CLIMATE CHANGE AND GREEN ROOFS – The example of three cities
Climate change is presenting many challenges to cities around the world and they will have to cope with decades of rising temperatures and changing rainfall patterns, among other effects. Some world cities have started to prepare for climate change and are looking for suitable adaptation measures to put in place to manage its impacts. This article looks at how green roofs can be an effective mechanism to reduce the impacts of climate change in urban areas. Green roofs are vegetated roofs, or roofs with vegetated spaces. Three case studies are provided to highlight how green roofs are being implemented in cities.
Changing climate risks and the benefits of green roofs
Our climate is changing due to man-made emissions of greenhouse gases, and we are faced with many years of continuing unavoidable change. Climate change is presenting many challenges to cities around the world and they will have to cope with its impacts. Even if we were to make significant reductions in greenhouse gas emissions tomorrow, the inertia in the climate system means that we will need to cope with a changing climate for the next 40-plus years, due to emissions we have already put into the atmosphere. Action needs to be taken both to reduce our emissions (mitigate) and adapt to inevitable climate change. There is no choice between mitigation and adaptation – complementary actions on both need to be pursued. Some world cities have started to prepare for climate change and are looking for suitable adaptation measures to put in place.
In Europe, climate change is resulting in:
* Hotter summers,
* Milder winters,
* Changing rainfall patterns, with increasing winter precipitation in many regions and, in some areas, reductions in summer rainfall,
* More frequent extreme high temperatures,
* More frequent heavy downpours of rain,
* Sea level rise and increases in storm surge height,
* Possible higher wind speeds.
Climate change is already leading to increasing temperatures. Higher average temperatures are being experienced and also more extremely hot days. Summer heatwaves are becoming more frequent and more intense. Higher temperatures are aggravated in cities by the Urban Heat Island (UHI) effect. Green roofs are one mechanism by which these overheating risks can start to be addressed. Green roofs reduce summer overheating by reducing building heat loss and increasing evapotranspiration. By providing cooling in summer and thermal insulation in winter, they also result in reduced energy consumption, greenhouse gas emissions and fuel costs.
Climate change is also increasing the magnitude and frequency of intense rainfall events that cause flash flooding. The loss of natural infiltration of rainfall into soil in urbanised areas exacerbates this risk. Green roofs provide a mechanism by which this risk can start to be addressed and are considered a Sustainable Urban Drainage System (SuDS) technique. They provide an alternative to traditional piped drainage approaches to managing runoff from buildings, mimicking natural drainage patterns and attenuating surface water runoff. This has benefits for developers as the number of drainage outlets required on a building can be reduced.
Other benefits of green roofs include creating natural green spaces in urban areas, benefits for biodiversity and reduced air pollution. Extended roof life is a further advantage, since a green roof protects a roof’s waterproofing membrane, almost doubling its life expectancy.
Three examples of cities where green roofs have been implemented are briefly described below. The drivers behind these initiatives have not solely been climatic, but climate concerns have been a factor and will provide more of an imperative for green roofs in the future.
The city of Basel in Switzerland began constructing green roofs in the 1970s. The main drivers behind their implementation were to assist in reducing storm-water runoff and overheating, deliver energy savings, and benefit nature conservation. There have been two campaigns in Basel to provide subsides for installing green roofs. Basel’s Building and Construction Law was amended in 2002 to require all new buildings with flat roofs to have green roofs. This amendment was made following research demonstrating the potential for green roofs to support biodiversity and species conservation. Specific guidelines for their implementation have been developed, which detail how to maximise their nature conservation potential.
Before the first green roof campaign a study was undertaken that evaluated the benefits of green roofs. The results showed that green roofs provided benefits both in terms of reduced energy needs for heating in winter, as well as keeping buildings cooler in summer. SFr14,000,000 (€8,763,000) was invested in green roofs, initiated by funding from the first campaign of SFr1,000,000 (€626,112).As a result of the first campaign it has been estimated that approximately 4 million kWh of energy is saved each year in Basel(1).
In 1998, an analysis was undertaken of the area of flat roofs in Basel, and of the percentage of these that had green roofs . The analysis showed that 10% of the city had flat roofs and of these flat roofs, about 0.29 km2 (290,000 m2) had green roofs. Approximately one-third of these green roofs (85,000m2, equivalent to 8 football fields) had been constructed on some 120 buildings as a result of the first green roofs campaign, the remaining two-thirds having been in existence prior to the campaign. A second green roofs campaign has led to additional 10,000m2 (2) of green roofs being installed onto existing buildings. This retrofitting is still continuing and at present, approximately 20% of Basel’s flat roof area is green roof.
A key feature of Basel’s green roofs is that their design criteria stipulate the creation of different habitats, with varying substrate thickness, as well as requiring that the roofs use natural soil from the region. By varying the substrate depth, different habitat conditions are created. As the roof evolves over time, an increasing number of species colonise it (3). These green roofs may help to provide ‘corridors’ for species’ movements in the face of climate change.
Basel has been successful at implementing green roofs and experience has shown that this success has been achieved by close cooperation between the local authority and nature conservation experts, as well as architects, construction and landscape planners, green roof companies and contractors.
Linz is the capital of Upper Austria, one of the nine counties of Austria, with about 180,000 inhabitants. Green roof policies were introduced in 1985 as part of its Development Plan. The main driver behind this policy was to counteract the large and rapid loss of green areas in the city to commercial and industrial development and to address air quality concerns. The city’s 1984 Green Space Plan included objectives to increase greening in built-up areas. At present, Linz has about 400,000 m2 of green roofs.
The city provides subsides for green roofs as long as the roofs are properly maintained. Concerns were raised initially about the fire risk of green roofs during hot, dry summers but since the start of the green roof policy in Linz there has been only one fire accident on a green roof in Germany. The incident occurred due to poor maintenance as there had been a long dry period and the grass had been left uncut. Technical rules were developed after the accident and obligatory regulations for issues like maintenance, distance between plant areas and buildings have been passed to prevent such accidents happening again.
Since the inception of the subsidy program in 1989 until the end of 2001, 237 projects received green roof subsidies. At present, Linz has about 400,000 m2 of green roofs and subsidies totalling Euros 4.77 million have been provided (4).
Toronto has a very new green roof strategy, which was implemented in early 2006 (5). The City recognised that green roofs could provide benefits including managing stormwater runoff, saving energy, reducing the urban heat island effect, and adding more green to the built environment. The council is committed to installing green roofs on new and existing buildings owned by the City wherever practical to do so. For example, green roofs are to be considered for existing municipal buildings when roofs are due to be replaced. For new City-owned buildings, the Green Roofs strategy sets a target of green roofs covering 50% to 75% of a building's footprint. The strategy also includes a pilot programme of financial incentives to assist others in constructing green roofs. A cost benefit analysis for green roofs was undertaken prior to commencement of the programme. This analysis was based on the assumption that 100% of available green roof area would be used across the city. The total available green roof area city-wide was determined to be 5,000 hectares (50 million m2), which is 8% of the total land area of Toronto. It demonstrated the following benefits (in Can$) (6):
Benefits from stormwater flow reduction:
* Infrastructure savings (i.e. infrastructure measures that could be replaced by green roofs, including pervious pavements and underground stormwater storage) worth $79 million,
* Erosion control measure savings worth $25 million,
* Pollution control cost avoidance worth $14 million.
Benefits for Combined Sewer Overflows (CSOs):
* Reduced requirement for underground water storage, saving $46.6 million,
* 3 additional "beach open" days per year, worth $750,000.
Air quality benefits:
* Reduction in levels of pollutants with an associated value of $2.5 million in terms of improvements to health and the environment,
* Reduction in carbon dioxide emissions.
Benefits from reduced building energy consumption:
* Cost avoided due to reduced demand at peak times of $68.7 million,
* Citywide savings from reduced energy for cooling of about $21 million per year.
The reduction in the UHI was estimated to have the following benefits:
* Reduced local ambient temperatures by between 0.5 to 2ºC, depending on the time of year,
* Citywide savings from reduced energy for cooling of $12 million, equivalent to 2.37 kWh/m2 of green roof per year,
* Cost avoided due to reduced demand at peak times of $80 million.
(1) Stephan Brenneisen, 2006, Pers.Comm,
(3) Brenneisen, S. ‘Green Roofs and Biodiversity – International Context.’ Contribution to conference: Delivering Sustainable Buildings, 21-22 April 2005 Birmingham.
(4) Linz. 2002. Grünflächenplan der Stadt Linz 2001. Available from Magistrat
der Landeshauptstadt Linz, Planungsamt.
(6) Banting, D., Doshi, H., Li, J., Missios, P., Au, A., Currie, B.A., Verrati, M.
October 31, 2005. Report on the Environmental Benefits and Costs of Green Roof Technology
for the City of Toronto, Dept. of Architectural Science, Ryerson University,
Toronto, Ontario, Canada.
Amy Hutchins, Risk Manager