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Green Roofs

Green roofs (also known as green roof tops, vegetated roofs, planted roofs, rooftop gardens, or eco-roofs) utilize a variety of techniques for growing vegetation on a building rooftop. Extensive green roofs are usually lightweight green roof retrofits on existing building roofs, and cannot usually accommodate foot traffic. Intensive green roofs usually involve a deeper soil layer, and are easier to incorporate into new building designs.

The Challenge

While an interest in green roof technologies continues to grow in Alberta, a lack of information is hindering their widespread development, including:

  • Data related to how they perform in our climate,
  • Information on costs, and
  • Maintenance procedures

To address these issues, a collaborative partnership known as the Alberta Ecoroof Initiative was formed to increase public awareness of the importance of eco-roof (green roof) technologies through community outreach, and to undertake local scientific research on the performance of green roofs and prove that green roofs are a suitable green building strategy for Alberta. The information gathered through this initiative will be useful in developing new green roof projects around the province and in particular for Calgary with its unique climatic conditions.

Project Description

Situated at the University of Calgary’s Alastair Ross Technology Centre in Alberta, this project not only demonstrates that green roofs can help address a number of urban environmental challenges but that they are viable in Alberta’s unique climatic conditions.

The green roof covers an area of 3000 square feet (280 square meters) and spans two existing roofs on a one story building. Eight green roof plots, with varying depths and situated along with four different green roof assemblies, were constructed and planted with volunteer teams lead by the core project team.

The project was developed over a number of phases starting in September, 2005. The rooftops were planted in August, 2006. A number of pre-grown green roof systems have since been added to observe the long-term viability of these systems in the Calgary region. An elevated structure was also added to measure stormwater runoff from two different green roof systems as compared to a conventional roof.

In contrast to many green roof projects that predominately use sedum plants, this project uses a diverse list of plant species native to the region. Over 2,000 native plants, including wild alpine strawberry, yellow penstemon, Indian blanket flower, mountain goldenrod and mountain Jacob’s ladder, were planted on the roof in 2006. These were grown in local nurseries (ALCLA Native Plants and Bow Point Nursery). Infill plants such as sedums were added in the following year. Plants that can grow in shallow, nutrient poor soils (e.g. alpine plants) were of particular interest. These plants are likely amongst the best to endure the harsh, exposed conditions of a rooftop condition and are the best adapted to the low levels of moisture that characterize the Calgary region.

Project research focused on three main areas important to the success of green roofs in Alberta:

  • Plant species and substrate depth and type,
  • Stormwater retention and runoff quality, and
  • Thermal performance and energy efficiency.

Overcoming Barriers

The lack of information on green roof technologies in cold climates has been the limiting factor in the development of green roof projects in Alberta. This project sought to better understand these challenges to increase the chance of success of green roofs in Alberta, and provide data related to how green roofs perform in cold and dry climates, information on costs, and maintenance procedures.

In colder climates, the success of green roofs can be impeded by cooler temperatures, low precipitation levels, high evaporation rates, limited selection and availability of hardy plants, and a short growing season. In the Calgary area, chinook winds also pose a challenge since they often melt insulating snow cover. This can result in freeze/thaw cycles and can bring plants out of dormancy; plants often start to form new growth but dry out and die when winter weather returns.

Successes and Outcomes

The project has provided the building community and the public with an opportunity to get involved in building and planting a green roof. In addition to having a diverse group of volunteer builders participate in the first phase of construction, the project team invited interested parties to come and visit the building while constructing the second phase (July 2006) and during the planting phase (August 2006).

The project has been featured in numerous presentations at events across the province, as well as at the annual Green Roofs for Healthy Cities conference in Boston (2006) and Minneapolis (2007). The site has attracted the attention of and been toured by the public, professionals, and developers alike.

The project has also received several awards including: Calgary Award – Honorable Mention for Environmental Achievement for a Not-for-Profit Organization 2005; BOMA Go Green Certification; TOBY Award (The Building of the Year 2006); 2007 Emerald Award Nominee; Project team member nominated for a Green Roofs for Healthy Cities Civic Award in 2008.

Economics

Green roof building costs can be quite variable depending on the design and type of the green roof (simple or complex), accessibility and depth of material used, and whether the project is new construction or a retrofit to an existing structure. In Alberta, a green roof can cost between $10–40/square foot, compared to the cost of installing traditional residential waterproof roofing membrane ($6/square foot). But as more builders become familiar with green roofs and embrace this technology, the cost of constructing green roofs will decrease.

The life of a green roof can be double the life span of a conventional roof. So, the full life cycle cost of a green roof can also make it a more attractive alternative, especially when the building is intended for long-term use.

For more information about green roof initiatives across North America visit Green Roofs for Healthy Cities

For more unique and interesting examples of stewardship in action in your community, request a copy of the Green Communities Guide today.

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Photo credit R. Thornton.

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