Environmental benefits
Building owners decide to install a green roof for many reasons: environmental, energy savings, increasing the useful life of the roof, increasing the value of the building, marketing power and to block out surrounding buildings.
Reducing the urban heat island effect
Researchers from the Institute for Research in Construction (IRC) of the National Research Council (NRC) built and compared at their research facilities in Ottawa a traditional modified bitumen roof system and a green roofing system (15 cm (6 in.) of growth medium and grass cover) to study the properties of green roofs in terms of thermal resistance and energy efficiency according to canadian climatic conditions.
Research results showed that in very hot weather, green roofs significantly reduced spending on energy for air conditioning in urban areas.
In this way, installing a vegetated roof for at least 50% of the roof area qualify for LEED credit SS7.2 regarding the reduction of the urban heat islands.
During the IRC research, observations were made that the green roof reduced temperature and daily temperature fluctuations considerably on roofs during spring and summer. The membrane installed on the traditional roof reached nearly 70°C (158°F), while the membrane on the green roof did not exceed 25°C (25°F).
The study also showed that the heat flow crossing the roofing system was moderated substantially. During a two-month period, the green roof reduced by 95% the heat gains and by 26% the heat losses compared to the modified bitumen reference roof. Thanks to shade, evapotranspiration and improved insulation readings, the daily average heat flow through the roof was reduced by more than 75%. In summary, although a green roof reduces heat gains considerably in spring and summer, it is less efficient at reducing heat losses during the autumn and winter.
Increasing the useful service life of roofing
The green roof protects the building’s waterproofing membrane. Reduction of the significant temperature variations between day and night increases membrane service life span substantially since heat expansion and contraction are the main causes of early failure. A green roof protects the roof from UV rays and avoids refuse build-up on the membrane.
Water runoff
Green roofs are increasingly used to manage stormwater runoff in urban areas. Since some of the rain is kept in the growth medium to be used by plants and later released into the atmosphere through evapotranspiration, green roofs delay and reduce the runoff flow rate into sewer systems. This reduces the risk of overflow in ageing stormwater sewer systems.
Benefits of air filtration
In addition to absorbing heat, green roofs reduce thermal flow and filter air circulation. A surface area of 1 m2 (imperial) covered by grasses can eliminate 0.2 kg (imperial) of air-suspended particles annually.