Green Roofs - The Trend for the Future

Environmental and Science Engineering , January 2004
Mark Hagesteijn, P.Eng. & Sam Beckett, P.Eng.

Inaccessible extensive green roofs at York University's Computer Science Building.

Green roofs, or vegetated roofs, although relatively new in Canada, have been well established in several European countries since the 1980s.

R.V. Anderson Associates Limited's first of several green roof projects was part of the site servicing and grading assignment for the Computer Science Building at York University, Toronto, constructed in 2001. For this project an inaccessible extensive green roof was selected to become part of the site's stormwater management solution. The green roof covered approximately 1,875 m2 or about 66% of the total roof area and consisted of a Sopranature green roof system with 140 mm of growing medium covered with alpine grasses. Approximately 270 m2 of the green roof was also vegetated with wild flowers.

Compared with a standard flat roof without roof drain controls, the green roof reduced the peak stormwater flow from the total roof by about 32% and the total runoff volume from the roof by about 29 m3 during the 2-year storm event. These reductions resulted in smaller on-site storm sewers and stormwater retention facilities to control the runoff from the site.

Notwithstanding these significant stormwater management benefits, as well as other environmental and social benefits of green roofs, many clients remain reluctant to invest in green roofs due to the added cost over a traditional roof. Some of the green roofs' costs will be recovered over the long term. Other costs, which will benefit the community, may not be recoverable by the building owner.

A typical green roof section consists of a waterproofing membrane with root repellent, which is overlain with a drainage layer, filter membrane, and growing medium with vegetation. The growing medium is a soil mixture designed to be light, erosion resistant, permeable and has the ability to retain water to support the vegetation.

• Accessible versus Inaccessible Green Roofs - An accessible green roof is a flat outdoor green space which is intended for use by people, and can, therefore, have pathways and play areas. An accessible green roof must also meet building code safety requirements such as live loads, guardrails and lighting.

An inaccessible green roof can only be viewed, and is only accessible for maintenance.

• Extensive versus Intensive Green Roofs - Accessible or inaccessible green roofs can be further classified as either extensive or intensive.

Extensive green roofs are characterized by their relatively low weight, low cost and low maintenance. The growing medium is typically 5 to 15 cm thick and due to the shallowness of the growing medium, plants must be low and hardy such as alpine type grasses and flowers. After the plants have been established, about three visits per year will be required for weeding, maintenance and safety inspections.

Intensive green roofs are characterized by their increased weight, increased cost, intensive planting and higher maintenance. The growing medium is typically 20 to 60 cm thick and due to the increased soil depth, the plant selection can be more diverse (including trees and shrubs). After the plants have been established, the green roofs will require regular maintenance and watering like a typical garden. Intensive green roofs are often designed as accessible roofs, because the increased soil depth allows a greater variety of landscape features, which could transform the roof into a garden or park.

There are numerous environmental and several cost benefits of installing green roofs including:

• Reduction of Urban Heat Island Effect - Typical roofs are hard, dark surfaces which re-radiate most of the sun's energy as heat, resulting in a significant increase in air temperature above the roof. Vegetated roof surfaces re-radiate only a small portion of the sun's energy as heat, resulting in a reduced increase in air temperature.

The outside air temperatures will be further reduced due to the heat insulation properties of green roofs. This insulation property results in less energy being required to cool the building, which will result in less hot air being emitted from the mechanical cooling systems.

Typical section of a green roof. Source: National Research Council,
Institute for Research in Construction.

• Improvement of Air Quality - Green roofs reduce outside air temperature during the summer, which reduces rising hot air, and thereby reduces the suspension of dust particles from the ground, hence improving air quality.

The air quality will be improved further by the vegetation's ability to filter the air, removing dust particles, and through photosynthesis when the vegetation converts carbon dioxide into oxygen. Also, the trapped air within the plants, growing media and drainage panels will act as a heat insulator, and the prevention of the sun's energy from making direct contact with the building roof, will reduce the building's internal temperature fluctuations. This reduction in temperature fluctuation will reduce the energy required to heat and cool the buildings thereby reducing greenhouse gas emissions.

Green roofs could extend the lifespan of the roof membranes, because green roofs protect the membrane against ultra-violet radiation, extreme temperature fluctuations, and physical damage from roof maintenance or use. The increased lifespan reduces roof membrane replacements or repairs and thereby reduces waste sent to the community's landfill site.

• Reduction of Storm water Runoff Standard flat roofs with roof drain controls can reduce the runoff rate and, in some degree, delay the runoff. However, green roofs with their ability to retain water, can also reduce the runoff volume.

The National Research Council in Ottawa conducted research which demonstrated that the total roof runoff volume from a green roof during the summer of 2002 was approximately half the runoff experienced from a standard flat roof. This significant reduction in runoff volume will reduce the flow in the storm sewers and creeks, and could, therefore, reduce the incidence of surcharged storm sewers, combined sewer overflows and creek erosion.

Currently, the Toronto and Region Conservation Authority is also conducting a similar study on the green roof of the Computer Science Building at York University. The results, which are expected in the spring of 2004, will be used by the City of Toronto to interpret the benefits of green roofs over large urban areas.

Green roofs are a relatively easy method to improve the aesthetics of a building. The improved building aesthetics will be valued the most by those residences or employees who can look down onto the green roof or visit an accessible green roof.

Jobs will be created in design, growing, manufacturing, installation and ongoing maintenance of green roofs.

Contact with nature is believed to reduce the stress of urban living, which could lead to a healthier and more productive community.

The additional green space within a city could make an urban area more attractive to live in and could thereby reduce commuter traffic and related greenhouse gas emissions.

Green roofs can be designed to mimic different habitats but the type of habitat will depend on factors such as depth of growing medium, exposure and degree of irrigation. Also strategically located green roofs could connect isolated habitats and other green roofs by air, thereby creating wildlife corridors for birds within urban areas.

An inaccessible extensive green roof with a 150mm thick growing medium covered with alpine grasses generally costs about $12/ft2 more than a standard flat roof. This additional cost excludes structural upgrades which may be required to support the green roof. An accessible intensive green roof could be three times the cost of an inaccessible extensive green roof. The additional cost to construct green roofs is perceived as one of the main barriers which green roofs must overcome to become a standard practice in the roofing industry. Such costs would likely reduce as the use of green roofs increases.

Roofs within cities appear to be an untapped resource which could improve living and working conditions. Some cities and countries have realized this resource and have provided incentives and/or regulations to encourage the construction of green roofs.

For example, in Germany over 40% of the cities are now providing direct incentives and/or using regulations in support of green roof implementation. Some cities in Germany passed municipal by-laws which require the installation of green roofs on all flat-roofed industrial buildings, and other cities compensate builders up to $6.20/ft2 for the green roof. These incentives and regulations resulted in the construction 6f 13.5 million square metres in 2001, which is up from the 9 million square metres constructed in 1994. Currently, about one in ten flat roofed buildings in Germany have green roofs.

In Tokyo, law requires that all buildings with more than 1,000 m2 of floor space need at least 20% of their roofs to be greened to counter the Urban Heat Island Effect.

In Portland, Oregon, incentives to encourage the construction of green roofs came into effect in 2001. The incentives provide developers with additional floor area based on the proposed area of the green roof.

In Chicago, City Council passed its first Energy Conservation Ordinance which came into effect in 2002. This ordinance includes a chapter which requires all new and replaced roofs to meet minimum standards of solar reflectance and emissivity using ASTM testing methods. This ordinance can be met by installing green roofs.

In the Province of Quebec, on September 26, 2003, the Quebec Energy Board approved a $1/ft2 incentive for green roof implementation to promote green roofs as an energy conservation measure.

In Toronto, in September 2003, City Council voted to establish a Green Roof Task Force to investigate and recommend policies and incentives to encourage the construction of more green roofs on new buildings and retrofits of existing buildings. The task force will make their recommendations to City Council in January 2005.

North American cities have become more interested in the benefits of green roofs and as a result are developing incentives and/or regulations to encourage their construction. It is believed that this trend will continue and expand in North America.

Building owners, particularly in cities like Toronto where incentives and/or regulations are considered, should prepare themselves for the potential green roof requirements" specified by the City. Designing the new building structure to support the future green roof, and providing a maintenance access to the future green roof can accomplish this. When this is not incorporated during the design stage of the building, it could become an expensive modification to an existing building.

However, once the building layout and structure are designed for future green roofs, the building owner will have the opportunity to gain the benefits of the green roofs at a minimum cost when the City's incentives and/or regulations for green roofs are implemented.