likely to be higher than rural and suburban areas. In urban areas, heat waves get exacerbated by urban heat-island effect. This has the potential to negatively influence health and well-being of urban residents. Due to rapid urbanisation the impact of weather on human health has become an issue, particularly in the context of global warming.

Urban heat island means any urban area which is significantly hotter than the areas next to it. Such islands are highly noticeable during the winter and summer and the temperature diversity is often greater at night than during day. Increased temperatures during summer in cities intensify energy demand for air-conditioning. Research finds that electricity demand for air-conditioning increases in the ranges of 1.5 to 2.0 per cent for every 0.6°C increase in air temperature, which accounts for about 5.0 to 10 per cent more electricity. The increased demand for air condition during summer also increases the energy bills of households contributing to more load-shedding in the city. Urban heat island also leads to the formation of ground level ozone and acid rain. Sensitive and vulnerable people feel discomfort, fatigue, respiratory problems, headaches resulting in heat stroke, heat cramps and heat-related mortality. Even runoff water into the drainage system gets heated as it is discharged into ponds, streams, rivers, lakes and oceans, which results in thermal pollution and the increased water temperature affects the aquatic system - the reproduction and metabolism of aquatic species and can be even fatal to aquatic life.

Though urban heat island has many adverse effects, there are also some solutions to the problem. By using light-colored concrete and white roofs, energy-efficient appliances and equipment; building green roofs and vegetation cover; planting trees in cities, making green parking lots and also by implementing heat reduction policies and rules heat island effects can be significantly reduced. Here the focus is on green roofs and vegetation cover.

Green roofs present an effective method of lessening the impacts of urban heat island. Green roofing is planting of vegetation on a roof. Plants on the roof are excellent insulators during summer and reduce the overall urban heat-island effect. Plants cool the surrounding environments thereby reducing air conditioning demands. Many home owners find that they can enjoy the luxury of functional fashion while still remaining eco-friendly in their daily life. A green roof is actually a cover for roof that uses grasses, herbs and plants to cover the top of the house. This process usually involves water-proofing of existent roof, applying a root repulsive system and a drainage system. A filter cloth on the roof has to be placed, next it has to be covered with a medium and then finally plants have to be secured onto the roof.

Installation of green roofs can be expensive but they are very unique and beautiful. Green roofs form a layer of insulations for a home. This means that home owners will probably use less insulation on their roofs when they build and they will also use less energy to both heat and cool their homes. In fact, studies have shown that a green roof can reduce cooling costs by 25 per cent during the summer months alone. Those savings are massive when it all starts to add up. By installing a green roof, inhabitants will quickly notice the benefit of even partial sound proofing that is provided. Green roofs are energy-efficient, reduce heat instability and less energy for cooling or heating can lead to significant cost savings. In summer, the green roof protects the building from direct solar heat and in winter, the green roof minimizes heat loss through added insulation on the roof. The greenery also burns fewer crude oil that is fewer greenhouse gases, which is great for everyone. Green roofs can reduce material waste from re-roofing and less frequent re-roofing is needed. In Dhaka city, there are 6,000 rooftop gardens. To encourage people to set up such gardens, the two Dhaka city corporations may take a decision to reduce holding tax if any family goes for rooftop gardening.

Green roofs make the most of unused space within the increasing density of our cities. Rooftops can be urbanised into social and recreational spaces and used for urban agriculture. The green space can be used for daycare, meetings, and recreation. Its improved aesthetic view makes a unique and beautiful sight for neighbours in adjacent buildings. In the office premises it will improve worker productivity and creativity. A large-scale rooftop gardening also has potential to contribute to urban food security through food production.

The many benefits of rooftop gardens in urban zone to public, private, economic and social sectors, as well as to the global environment are really inclusive and sustainable. Eco-friendly rooftop garden can generate oxygen which is an excellent way to ensure public health. A policy needs to be formulated to make cities in the country green for generations to come.

Shishir Reza, an environment analyst, is associate member of the Bangladesh Economic Association. Sharmin Akhter is an urban planner.

[email protected]