Green Roofs: Overview
A green roof system is an extension of the existing roof comprising a high quality waterproofing and root repellent system, a drainage system, a filter cloth, a lightweight growing medium and plants.
Green roof systems may be modular, with drainage layers, filter cloth, growing media and plants already prepared in movable, interlocking grids. Alternatively, each component of the system may be installed separately. Green roof development involves the creation of "contained" greenspace on top of a human-made structure.
This greenspace could be below, at or above grade, but in all cases the plants are not planted in the "ground". Green roofs can provide a wide range of public and private benefits.
RATIONALE
In North America, the benefits of green roof technologies are poorly understood and the market remains immature, despite the efforts of several industry leaders. In Europe, however, these technologies have become very well established as a direct result of government legislative and financial support, at both the state and municipal level. Such support recognizes the many tangible and intangible public benefits of green roofs. This support has led to the creation of a vibrant, multi-million dollar market for green roof products and services in Germany, France, Austria and Switzerland among others. In Germany, for instance, the industry had 700 million DM in sales in 1997, up from 500 million DM in sales in 1994. The industry continues to experience growth, with 13.5 million square meters of green roofs constructed in 2001, up from 9 million square meters built in 1994.
Water from precipitation that flows over nonporous surfaces into sewer systems or receiving water bodies.A mixture of decayed plants and other organic material that is used to enrich soil with nutrients.A term used for significant changes over an extended period of time in the Earth's natural environments. Climate change can be produced by natural processes (e.g. changes in the Earth's orbit) or by the impact of human behavior (e.g. deforestation). See GLOBAL CLIMATE CHANGE.The absorption of heat by dark, non-reflective hardscapes (including pavement and buildings) and its radiation to surrounding areas. A problem in urban areas, the effect is exacerbated by vehicle exhaust, air-conditioners, and street equipment.Greenhouse gases are a part of the Earth's atmosphere and are both naturally occurring and the result of human chemical processes. The most common greenhouse gases are carbon dioxide, water vapor, methane, nitrous oxide, ozone, and chlorofluourocarbons. These gases trap heat and thus contribute to the warming of the planet. See also CFCS and GREENHOUSE EFFECT.The ability or potential of a physical body to do work. The most common forms of energy are heat, light, mechanical (moving parts), and electrical.
EFFORT REQUIRED
Before installing a green roof, it is necessary to determine many factors, including the slope, structural loading capacity and existing materials of the roof; the nature of any drainage systems, waterproofing and electrical and water supply in place; who will have access to to the green roof, how maintenance will be handled; and what kind of sun and wind exposure the roof gets. Inclusion of a Green Roof Professional on a project will reduce the learning curve for the project team and lead to greater likelihood of success.
BENEFIT
Economy
Green roofs protect the roof membrane, resulting in a longer material lifespan (it is estimated that green roofs will last up to twice as long as conventional roofs), and thus decreased maintenance and savings in replacement costs.
Green roofs reduce energy heating and cooling costs, depending on the size of the building, climate and type of green roof. Using a Micro Axess Simulation model, Environment Canada found that a typical one story building with a grass roof and 10 cm (3.9 inches) of growing medium would result in a 25% reduction in summer cooling needs. Field experiments by Karen Liu in Ottawa, Canada, found that a 6 inch extensive green roof reduced heat gains by 95% and heat losses by 26% compared to a reference roof. Public policies that support green roof installations will create jobs for the following:
- Suppliers and manufacturers of roofing membranes and root repellent layers
- Suppliers and manufacturers of drainage layers, landscaping cloth, curbs, irrigation systems and other specialty products
- Suppliers and manufacturers of substrate, light-weight soils and amendments
- Garden nurseries specializing in plants specifically for green roofs
- Design and engineering professionals
- Contractors and landscapers
- Companies supplying maintenance contracts
Although no exact figures exist, it is estimated that the roofing industry in Germany employs approximately 12,000 people, and if all flat roofs were to be greened, this figure would increase to approximately100,000.
Community cost savings opportunities include:
- Cost savings from increased stormwater retention and decreased need to expand or rebuild related infrastructure
- Decreased cost of meeting greenhouse gas reductions and adapting to climate change by reducing the "Urban Heat Island Effect" and the need for interior building insulation
- Decreased need for health care services from reductions in ground level ozone resulting from a reduction in the urban heat island
- Increased worker productivity and creativity
- Decreased need for health care services and medication due to the benefits of passive experiences with nature and vegetation
- Extending the lifespan of landfill sites by reducing re-roofing material waste
- Opportunities to recycle aggregate and compost
Food Production
Green roofs can provide new opportunities for urban agriculture. One example of this is the Fairmount Waterfront Hotel in Vancouver who uses their green roof to grow herbs, flowers, and vegetables on its accessible roof, saving its kitchen an estimated $30,000 a year in food costs.
Improved Air Quality
Filtration of Airborne Particulates: A green roof will not only absorb heat, decreasing the tendency for thermal air movement, but will also filter the air moving across it. 1 m2 (10.76 ft2) of grass roof can remove up to 2 kg (4.4 lbs) of airborne particulates from the air every year, depending on foliage type.
Carbon Dioxide/Oxygen Exchange: Through the process of photosynthesis, plants convert carbon dioxide, water and sunlight/energy into oxygen and glucose. This cyclical process supplies animals and humans with oxygen and food. 1.5 m2 (16.15 ft2) of uncut grass produces enough oxygen per year to supply one human’s yearly oxygen intake requirement.
Temperature Regulation
Moderation of the Urban Heat Island Effect: Through the daily dew and evaporation cycle, plants on vertical and horizontal surfaces are able to cool cities during hot summer months. In the process of evapotranspiration, plants use heat energy from their surroundings (approximately 592 kcal per L of water) when evaporating water. One m2 (10.76 ft2) of foliage can evaporate over 0.5 liters of water on a hot day and on an annual basis the same area can evaporate up to 700 liters of water.
This process reduces the summer “Urban Heat Island Effect” (UHI) - the difference in temperature between a city and the surrounding countryside caused by cities’ expanses of hard and reflective surfaces, such as roofs, that absorb solar radiation and reradiate it as heat. Reduction of the UHI also reduces the distribution of dust and particulate matter throughout the city and the production of smog. This can play a role in reducing greenhouse gas emissions and adapting urban areas to a future climate with warmer summers.
Green roofs can play a role in reducing the UHI, augmenting the existing vegetation, but the precise amount of coverage is still uncertain. Dr. Brad Bass, in collaboration with a modeling group at the University of British Columbia under the direction of Dr. Roland Stull, ran a mesoscale atmospheric simulation for the City of Toronto with green roofs. The city's vegetation reduced the UHI by up to 1 degree C over approximately one quarter of the City. Using green roof coverage of 50%, this cooling was extended to approximately one third of the City and increased the maximum cooling to 2 degrees C.
Although the green roof coverage was high, Dr. Bass estimated that only 6% of the roofs were fully irrigated as the model decreased irrigation to those parts of the city that were not fully urbanized, decreasing it to zero in totally natural areas of the city. This suggests that the actual green roof coverage to obtain these results could be much smaller than 50%, although the exact requirement is difficult to determine due to a number of uncertainties that emerged in this modeling exercise.
Water
Stormwater Retention: Water is stored by the substrate and then taken up by the plants from where it is returned to the atmosphere through transpiration and evaporation. In summer, depending on the plants and depth of growing medium, green roofs retain 70-90% of the precipitation that falls on them; in winter they retain 25-40%. For example, a grass roof with a 4-20 cm (1.6 - 7.9 inch) layer of growing medium can hold 10-15 cm (3.9 - 5.9 inches) of water.
Water Filtration: Green roofs not only retain rainwater, but also moderate the temperature of the water and act as natural filters for any of the water that happens to run off.
Temporal Delay of Stormwater Runoff and Reduced Runoff Volume: Green roofs reduce the amount of stormwater runoff and also delay the time at which runoff occurs, resulting in decreased stress on sewer systems at peak flow periods.
RISKS
Failure to use licensed professionals to determine the structural capacity of a roof can result in catastrophic failure. Success can be greatly aided by using a Certified Green Roof Professional. A GRP has passed a written exam showing a body of knowledge about green roof design, installation, and management and can assist project teams in successful installation of a green roof.
ACTION AGENT(S)
- Landscape architects and contractors frequently design green roofs and can install the living components (i.e., growing medium and plants).
- Because green roof systems include materials not found on conventional roofs, a qualified roofing contractor with experience in green roof installation should install the non-living components (e.g., vapor barrier, waterproof membrane).
- A roofing consultant can assess roofing design needs, ensure quality control and recommend a suitable green roof system.
COST
The cost of a green roof varies considerably depending on the type and factors such as the depth of growing medium, selected plants, size of installation, use of irrigation and whether they are to be accessible or inaccessible - intensive, semi-extensive or extensive. Intensive green roofs typically require greater investment but confer the benefits of accessibility. An installed extensive green roof with root repellent/waterproof membranes may be installed for $10-$24 per square foot.
While green roofs typically require a greater initial investment than conventional roofs, they can extend the life of the roof membrane and reduce the heating and cooling costs of the building.
