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Green Roofs



Faced with increasing environmental challenges such as hotter summers and large storm events, many cities in North America are following Europe’s example and implementing green roofs as part of their urban greening strategy. Though more common here than a few years ago, green roofs are still new enough that not everyone is sure what they are. Below are answers to commonly asked questions about green roofs that provide a general overview. For more detailed information and resources, please see the resources listed below or click on the links provided.


A green roof is a layer of vegetation on a roof, as opposed to planters on a deck, which is more commonly called a roof garden. Though this definition has been blurred a bit in recent years with the advent of green roof “blocks”, or low-level containers that when placed side-by-side form a continuous layer of vegetation but are easier to maintain, most green roofs are still some kind of layer of vegetation on a roof. There are many variations, but the most standard green roof structure consists of:


Growing medium

Drainage/Filter membrane

Root barrier

Waterproofing Membrane

Click here to see a diagram of a typical green roof structure:


There are three main types of green roofs- extensive, intensive and semi-intensive. The easiest to install due to the lighter weight are extensive green roofs, whose depth of material is 6” or less. The type of vegetation on extensive green roofs are most often sedums, or small, hardy alpine and desert plants, though sometimes grasses can also be used. Extensive green roofs are often inaccessible.

Extensive green roof over parking garage in Germany- Xeroflor
Extensive green roof over parking garage in Germany- Xeroflor

Semi-intensive green roofs are a little thicker, usually a bit above or below 6”. They can have small bushes, grasses, and a range of flowers, and are sometime accessible.

Intensive green roofs have a much thicker depth of material- over 6”- and can support trees, shrubs, and many different types of vegetation. Often these types of green roofs are over parking garages or transit and may be at ground level, such as Millennium Park in Chicago, or Vancouver’s Robson Square, and are usually accessible.

Semi-intensive green roof- Ontario
Semi-intensive green roof- Ontario

Because green roofs need to be structurally supported, one of the most important considerations is the weight of the green roof when fully saturated. The heavier the green roof, generally the more expensive, though this cost is reduced if the green roof is part of the original design and the roof is designed to withstand the added weight.

Kunterwasser House Intensive Green Roof
Kunterwasser House Intensive Green Roof


There are numerous benefits of green roofs, most of which fall under public benefits, such as a reduction in air pollution, or private benefits, such as reduced heating and cooling costs. Below is a brief outline of the most common benefits.


Reduction of the Urban Heat Island effect and associated cost savings

Green roofs can reduce the urban heat island effect (UHI) by keeping the surfaces of roofs cooler than a black tar roof. For example, the building that houses the Chicago City Hall on one side and the Cook County offices on the other is only covered by a green roof on the City Hall side. When temperatures reach 150 degrees Fahrenheit (F) (65 degrees Celsius (C)) on the Cook County side of the roof, the City Hall green roof is only 90 degrees F (32 degrees C) (City of Chicago). This cooling benefit of green roofs also reduces the cooling costs of buildings, particularly on the upper floors. Keeping cities cooler in turn reduces the electricity demand during hot summer days, thus reducing the load of coal-fired plants and reducing the chance of black-outs.

A study done by Environment Canada found that a 25% green roof coverage could reduce the UHI by up to 1.8 degrees F/1 degree C for 1/4 of the city; while a 50% coverage increased this to 3.6 degrees F/2 degrees C. Another Environment Canada study found that the energy demand associated with even a 1 degree C/1.8 degree F increase in temperature is 600 MW (Doshi et al 2006), while the U.S. Environmental Protection Agency (EPA) found that in cities with a population over 100,000 the increase in peak demand electricity was 1.5-2% for every temperature increase of 1 degree F/0.6 degree C (EPA 2002). What does this all mean? At 2005 energy rates, the cost of the UHI effect on electricity demand could be as much as USD 1,000,000/CAD 1,174,000 per hour, or over 1,000,000,000/CAD 1,174,500,000 annually (EPA 2002, cited in GRHC 2005 Policy Workshop manual).

Reduction of Air Pollution

Vegetation, and in particular the urban forest, has long been shown to absorb particulate matter and improve air quality. Researchers in Toronto have used an urban forestry model to estimate the equivalent air pollution mitigation possibilities of green roofs, and found that green roofs, when combined with grasses, shrubs and trees, could play a significant role in mitigating urban air pollution (Bass and Currie 2006). If only 10% of Chicago’s roofs (6540 ha) were greened, 17,400 tons of NO2/y would be removed, resulting in city-wide public health benefits from reduced air pollution of $29.2 million to $111 million annually. (Clark et al 2001 ).

Stormwater Management: quality and quantity

Green roofs reduce the quantity of stormwater that runs off the roof during rain events by absorbing up to 60% of total runoff and up to 85% of the first wave of rainfall (Moran et al 2004) (Hutchinson et al 2003). By reducing the amount of stormwater runoff through green roof absorption, cities can reduce the pressure on ageing pipes, reduce stormwater overflow, and save potentially millions per year on avoided big pipe solutions or upgrades. It is estimated that if all of New York’s flat roofs were greened they would divert 13 billion gallons of stormwater that would otherwise head into the city’s ageing sewer system (Norquist 2008).

Green roofs also improve the quality of stormwater runoff and have been found to filter many of the metals, polycyclic aromatic hydrocarbons (PAHs) and other pollutants (Clark et al 2001).

Increased food security

Green roofs have the potential to grow food, and have been used by both community groups looking for additional garden space, as well as restaurants and hotels who use the roofs to provide fresh herbs and lettuces.

Roof Garden on Earthpledge's offices, NYC
Roof Garden on Earthpledge’s offices, NYC

Increased greenspace in the city

There are many benefits association with an increase in greenspace in the city; a few of the major ones are outlined below.

Increased liveability

Many studies have been done outlining the increased liveability and quality of life of neighbourhoods with access to greenspace. Access to greenspace increases property values, fosters a sense of community, and has numerous health benefits (see references below).

Biodiversity and habitat

There has a been a recent interest in the potential for green roofs to link with other urban natural corridors and provide islands of biodiversity and habitat. This is particularly true for dense urban cores where there is little, if any, greenspace. Providing habitat has become important since many cities have committed to intensifying land use through infill development. Legislation which protects the habitat of the Black Redstart in London, England, helped to promote green roofs in London. As vacant lots that had become the home of the Black Redstart were being developed, crusaders such as Dusty Gedge argued for rubble or green roofs as an alternate habitat for the birds, spearheading the green roof movement in the UK (Gedge 2004). Stephan Brenneisen, out of Switzerland, has done research for years on biodiversity and green roofs and found that they can be provide excellent habitat to birds and invertebrates (Brenneisen 2003). There is also a movement to use moss and microbes as a more ecologically-viable foundation for green roofs and biodiversity.

Social and Health Benefits

There is little published research on the social or health benefits of green roofs per se. Most of the research currently cited is based on work done by social scientists that has shown that contact with nature improves our health, well-being, and socialization and community, and then extended to apply to green roofs. Green roofs have been put on hospitals, condos and women’s shelters to provide healing gardens, amenity space, and safe play areas when greenspace on the ground is either not feasible or available. My doctoral research examines the potential health and well-being benefits of green roofs that are visually or physically accessible to office workers in the central business districts of Chicago and Toronto. The results will be out in the summer of 2009. Please see a list of resources below for more information on the health benefits of nature.

Private benefits

Energy Efficiency

Green roofs, by providing an extra layer of insulation on the roof, have proven particularly effective at reducing heat gain in hot summer months. A study by the National Research Council of Canada found that a green roof reduced the daily demand for air conditioning by 75% (Liu et al 2003). Due to a more moderate roof temperature, green roofs also increase the effectiveness of HVAC (heating, ventilation and air conditioning) systems, as well as solar panels and photovoltaics, all of which operate more efficiently at moderate temperatures (Leonard and Leonard 2005).

Extension of roof life

A green roof covers the roof membrane which otherwise would be subject to harsh weathering and more rapid deterioration. Typical roofs in North America have an average lifespan of 10-15 years (Hutchinson 2001). Protected membranes, which are buried beneath insulation, have lifespans closer to 20-25 years, and this is considered to be closer to the lifespan of roof membranes under a green roof (Mutton 2004).

Noise Reduction

Green roofs, through their insulating properties, can reduce sound by between 40-50 decibels, depending on the type of green roof (Peck et al 1999, cited in Green Roof Policy Development Workshop, GRHC, 2006).


Green roofs are a highly visible symbol of a green project, and many companies and cities have found them to be an effective way to point to other green initiatives they may be engaging in that are less visible.


The cost of a green roof varies, but an extensive green roof can run between $10-24 a square foot ( Usually the thicker the depth of the green roof, the more expensive it is, particularly if the building needs to be structurally reinforced to support it.


Yes, usually from a municipality who is trying to encourage the implementation of green roofs. Chicago and Toronto, for example, both have Green Roof Grant Programs that helps offset the cost of green roofs to selected winners, while Chicago also has fast-track permitting and other incentives for developments that are green.

Click here to go to the City of Chicago’s Green Roof Policies and Programs:

Click here to go to the City of Toronto’s Green Roof Policies and Programs:


Yes- green roofs are one option that developers and architects can use to reduce stormwater runoff, decrease the urban heat island effect, and provide native landscaping under the U.S. Green Building Council’s LEED system (Leadership in Energy and Environmental Design). They can also sometimes be used as innovation point. Green roofs can get a development up to a maximum of 15 points when in conjunction with other green building technologies for the following:

  • Reduced Site Disturbance, Protect or Restore Open Space
  • Landscape Design that Reduces Urban Heat Island Islands, Roof
  • Stormwater Management
  • Water Efficient Landscaping
  • Innovative Wastewater Technologies
  • Innovation in Design

Click here to be directed to the U.S. Green Building Council’s (USGBC) LEED for New Development criteria:


Green Roof References

References on Benefits of Nature

Green Roof Associations:

Chicago’s Green Renaissance

*Due to demand I’ve re-posted this page- enjoy!

Mayor Daley’s leadership puts Chicago on the map for urban greening initiatives

Chicago has become nationally known for its leadership in the environment…. It improves public health; it beautifies the city; it enhances the quality of life; it saves money; and it leaves a legacy for future generations.
Mayor Daley, (City of Chicago: Department of the Environment 2006)

Chicago has been getting greener thanks in large part to the leadership from Mayor Richard M. Daley. Traditionally known more for its architecture, gritty industrial roots and gangster history, Chicago has been showing a greener side to both visitors and locals alike. In addition to having one of the largest public-access waterfronts in North America, Chicago has recently been adding seasonal planters to main boulevards, planting millions of trees, greening schoolyards, increasing parkspace, and greening rooftops.

Chicago Cultural Centre green roof overlooking Millennium Park
Chicago Cultural Centre green roof overlooking Millennium Park

Originally intended to beautify the city, the City is realizing that greening the city provides multiple benefits that go beyond aesthetics. Green initiatives are now a cornerstone of Daley’s administration, and have won him both international leadership awards and recognition. Daley was initially criticized for spending money on planting trees when some of Chicago’s poorest and most segregated neighbourhoods face significant challenges in violence and unemployment. In light of both the multiple benefits these urban greening projects provide and the international recognition they garner, however, even some of Daley’s critics admit that these greening initiatives have proven to be a powerful symbol of change and revitalization.

Green = Investment and Revitalization

Adding greenspace to the city has proven to be a smart approach to signify investment and pride in neighbourhoods in Chicago. Developers hunting for the next up-and-coming neighbourhood keep a close eye on investment by the City in the form of planters along boulevards, upgraded parks, and street beautification. With the largest number of TIF (Tax Incremental Financing) districts in the U.S., City Hall has also been able to mandate green roofs, increased greenspace and green building features on schools and new developments that receive money from the city. Landscape ordinances mandate that parking garages be covered with vines or vegetation, while laneways are being greened through the City’s Green Alley Program. Bringing this all together is the City’s new Green Urban Design Guidelines (GUD) (2007) that form the first comprehensive, interdepartmental greening plan. The GUD lays out urban greening criteria based on hydrology, neighbourhood, air quality and other factors that influence the required level of greening for a particular devleopment. Chicago has also been selected to pilot one of a handful of Leadership in Energy and Environmental Design Neighbourhood Design (LEED ND) projects in the U.S. which aims to blend green building and smart growth objectives. With these initiatives urban nature is beginning to be equated with revitalization, care, and investment, turning a midwest city into a symbol of progress, innovation, and health.

Along Michigan Avenue
Along Michigan Avenue

Green Roofs

“We do this not because it’s fashionable, but because it makes sense. It improves public health; it beautifies the city; it enhances the quality of life; it saves money; and it leaves a legacy for future generations.”

Mayor Richard Daley

Perhaps the most famous of Daley’s greening initiatives are Chicago’s green roofs. Leading the way with an award-winning green roof on their City Hall, Daley’s tough green roof incentives and requirements have made Chicago the North American leader for green roof implementation for four years running (Green Roofs for Healthy Cities). Known for their ability to reduce stormwater runoff and reduce the urban heat island effect, green roofs have also proven to be perhaps the most symbolic of all Daley’s initiatives. Other cities wishing to implement green initiatives have admired Daley’s use of green roofs as a symbolic figurehead for his other greening initiatives, and is perhaps a good lesson in public perception. Green roofs are sexy and innovative, and easily capture the public imagination. The most famous green roof in Chicago may not even be recognized as one- Millennium Park.

The multi-million dollar green roof/park that covers sections of the railway that had previously cut off part of the lakefront from downtown has boosted tourism and become a centerpiece of the Loop. Combined with favourable incentives to bring housing downtown, the loop has for the first time been seen as a popular place to live and the residents are moving in. Though green roofs have also been criticized as merely symbolic, they are but one of a number of features that can make a building green* and have paved the way for further greening initiatives such as the current LEED-Gold standard for all new City buildings and LEED certified for all renovations for City buildings.

View from green roof of the Chicago Center for Green Technology, first LEED Platinum building
View from green roof of the Chicago Center for Green Technology, achieved LEED Platinum

Still dependent on coal and suffering from raging sprawl outside the city like many other North American cities, along with a still-lagging recycling program, Chicago still has a long way to go before being considered truly “green.” With the increasing awareness and acceptance of Daley’s green agenda by Chicagoans and the international community alike, however, Chicago may be a good lesson for other cities wishing to move green initiatives forward- don’t forget the vegetation in green.


Chicago’s Green Roof and Green Building Policies

Chicago’s Green Roof Policies

Chicago’s Environmental Initiatives

Urban Land Institute Healthy Places report

The Urban Land Institute has prioritized health and place for the next two years. As part of this process, they have published Intersections, a comprehensive report on the link between health and place. Topics covered include access to healthy food, encouraging active living, transportation, and materials. BHP_Intersections2013_Cover-230x300You can download the report for free here.

Prairie-style green roofs may be more fascinating, promote more creativity, problem-solving

Research on the aesthetics of green roofs indicates that wilder, more ‘messy’ style green roofs may not always be as well-liked, but tend to be more fascinating, more associated with ‘nature’ and nature experiences, and promote creative thinking and problem-solving. See the synopsis video here.

Though this 'prairie-style' green roof may look messy and unkempt, it may also help your well-being and promote creative thinking and problem-solving
Though this ‘prairie-style’ green roof may look messy and unkempt, it may also help your well-being and promote creative thinking and problem-solving

Further research links exposure to glyphosate to premature birth

Further research has shown that exposure to glyphosate- a key ingredient in Monsanto’s Round Up Ready- is linked to premature birth rates and lower birth weights, all of which have been associated with long-term health problems. It is due to studies like these that the International WELL Building standard includes precautionary water testing for pesticides under their Water Feature. You can read the full article here.

Research finds that key to creative energy and productivity is real time away and rest- both internal and external

Interesting research is demonstrating what we have suspected for a while- that it isn’t enough to just stop working to really recharge (especially if we keep thinking and worrying about work while ‘resting’). What is really needed is both internal rest, such as doing something mindless and restful, like daydreaming, reading a fiction book, etc, and external rest, such as gardening or hanging out with friends. This counteracts the prevailing American ideal of work and productivity, which views constant work, sleep sacrifice, and burnout as hallmarks of productivity. Recent books such as Arianna Huffington’s Sleep Revolution, Carl Honore’s In Praise of Slow , and the WELL Building Standards’s MIND Feature  have challenged this ideal, showing in fact that this constant busyness is counterproductive and ineffective. This research supports this growing trend of acknowledging the role of rest for renewal and productivity, and health and well-being. See the full article here.

Chemical exposure during childhood linked to cancer

about-the-smart-human-image-300x269New research is solidifying the link between childhood chemical exposure and the increased risk of cancer, often from daily household products. Limiting exposure by using products known to be less harmful and can help reduce this risk. Standards such as the Living Building Challenge Red Petal list, and the WELL Building Standard, are addressing these risks to health. See the full article here.

Views of green roofs can have cognitive benefits in less than 40 seconds


A new study published in the Journal of Environmental Psychology has found that even simulated views of green roofs improved cognitive tasks completion in participants in 0-40 seconds. Participants were given a difficult cognitive task, and then half were shown the image on the left of a concrete roof, and have the image on the right, of a green roof in flower. Those who saw the concrete roof performed worse on the cognitive task the after viewing the concrete roof, while those who saw the green roof image performed better. Researcher Kate Lee reports: “this indicates higher sub-cortical arousal and better attention control after viewing the green, rather than the concrete roof.” Click here to see the original article, and here for the journal.