The surface area of green roofs installed in 2013 grew by 10 per cent over 2012, Green Roofs for Healthy Cities (GRHC) reports.

GRHC, a member-based nonprofit industry association for green roof and wall professionals in North America, conducts an annual survey of its members.

According to its survey, GRHC members installed 596,580 square metres in 2013 on 950 projects, up from 519,151 square metres installed in 2012 on 982 projects. The area installed on public projects grew compared to that on private projects, whereas in past years the public/private balance had been more equal. The number of private projects installed was far higher, indicating that the public projects were much larger than the private projects.

The group believes the actual number of green roofs installed each year is far higher than their survey indicates.

“We estimate that the data in this report understates market activity by anywhere from 25 to 50 per cent given that not all firms in the industry are members and not all members are able or willing to participate in the survey,” GRHC said.

Green roofs in Australia now have some official backing, as the city of Sydney has created a green roofs and walls policy to encourage their development. The city now offers a resource guide to interested parties, as well as a commitment to encourage the construction of high-quality green roofs and walls. Sydney currently has about 80 green roofs, with 50 more planned.

Both academia and industry are working to grow the field and establish standards and education. In addition to the annual member survey, GRHC has also developed the Green Roof Professional (GRP) training and accreditation program to establish and promote best practices in green roof design, installation, and maintenance. Industry professionals can earn continuing education credits for courses like the Net Zero Water Boot Camp and Rooftop Urban Agriculture, for example.

The University of Toronto’s Green Roof Innovation Testing Laboratory (GRIT Lab), established in 2010, is used to document the performance of green roofs, green walls, and solar photovoltaic (PV) systems. The lab has built, on the roof of the John H. Daniels Faculty of Architecture, Landscape, and Design, more than 30 green roof test beds and three green walls. The test beds are used to compare growing media type, growing media depth, the vegetation types, and methods of irrigation.

With 270 sensors installed, the lab monitors a variety of data, including rainfall, humidity, temperature, soil moisture, flow rates, and wind. Phase I, which continues through 2016, will help researchers to evaluate green roof construction standards as they relate to Toronto’s Green Roof Bylaw Construction Standard, and in relation to stormwater retention, evaporative cooling, biodiversity, and life cycle cost.

In addition, the project provides an opportunity for multi-disciplinary cooperation amongst professionals and students in biology, hydrology, building science, and landscape architecture. This collaboration is expected to lead to innovation, commercial advances, knowledge transfer, policy, and guidelines.

Another professional test bed is the headquarters of the American Society of Landscape Architects (ASLA) in Washington, D.C. Completed in 2006, the roof’s performance monitoring has provided data about building temperature and rainfall, such as:

● After installation, the building’s energy use decreased by 10 per cent in the winter months.
● On the hottest summer days, the green roof’s temperature can be up to 59 degrees Fahrenheit cooler than neighboring conventional roofs.
● The green roof intercepted about 78 per cent of all rainfall during a 10-month monitoring period, or more than 104,000 litres.
● Runoff from the green roof contains much less nitrogen and other pollutants than typical urban runoff.
● Most rainfalls of under 25 millimetres result in no runoff at all.

Green roofs provide widely acknowledged benefits to urban environments, including:

  • Stormwater management, sequestration, and filtering
  • Moderating the urban heat island effect
  • Improved air quality and aesthetics
  • Increased biodiversity
  • Urban food production

In addition, benefits of green roofs often outweigh their increased installation costs. By protecting the roof membrane from ultraviolet radiation and harsh weather, green roofs can extend the life of the membrane and the roof overall.

Elements of green roof systems generally include a waterproof membrane with root barrier, specialized drainage layer, often with built-in water reservoirs, filter/landscape cloth to contain the growing medium, and the growing medium itself.

Green roofs are classified as intensive or extensive, though they can combine elements of both systems. Extensive roofs use a thin layer, five to 15 centimetres, of lightweight growing medium to sustain the plant mixture. At a weight increase of between 72.6 to 169.4 kilograms per square metre, extensive roofs offer low cost, low weight, low maintenance, and low plant diversity. Costs vary but range from $108 to $248 per square metre.

An intensive roof, by contrast, can incorporate a greater variety of perennial plants, shrubs, and even trees, and may include gardens and park-like areas. The growing medium is soil based, with a depth from 20 to 60 centimetres and a saturated weight increase of 290 to 967.7 kilograms per square metre. Cost for intensive green roofs range from $355 to $2,368 per square metre.