Green Roofs — Benefits and Challenges
Posted: November 25, 2022 | Project Management
Green roofs (sometimes referred to as “vegetated roofs” or “eco-roofs”) consist of a waterproofing membrane, growing medium (soil), and vegetation (plants) overlying a traditional roof. Green roofs can help mitigate the problems that cities create by bringing the natural cooling, water treatment, and air filtration properties that vegetated landscapes provide to the urban environment.
Architects and planners can use green roofs to help solve environmental problems by bringing nature back to the city in key ways. Green roofs — properly designed, constructed, and maintained — are beneficial socially, environmentally, and fiscally. They are an important tool to increase sustainability and biodiversity and decrease energy consumption, urban heat island impacts, and greenhouse gas generation.
Green Roofs in the United States: Green roofs have been around a long time. Prairie homesteaders built sod houses when settling the frontier. There have been green roofs on U.S. government buildings and parking structures for a century: Green roofs installed on several federal buildings in the Washington, DC region have not been replaced since their installation in the 1930s.
Although they have been used, albeit infrequently, for decades, green roofs are now being revived and studied for their environmental benefits. The growth of green roofs in the U.S. mirrors their use in other countries, like Germany, where they are more commonly seen. The General Services Administration (GSA) has designed and maintained green roofs for decades and finds them to be economical amenities that make fiscal and environmental sense.
Green Roof Types: A typical green roof includes a waterproof barrier to protect the building, a drainage layer to store and direct runoff, a soil or growth medium layer, and a plant layer. There are two main types of green roofs: extensive roofs, which are relatively inexpensive to install and are used mainly for environmental benefit, and intensive roofs, which allow a greater variety and size of plants, such as shrubs and small trees, but are usually more expensive to install and maintain (partly due to the need for irrigation). Commercial and public buildings tend to use extensive roofs unless the roofs are intended primarily as occupied garden amenity spaces.
Extensive roofs have a thin soil layer and feature succulent plants like sedums that can survive in harsh conditions. Extensive roofs require little maintenance once they are established, and are generally cost effective, particularly in buildings with long life spans.
Intensive roofs have a thicker soil layer and should be considered a landscape with plants found in parks and gardens. These plants may require irrigation during dry periods. Because of their thicker soil, intensive roofs require greater structural support than extensive ones. However, intensive roofs also have greater potential for ecological benefit and amenity use than extensive roofs.
Green Roof Costs & Benefits: The environmental benefits of installing green roofs on commercial and public buildings include improved water quality due to reduced stormwater runoff and fewer overflows of combined sanitary and stormwater sewage systems; increased habitat, which promotes biodiversity; lower temperatures for roofs and the air above them in most climates; reduced energy consumption in some climates; improved sound absorption in the top floors of buildings; and improved air quality.
The economic costs of installing green roofs on commercial and public buildings also include the cost of installing and maintaining the roof. The economic benefits of installing green roofs include lower energy costs due to the cooling effect of plant respiration; the insulation, shading, and thermal mass of the plant and soil layers; a less frequent roof replacement schedule due to greater durability than conventional roofs; reduced stormwater management costs; and the creation of job opportunities in roof installation and maintenance and in the emerging field of urban agriculture.
Green roofs provide a payback (based on a 50-year average annual savings) of approximately 6.2 years nationally (internal rate of return of 5.2%) and 6.6 years in Washington, DC (internal rate of return of 4.2%). Conservative analysis puts the average life expectancy of a green roof at 40 years, versus 17 for a conventional roof.
Challenges to Green Roof Construction: The main design, installation and management challenges of green roofs include:
• Ensuring the building can support a green roof
• Quality installation and leak prevention
• Maintenance requirements
• Potential plant loss due to environmental conditionsor mismanagement
Designers can maximize the benefits of green roofs by properly selecting plants, growth medium, drainage layers, and other features tailored to the local climate and the building’s surroundings.
Research has identified the following 10 benefits of green roofs.
1. Stormwater Management. In a typical urban area, rain that falls onto paved or built-on surfaces quickly flows into storm sewers and out into a nearby body of water. This excess stormwater runoff can cause numerous environmental problems, including damaging water quality by sweeping urban pollutants into water bodies, eroding river banks, and flooding.
In cities that built sewer systems before the 1930s, these storm drains are typically combined with the sanitary sewers that carry wastewater away from buildings to water treatment plants. This can be a problem during storms, when the large volume of stormwater can cause a sewer overflow and lead to the discharge of untreated sewage into rivers and lakes. Particularly in urban areas with high-density development, green roofs may be the most practical way to address wet weather flows, especially when existing buildings must be retrofitted.
Green roof stormwater retention is affected by roof construction, size and slope, and the plants, drainage layer, and growth medium used. Green roofs can form a key part of a site-level stormwater management plan, reducing peak flow rates by up to 65% and increasing the amount of time it takes for water to flow from a site into the sewer by up to three hours, depending on the size of the roof and the distance the water must travel. Green roofs reduce runoff rates after both large and small storms. Installing a green roof at least 3 inches thick on a large enough area can reduce the frequency of sewer overflows during the summer season.
2. Biodiversity and Habitat. Biodiversity is a measure of the variety of plants and animals in an area. Green roofs provide new habitat for beneficial plants and animals in urban areas, helping to increase biodiversity. Increased biodiversity can help ecosystems continue to function even when they are disturbed by development or in other ways.
Green roofs, particularly intensive ones, can be designed to integrate multiple habitats and microclimates, thus providing appropriate conditions for a variety of plants and animals to thrive. They can also be designed to mimic local native habitats, extending the area available for native species to colonize. They can also simulate early succession patterns of ground-level habitats, which can allow gains in biodiversity over time.
3. Urban Heat Islands. Urban heat islands are highly builtup areas that are generally warmer than surrounding rural or suburban areas. This is due to the absorption of solar radiation by buildings and other man-made surfaces and the lack of natural cooling from vegetation. Heat islands cause increased energy consumption, greater rates of heat-related illness and death, and increased air pollution. Reintroducing vegetation to urban areas through green roofs is one of the most promising solutions to mitigate the problem of heat islands.
Green roofs absorb less sunlight than dark roofs, through the process of evapotranspiration (the process by which water is transferred from the land to the atmosphere by evaporation from the soil and other surfaces) and by transpiration from plants (the process by which water exits through pores in the leaves of plants), and by providing a shading effect to buildings. In the summer, green roofs cool buildings and the air around them through both evapotranspiration and transpiration. It takes energy to turn the liquid water into vapor, and the process of evapotranspiration therefore cools the plant. This creates a cooling effect on and around buildings. The peak temperatures of both green roofs and the air above them are typically lower than for black roofs.
Studies have shown green roof surface temperatures approximately 30–40°F cooler than black roof surface temperatures in the summer. A green roof program covering at least half the roof space in a city could result in citywide cooling throughout the day and during peak summer energy demand periods, particularly when combined with street tree planting and other large-scale greening efforts.
4. Energy. Green roofs reduce a building’s energy use in hot and (to a lesser degree) cold seasons. As discussed above, the process of evapotranspiration cools green roofs in the summer, leading to reduced air conditioning needs.
In summer, green roofs can also act as an insulating layer, reducing heat flux, or the transfer of heat from a building’s exterior to its interior through the roof by up to 72%. They also make roofs more energy efficient by reducing summer air temperatures directly above a building as cooler ambient air can reduce energy consumption related to building cooling.
In the winter, the insulating effect reduces heat loss through green roofs as compared with black roofs. Green roofs can therefore also reduce building heating demand in the winter, although the marginal benefit is lessened with higher coderequired insulation levels.
It is important to note that the insulating effect is not the primary energy benefit of green roofs — the evapotranspiration effect, thermal mass, and shading effects are their primary contribution to energy savings.
5. Urban Agriculture. Compared with plants in a groundlevel field, plants grown on rooftops are less subject to damage from insects, rodents, and deer. Growing plants on rooftops also contributes to job generation. Urban agriculture can potentially increase property values and building marketability.
It can also provide easier access to fresh produce and a way to educate the local community about food production and seasonal variety. Urban agriculture may also reduce carbon emissions associated with food distribution.
6. Acoustics. Green roofs are better at absorbing sound than conventional and concrete roofs. When used on buildings without ceiling insulation, they can reduce the amount of noise transmitted inside the top floors of a building, particularly in areas with heavy air or automotive traffic.
7. Air Quality. Plants have long been used in the urban environment to remove air pollutants like carbon dioxide and carbon monoxide, smog-forming compounds, and particulate matter. A green roof’s effectiveness at improving air quality depends on the type of plant grown and the depth of soil used. For example, the greater the leaf surface area, the more particulate matter can be captured.
Because green roofs also reduce a building’s energy use, they can potentially reduce the amount of CO2 and smogcausing pollutants emitted by power plants. The reduction in the urban heat island also contributes to smog reduction.
8. Aesthetics & Quality of Life. Both intensive and extensive green roofs can create an attractive space for building occupants and views for those in neighboring buildings. Intensive roofs can offer a place of refuge and relaxation for people who work in a building, thus reducing stress and boosting worker productivity.
9. Job Generation & Economic Development. Green roofs offer potential long-term job opportunities for both skilled and unskilled workers. They can also offer building developers and owners a more marketable building as compared to those lacking green roofs. Some evidence suggests that higher rental occupancy, purchase prices, and faster sales may result from the presence of a green roof.
10. Roof Longevity. Properly installed green roofs more than double the number of years typically needed before a roof must be replaced, as compared with conventional and white roofs.
Energy efficiency and temperature reduction benefits are a key contributor to the growing popularity of green roofs in the United States. In 2016, the North American green roof industry was estimated to have grown by more than 10% over 2015, continuing the industry’s growth trend over the past decade. The global green roof market size was valued at USD $1.1 billion in 2019 and is expected to grow at a compound annual growth rate (CAGR) of 17% from 2020 to 2027.
For more information on the green roof industry, visit gsa.gov.