Green Roofs
Basics of a resurgent building technology
Living Biological Machine
The green roof of Electronic Arts, Phase II. Musson Cattell Mackey Partnership, Vancouver.
by Bruce Hemstock .The use of green roofs for aesthetic and recreational purposes can be traced back to the Hanging Gardens of Babylon which were built in the 7th century BC as a diversion for the aristocracy. Later, more practical applications emerged in Scandinavia and North America where sod roofs, laid on a waterproof layer of bark, were used to insulate indigenous dwellings against the extreme cold.
The current interest in green roofs as an environmental strategy toward creating a living biological machine, originated with the ecological movement in Germany in the early 1970s. In North America the focus, until recently, has been on aesthetic/recreational applications mostly on flat roofed, high rise and low rise structures. However, the concept of global warming has brought environmental concerns back to prominence.
There is a general interest in addressing the problems associated with urban development and a changing world climate. With roofs occupying a substantial percentage of a city’s hard surfaces [that are contributing to climate change], the introduction of sustainable roof specifications has been seen as a way to help address some of these concerns.
Benefits of Green roofs
Energy Consumption
The insulating value of growing medium and plant material contributes to a reduction in the energy consumption of buildings, keeping them warmer in winter and cooler in summer. For example, as little as 10cm of growing medium substrate with grass cover was found to reduce the summer air conditioning needs in a one-storey building in Toronto by 25%, due to a reduction of indoor air temperatures in the range of 3-4 °C.
Actively growing roof plants offer an effective insulating layer against heat transfer through roofs. Increased plant leaf mass provides more shading of the surrounding roof area, while growing medium and plant matter trap air in pockets much like conventional insulation. The trapped air contributes to the insulating qualities of the roof.
Reduction of the Urban Heat Island Effect
The urban heat island effect [UHI], or overheating of the urban and suburban environment relative to the surrounding vegetated countryside, has become a global concern. Temperature differences of up to 6 °C have been measured between urban and rural areas in some cities. Compounding the problem is that the UHI increases the concentration of ground level pollutants such as ozone, smog and nitrous oxides. The UHI also contributes to the increase in energy consumption through the operation of air conditioning systems.
Green roofs can reduce the heat island effect by moderating the heat gain on roofs and reducing the heat transfer into buildings, mindful that a reduction in ambient room temperature of less than 1°C results in a 1.2% reduction in the energy needed for cooling.
In addition to the insulating qualities of the soil and plant materials, the plants themselves shade the roof surface and as they transpire they release water which in turn cools the air and reduces the ambient air temperature. A large number of smaller roof areas evenly distributed in an urban area have greater impact on heat island reduction than a smaller number of larger roofs
Where summer rainfall is not sufficient to ensure minimal plant function the addition of a high efficiency irrigation system to the planted roof increases the amount of evapo-transpiration by making water readily available. Depending on the amount of water and the plant type the temperature decrease can be as much as four times that of a non- irrigated roof.
Extension of Service Life
The surface temperature of a conventional roof can reach 50 - 70°C on a summer afternoon, while on an adjacent green roof, the membrane will maintain a more stable temperature that is approximately half of this value. The fluctuation in diurnal temperature of a conventional roof causes physical stress on the membrane due to expansion and contraction.
Green roofs reduce the heat gain and surface temperature fluctuations, and provide protection from ultra violet degradation thereby extending membrane life by two or three times. The green roof also provides protection against physical damage by work and maintenance crews walking on the roof surface.
Storm Water Management
The most dramatic impact of green roofs, especially in wet temperate climates such as the Lower Mainland of British Columbia, is the reduction of storm water runoff. Upwards of 90% of the rainwater falling on urban areas runs off hard surfaces and is collected by a storm water system compared with the 5% runoff typical in a forested environment. This increased runoff creates several problems including soil erosion, turbidity in water bodies, contaminated runoff and larger more expensive stormwater infrastructure. Green roofs have the ability to positively address all of these issues.
Green roofs act as ecological sponges absorbing and detaining rainwater, utilizing it in the plant growth process and or releasing it slowly over an extended period of time. In the Vancouver area a green roof with 30cm of growing medium will trap and hold on average 3.3 cm of storm water. This accounts for 90% of the storm events in a typical year. Roofs with lesser thicknesses of growing medium also contribute to reductions in rates of run off. Their effectiveness in any given region will depend on the local rainfall profile.
Cleaning of Urban Air
Trapping particulate matter from vehicle engine emissions as well as trapping carbon dioxide and giving off oxygen are other benefits of green roofs that contribute to the cleaning of urban air. Denser more diverse plant material is more effective in trapping particulate matter. Similarly, the deeper the growing medium the better able it is to trap contaminants. Irrigating green roofs further enhances the cleansing properties by ensuring active plant growth that creates more leaf density which generates more oxygen.
Continuity of Natural Systems
Urban growth typically leads to the fragmentation and loss of wildlife habitat, and the severing of natural corridors that wildlife need to survive. Green roofs can help restore ecological connectivity through the urban environment by providing small ‘green patches’ that link together large natural areas. In effect, green roofs can contribute to the re-creation of ecological functions within the city by providing habitat for a diverse range of plants, insects, micro-organisms, and birds.
Social Benefits
The social benefits of green roofs are likely the least recognized but equally valuable contributions to the urban environment. They provide appealing landscapes that are often visible not only to the individuals within a building, but to those in the surrounding buildings. Studies have shown that in the case of office workers and hospital patients, an overlook onto green roof landscapes reduces stress, creates a stronger sense of wellbeing, increases productivity and, in the case of hospitals, helps in the healing process.
Design
Generally green roofs are divided into two classifications that have their own advantages and disadvantages. Although the basic difference is growing medium depth, this one factor determines the overall aesthetic, use, contribution to sustainability and maintenance requirements.
Intensive
Intensive roofs typically utilize 20cm to 90cm of growing medium with weights ranging from 240 - 1200kg/sq. m [50 – 250 lbs/sf]. This kind of roof will require additional structural design. Intensive roofs have a medium to high capital cost but are able to support shrubs and trees as well as grasses. They require medium to high maintenance, including irrigation, but offer medium to high value for recreation.
Extensive
Extensive roofs have only 7.5cm to 15cm of growing medium, and consequently weigh only 90 – 180kg/sq. m [9-38 lb/sf). These loads may not require additional structural design. Extensive roofs have relatively low capital and maintenance costs, but are limited in the plant diversity they can support – typically only sedums, and drought-tolerant grasses and herbs that often require only seasonal irrigation. This kind of roof offers little or no recreational value but can provide very high habitat value.
Roof Systems
There are a number of roof systems commercially available and suitable for a full range of roof applications. The general components of the system include;
• Waterproof membrane: covers the entire roof structure or slab [often with an impregnated root barrier, or covered with a protection board],
• Drain mat: Ensures the positive conveyance of storm water to roof drain once the growing medium has become fully saturated [typically water is the heaviest component of the green roof],
• Filter cloth: Provides a layer separating the drain mat from the growing medium to ensure that the permeability and conveyance properties of the drain mat are not compromised.
• Growing medium: PWL Partnership’s green roof growing medium mix comprises [dry weight] 50 – 75% sand, 20% total fines [silts and clay], 15-20% organic matter [derived from composted wood, vegetable and food waste], 75% lava rock [mixed with medium and after medium is compiled].
A key consideration of the growing medium is that it be comprised of sustainable materials that form a living biological machine complete with insects, worms, fungi, and micro organisms.
Variations to the green roof build-up can come with the addition of proprietary elements such as:
• Expanded cell water holding components [improve storm water retention capabilities]
• Moisture retention mats
• Water delivery systems
There is a wide range of details that have been developed to allow the basic system to adapt to various roof design considerations including steep slopes, exposure to high wind, protection from heavy rain, termination at the building edge, conveyance of water, etc.
At this point green roofs are still seen as cutting edge technology in many parts of Canada. With failures in building envelop design still fresh in the minds of many, the concept of putting a landscape element that is actively growing and in need of water on top of a building is somewhat counter intuitive.
The recent concerns of the insurance industries Home Protection Office regarding the use of green roofs on wood frame condominium buildings is one example of how the lack of knowledge regarding proper green roof installation and maintenance has slowed its acceptance.
The success of a green roof installation is in the details. If designed, detailed and installed by competent professionals green roofs can provide significant benefits not only to the building envelope but to the ecology of its surroundings, truly making it a living biological machine.
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»»» Related Information:….
GreenSave Calculator:
a life-cycle cost-benefit tool for green roofs
by Steven Peck .Green Roofs for Healthy Cities, an industry association for the green roof industry with over 4,000 individual and 75 corporate members, has launched the GreenSave Calculator on its website, www.greenroofs.org.
The tool was designed under the leadership of Tremco’s Ralph Velasquez, the members of Green Roofs for Healthy Cities’ Life-Cycle Cost-Benefit Sub-Committee, and the Athena Institute*.
The GreenSave Calculator is the first web-enabled Life-Cycle Cost-Benefit tool that lets building professionals and owners compare the short- and long-term costs and benefits of green roofs and conventional roofs.
“The value of the GreenSave Calculator is that it allows users to be much better informed about the costs and benefits associated with green roofs and conventional roofing systems within a robust life-cycle cost analysis environment,” said Jamie Meil, co-founder and director of the Athena Institute.
The GreenSave Calculator facilitates detailed comparisons of up to three different roofing options over the defined life-span of the project [typically ranging from 25 to 60 years], providing a more accurate, comprehensive picture of the long range impacts of each option. Among the many elements that can be considered are the life-cycle study period, discount rate, general and energy price inflation, investment cost data, residual or resale value, recurring operating and maintenance and replacement costs.
The GreenSave Calculator is also capable of capturing other possible roof benefits such as storm water and heat island mitigation, extended roof durability, related energy costs and savings, development fee savings, marketing benefits, increased rental income and tenant health and productivity improvements that can be ascribed to a roofing system.
The GreenSave Calculator is currently in its first phase. Phase 2 will incorporate an expanded selection of the costs and benefits of green roofs.
*The not-for-profit Athena Institute undertakes and directs R&D activities that make it possible to factor both environmental and economic considerations into building design and operations through the provision of software tools, life cycle assessment databases and specialized consulting services [info@athenasmi.ca]. .