Green Roofs: Ecosystem Services and Research Frontiers

Green roofs are roofs with a vegetated surface.  In their simplest forms--called “extensive” roofs--they consist of an insulation layer, a waterproofing membrane, a layer of growing medium, and a layer of plants.  These extensive green roofs require as little as 2 cm of substrate, need little to no maintenance, and can generally be installed on buildings with  standard roof weight-bearing parameters (an additional 70-170 kg per m²) (Dunnet and Kingsbury 2004).  Extensive green roof plants are usually stress-tolerant, shallow-rooting species that can tolerate fluctuations in temperature and moisture levels.  Many of these plants are arctic-alpine in origin, such as Sedum and Sempervivum species.

Green roofs are also known as living roofs, since they are not always green.  They come in as many colours as a ground-level garden, given adequate structural support, substrate depth, and irrigation.  These more elaborate roof gardens are known as “intensive roofs,” and they have a venerable history:  the earliest documented roof gardens are the hanging gardens of Semiramis in what is now Syria, considered one of the seven wonders of the ancient world.  Extensive green roofs are a more modern adaptation of the ancient concept, and have their origin in early 20th century Germany as fire-retardant structures (Köhler 2003).  Germany remains the world leader in green roof coverage, with 13.5 million square meters (m²) of new green roofs added every year.

Ecosystem Services
The “green” in green roof refers as much to the ecological services the roofs provide as it does to the plants.  These ecosystem services stem from the components of the roof:  plants, substrate, and waterproofing membrane.  Benefits are derived at the level of the individual building, the city or municipality, and in less quantifiable arenas of human health and “liveability” of cities.

Energy Conservation
One of the foremost benefits of green roofs is energy conservation.  Green roofs reduce heat flux through the roof, which reduces building heating and cooling costs.  The plants and soil physically insulate and shade the roof, which also protects and extends the lifespan of the roof membrane.  In summer, evapotranspiration promotes cooling.  Research in Japan has shown that green roofs reduce heat flux by 50% per year (Onmura et al 2001).

Stormwater Management
Most cities are characterised by hard, non-porous surfaces that impede groundwater recharge after rain events.  Instead, excess water flows into sewers, where it must be treated at waste-water facilities before being released into lakes and rivers.  Green roofs reduce the amount of run-off after rain events by temporarily storing water on the roof and releasing it slowly over time.  A green roof also returns moisture to the atmosphere by evapotranspiration.  Green roofs are an ideal storm-water management technology because, unlike surface-area intensive storage reservoirs, constructed wetlands and sand filters, they require only existing roof space.  Depending on substrate depth, green roofs have been shown to retain 25-100% of rainfall. (Beattie and Berghage 2004), and reduce annual total building runoff by 60-79% (Köhler et al 2002).

In addition to creating stormwater management problems, impervious and dark surfaces in cities also contribute to the urban heat island effect, the tendency of urban areas to be up to 8ºC hotter than surrounding rural areas.  Replacing some of these surfaces with green roof vegetation increases evapotranspiration and cools cities.  One regional simulation model assuming 50% green roof coverage predicted temperature reductions of up to 2ºC in the city of Toronto (Bass et al 2003).

Urban Habitat Values
Green roofs provide important wildlife habitat in cities.  They are used by nesting and native birds, as well as insect and spider populations (Baumann 2006; Coffman and Davis 2005).  Rare species of beetles and spiders, as well as plants and lichens, have all been found on green roofs (Brenneisen 2006).  In addition to promoting plant and animal diversity, green roofs promote health benefits for city dwellers, such as relaxation, aesthetic visual relief, and stress-reduction (Hartig et al 1991).  Accessible green roofs augment living space and can be used for gardening and urban agriculture.  

Frontiers in Green Roof Research
Most green roofs use random assemblages of plant species from different regions of the world.  There is interest in using coevolved species from local, native habitats as green roof communities to promote local biodiversity, and possibly to achieve greater resource use and green roof functionality.  Early testing has shown that some native species cannot tolerate the stressful environmental conditions and shallow soil depth of some green roofs (Monterusso et al 2005), but much more research is needed in this area.  Likewise, the role of biodiversity in green roof performance warrants more investigation.

Another avenue for further research is the contribution of green roofs to air quality improvement.  Because of their deeper substrate, intensive green roofs show promise in offsetting carbon emissions, but the role of green roofs as urban carbon sinks has not been documented.  Green roofs could also potentially improve stormwater run-off quality if relatively inert substrates were used and plants were selected for their ability to uptake and store nutrients and contaminants.

Finally, it is important to debate the relative merits of green roofs as compared to other technologies.  For example, high-reflectance roof membranes may be superior to green roofs in reducing the urban heat island effect.  On the other hand, such membranes do not contribute to stormwater management or create urban wildlife habitat.  Green roofs may in fact perform more optimally when used in combination with other green technologies, such as photovoltaic panels.  We must consider the synergistic effects of using multiple green building technologies and work towards a holistic approach to solving urban environmental challenges.  Similarly, we must extend the value of such technologies beyond simple cost-effectiveness and technical efficiency, and consider benefits that are often missing from cost-benefit models, such as human health benefits, employee satisfaction, and increased community space.

References:

1. Bass B, Krayenhoff ES, Martilli A, Stull RB, Auld H. 2003.The impact of green roofs on Toronto’s urban heat island. Pages 292–304 in Proceedings of the First North American Green Roof Conference: Greening Rooftops for Sustainable Communities; 29–30 May, Chicago. Toronto (Canada): Cardinal Group.
2. Baumann N. 2006. Ground-nesting birds on green roofs in Switzerland: Preliminary observations. Urban Habitats 4: 37–50.
3. Beattie D, Berghage R. 2004. Green roof media characteristics: The basics. Paper presented at the Second Annual Greening Rooftops for Sustainable Communities Conference, Awards and Trade Show; 2–4 June 2004, Portland, Oregon.
4. Brenneisen S. 2006. Space for urban wildlife: Designing green roofs as habitats in Switzerland. Urban Habitats 4: 27–36. (14 August; www.
urbanhabitats.org/v04n01/index.html)
5. Coffman RR, Davis G. 2005. Insect and avian fauna presence on the Ford assembly plant ecoroof. Paper presented at the Third Annual Greening Rooftops for Sustainable Communities Conference, Awards and Trade Show; 4–6 May 2005,Washington, DC.
6. Dunnett NP, Kingsbury N. 2004. Planting Green Roofs and Living Walls. Portland (OR): Timber Press.
7. Frazer L. 2005. Paving paradise. Environmental Health Perspectives 113: 457–462.
8. Hartig T, Mang M, Evans GW. 1991. Restorative effects of natural environment experience. Environment and Behavior 23: 3–26.
9. Köhler M. 2003. Plant survival research and biodiversity: Lessons from Europe. Paper presented at the First Annual Greening Rooftops for Sustainable Communities Conference, Awards and Trade Show; 29–30 May 2003, Chicago.
10. Köhler M, Schmidt M,Grimme FW, Laar M, de Assunção Paiva VL,Tavares S. 2002. Green roofs in temperate climates and in the hot-humid tropics—far beyond the aesthetics. Environment and Health 13: 382–391.
11. Monterusso MA, Rowe DB, Rugh CL. 2005. Establishment and persistence of Sedum spp. and native taxa for green roof applications. HortScience 40: 391–396.
12. Onmura S, Matsumoto M, Hokoi S. 2001. Study on evaporative cooling effect of roof lawn gardens. Energy and Buildings 33: 653–666.