Green roofs
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Contents
Introduction
Green roof cross section concept. Source: EPA Green roofs are an extension of the existing roof that allows plant trees, shrubs, grasses, and other vegetation to grow in a light-weight growing medium. Generally green roofs are on top of a human-made structure and can be located below, at, or above grade. A green roof consists of vegetation and soil, or a growing medium, planted over a waterproofing membrane. Additional layers, such as a root barrier and drainage and irrigation systems may also be included. Among other environmental benefits, green roofs mitigate the effect of urban heat islands (Green roofs) . On hot summer days, the surface temperature of a vegetated rooftop can be cooler than the air temperature, whereas the surface of a traditional rooftop can be up to 90°F (50°C) warmer. Green roofs can be used in many applications, including industrial facilities, residences, offices, and other commercial property.
In Europe, green roofs are widely used for their stormwater management and energy savings potential, as well as their aesthetic benefits. This is the 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.
Benefits of green roofs
The cooling effect of vegetation. Trees, shrubs, and other plants to shade buildings, intercept solar radiation, and cool the air by evapotranspiration. Evapotranspiration occurs when plants secrete or "transpire" water through pores in their leaves. The water draws heat as it evaporates, cooling the air in the process. A single mature, properly watered tree with a crown of 30 feet can "evapotranspire" up to 40 gallons of water in a day, which is like removing all the heat produced in four hours by a small electric space heater. Deciduous trees shading the south and west sides of a building block the summer sun. The net cooling effect of a young, healthy tree is equivalent to ten room-size air conditioners operating 20 hours a day. Thus, studies by the U.S. Department of Agriculture show that that parts of cities without cooling shade from trees can literally be "heat islands," with temperatures as much as 12 degrees Fahrenheit higher than surrounding areas.
Mitigation of storm water runoff. As impermeable surfaces like buildings and pavement replace open space and vegetation, green roofs can play an increasingly important role in stormwater management. During rainstorms, green roofs act as a sponge, absorbing much of the water that would otherwise run off. 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 between 25-40%. For example, a grass roof with a 4-20 cm (1.6 - 7.9 inches) layer of growing medium can hold 10-15 cm (3.9 - 5.9 inches) of water. Green roofs thus 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.
Green roofs also filter pollution from rainwater. This is achieved by the root systems' bacteria and fungi, which utilize the natural filtering processes of bioremediation and phytoremediation. As a result, the non-point source pollutants, nitrogen and phosphorus, are broken down and detoxified. This beneficial process increases over time as rooftop plants and root systems mature.
Air purification. A green roof not only absorbs heat, decreasing the tendency towards thermal air movement, it also filters the air moving across it. One m2 (10.76 ft2) of grass roof can remove between 0.2 kg of airborne particulates from the air every year.
Sound insulation. Soil, plants and the trapped layer of air can be used to insulate for sound. Sound waves that are produced by machinery, traffic or airplanes can be absorbed, reflected or deflected. The substrate tends to block lower sound frequencies and the plants block higher frequencies. A green roof with a 12 cm (4.7 inches) substrate layer can reduce sound by 40 decibels; a 20 cm (7.9 inches) substrate layer can reduce sound by 46-50 decibels
Economic benefits. Green roofs protection roof membrane by eliminating exposure to the sun's ultraviolet (UV) radiation and extreme daily temperature fluctuations, resulting in a longer material lifespan. Green roofs may last up to twice as long as conventional roofs, resulting in decreased maintenance and savings in replacement costs. Green roofs decrease air conditioning and heating costs, depending on the size of the building, climate and type of green roof. Environment Canada estimates 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. Actual field experiments in Canada suggest that a 6-inch extensive green roof reduced heat gains by 95% and heat losses by 26% compared to a reference roof.
Amenity Aesthetic Value. Some green roofs provide amenity space for day care, meetings, and recreation, and otherwise increase overall aesthetic appeal, and thereby incrase the value of the property and the marketability of the building as a whole. Green roofs also offer aesthetic benefits to people looking down upon the roof from adjacent buildings.
Habitat for biodiversity. Rooftop habitats can play one of two roles: 1) a 'stepping stone' habitat connecting natural isolated habitat pockets with each other; or 2) an 'island' habitat remaining isolated from other habitats at grade. Green roofs can be specifically designed to mimic endangered ecosystems/habitats, including the prairie grasslands of the midwest US, the rocky alvars of Manitoulin Island and the Great Lakes Region in Canada. For example, the Toronto City Hall Demonstration Project features a black oak prairie ecosystem and native plant butterfly plot.
Green roofs designed for minimal maintenance are very protected and can provide homes to plants easily damaged by walking and to birds that nest on the ground. Since the soil on these green roofs is also less likely to be disturbed, it provides a safer habitat for insects, and the deeper the soil the more diversity the roof can support. In Germany, for instance, research has shown that green roofs can support anywhere from 10 to 40 different insect species and have even been found to harbor nesting bird species.
The London Biodiversity Partnership seeks to build an area greater than 100,000 m2 of green roofs as habitat for the endangered black redstart (Phoenicurus ochruros). Preliminary research in London and Swizerland suggest that green roofs can provide refuge for a number of nationally rare and scarce species.
An assesssment of green roof potential in the city of Toronto sugests some significant benefits. The study estimated the citywide benefits from the approximately 5,000 hectares or 50 million m2 of roof area eligible for green roofs in the City of Toronto. The calculation of benefits on a citywide basis are based on these assumptions about the green roofs:
- are greater than 350 m2 in size
- cover at least 75% of the roof area
- install over heated spaces
- excluded roofs over underground parking garages and other non-conditioned enclosed spaces at grade level
The estimated benefis include:
- Stormwater runoff
- Reduction in stormwater flow of 12 million m3 per year
- Infrastructure savings worth $79 million
- Erosion control measures savings worth $25 million
- Pollution control cost avoidance worth $13 million
- 3 additional "beach open" days per year worth $700,000
- Energy demand
- Citywide savings from reduced energy for cooling is $22 million, equivalent to 4.15 KWh/m2 per year
- Cost avoided due to reduced demand at peak times is $68 million
- Urban Heat Island effect
- Widespread greening of Toronto's roof would reduce local ambient temperature from 0.5 to 2 degrees Celsius
- Citywide savings from reduced energy for cooling of $12 million, equivalent to 2.37 kWh/m2 per year
- Cost avoided due to reduced demand at peak times of $80 million
Costs of green roofs
Currently, the up-front cost of an extensive green roof in the U.S. starts at about $8 per square foot, which includes materials, preparation work, and installation. In comparison, the cost of a traditional built-up roof starts at about $1.25 while cool roof membranes start at approximately $1.50 per square foot.
Extensive green roofs cost more than traditional roofs because they require more material and labor for installation. Another factor affecting price is that green roof contractors are limited in number. As the demand for rooftop gardens increases in the U.S., and as additional contractors come into business, up-front costs will likely decrease.
However, it is widely known that up-front costs do not tell the whole story. Taking into account future summertime energy savings at the time of purchase brings the price of a green roof closer to that of a traditional roof. Depending on local construction codes, it also may be possible to do without stormwater infrastructure investments.
Another factor reducing the cost of a green roof is that vegetation can extend the life of a roof. This is because less solar energy reaches the roof substrate, limiting damage from UV radiation as well as daily temperature fluctuations, which cause repeated contraction and expansion.
Further reading
- Report on the Environmental Benefits and Costs of Green Roof Technology for the City of Toronto, prepared by Ryerson University.
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