A roof with a view
Green roofs are fast becoming popular as a way to moderate the external climate, says Dr Bassam Abu-Hijleh.
The concept of green roofs is not new, humans have used natural features as a shelter to moderate climatic conditions since the dawn of humankind.
An example of that is living in caves, which not only provided safety but also maintained a reasonable level of indoor comfort. Green roofs build on this idea of using natural features to moderate indoor conditions.
Green roofs are fast becoming popular as a way to moderate the effect of external climate and to provide some relief from current problems given rise by the sprawl of high density urban centers, for example, the heat island effect. Many countries and organisations now acknowledge the benefits of the green roof and actively encourage it.
The proper installation of green roofs can actually help a building gain points toward achieving LEED accreditation.
Some of the advantages of a green roof include:
- Provision of an extra layer of thermal insulation on the top of the building reducing the heat gain from the ambient
- Provision of an evaporative cooling effect to the building from water evaporation from the plants
- Reduction of heat gain from direct solar radiation through plants on the roof reflecting part of the solar irradiance falling on the building's roof
- Moderation of temperature fluctuations on the micro climate level
- Provision of an extra level of ambient noise insulation
- Possiblity of the roof being used as a local park and enhancement of aesthetics of the building
- Extension of life of the material used in the construction of the roof
- Reduction of the amount and the maximum flow rate of storm water discharge in regions that experience frequent rain
Installing a green roof requires special attention during the installation and subsequent maintenance of the green roof. There are several considerations. First, a waterproof layer needs to be installed between the roof and the soil bed of the green roof to prevent the green roof's water from damaging the roof material.
Second, a tough and impermeable layer is also needed to prevent the plants' roots from penetrating the roof and damaging its material. Third, the type of plants selected must be suitable for the prevailing climatic conditions. Fourth, water consumption is a big issue when using green roofs, especially in hot and arid regions such as the Middle East.
The use of potable water needs to be kept to a minimum or even eliminated. Properly recycled gray water can be used effectively to irrigate the green roof.
Green roofs are most effective in low-rise buildings where the roof's area is of good proportion compared to the overall exposed walls of the building. In high-rise buildings the roof area is very small compared to the area of the sidewalls. This reduces many of the benefits of a green roof.
To compensate, designers can go the next step and install green "living" walls. Living walls extend the green roof concept to the side building's walls thus greatly increasing the green area and providing most of the benefits of a green roof. Living walls requires all attention points listed for green roofs and more, depending on the coverage area.
Green roofs and LEED
Green roofs can help contribute to the LEED rating of a building in conjunction with other building systems, under the following criteria:
- Reduced site disturbance, protect or restore open space
- Landscape design that reduces urban heat islands
- Storm water management
- Water efficient landscaping
- Innovative wastewater technologies
- Innovation in design
- History of Green Roofs
Modern green roofs are a relatively recent phenomenon, although green roofs have been used in Iceland and Norway for centuries. Green roofs are increasingly popular in the US and Europe, but still relatively new to the Middle East. The Chicago City Hall green roof is one of the best-known green roofs in the US.
The American Society of Landscape Architects (ASLA) recently built a green roof on its Washington DC headquarters. Designed by Boston-based landscape architect Michael Van Valkenburgh and Associates, the roof covers 306.5m2.
Dr Bassam Abu-Hijleh is senior lecturer at Atkins, and head of the sustainable design of the built environment programme, at the British University in Dubai.