Green roofs (also known as living roofs, green roof tops, vegetated roofs, planted roofs, rooftop gardens or ecoroofs) 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.
Green walls (also known as living walls, vertical greenery systems or vertical gardens) are walls intentionally covered in vegetation. They can be classified as green facades wherein the plants climb the wall but are rooted in the ground, and living wall systems wherein the plants are attached to the wall, typically with a modular structure that contains growing medium and/or soil. Green walls can exist on the exterior or interior of buildings; however, this chapter focuses primarily on those planted on the exterior of buildings.
Capture stormwater. Green roofs and walls can be used to help capture precipitation and stormwater runoff during rain events. The stormwater is then used for irrigation, is partially dispersed by evapotranspiration and is slowly dispersed into the storm sewer system at a rate that causes less erosion problems in receiving water bodies.
Range of uses. The type of green roof or green wall that is implemented depends on the desired outcomes of the system. A structural assessment of the roof or wall should first be done to determine how much additional weight it can support and what type of media and structures can be affixed to grow the plants in. Existing buildings may or may not require retrofitting or structural enhancement to support a green roof or green wall system. New buildings can be designed to accommodate the additional weight and watering systems of a more intensive green roof or wall if desired.
Preparation of a green roof or green wall typically requires a number of layers to prevent damage from moisture or roots. Green walls are often modular and affixed to the surface to prevent structural impacts from plant growth. The following are layers of a green roof recommended by the Toronto Green Roof Construction Standard:
- Structural support
- Roofing membrane
- Membrane protection and root barrier
- Drainage panel and filter fabric
- Growing media
Loading capacity. Adding a green roof or wall to an existing building requires that the structural loading capacity of the building be evaluated. The green roof or wall can then be designed to be within the structural capacity of the roof or wall, or retrofits can be done to increase the weight the building can support. The depth of the planting medium for a green roof can be thicker on parts of the roof that can carry additional weight, such as over elevator shafts.
Plant selection should target drought-tolerant plants that produce little biomass. Sedums are the most commonly used plant for green roofs because they are highly tolerant to the extreme temperatures and conditions of rooftops. For green walls, plant selection depends on whether the plants will be rooted in the ground and climb the wall or be planted in modules affixed to the wall surface.
Maintenance and operations requirements may be specialized. Irrigation and weeding is required for at least the first year to get plants established. Drains and membranes should be inspected regularly.
Public access. Where structural load and access structures permit, providing public access to the rooftop provides a good educational opportunity. Similarly, outdoor green walls and indoor green walls in public spaces may provide community members a chance to learn about this example of NBS. Allowing access and providing tours can enhance public appreciation and understanding of the concept of green roofs and walls and can lead to greater uptake and implementation of this NBS in other areas, including private properties.
The Business Side
Green roofs and walls can reduce heating and cooling costs. A Toronto-based green roof study estimated that the presence of a green roof reduced energy use by 4.15kWh/m2/year at an electricity rate of $0.11919 per kWh. If this calculation were applied to a 1000m2 roof in Edmonton, this could translate to savings of almost $500/year. Using a simulation model, Environment Canada found that a typical one storey building with a grass roof and 10cm of growing medium would result in a 25% reduction in summer cooling needs. Another study in Ottawa found that a 6 inch extensive green roof reduced heat gains by 95% in summer and heat losses by 26% compared to a reference roof.
Reduced stormwater infrastructure costs may result from the implementation of green roofs and walls, due to lower demands on infrastructure and the resulting extension of life of stormwater conveyance systems. A study from Winnipeg determined that the use of green roofs as an alternative means of stormwater retention could reduce the cost of sub-grade infrastructure by 10%. The model suggested that, in Winnipeg, the cost of green roofs are greater than using stormwater retention ponds, but almost half the cost of using underground stormwater storage tanks. In dense cities with high land costs, green roofs and walls could be a more cost-effective use of space.
Green roofs may also lengthen the life of a roof membrane by two to three times, due to the moderation of surface temperatures and the protection from the harmful effects of UV light.
The Nature Side
In cities with combined sewer overflow systems (CSO), stormwater detention and reduction in runoff due to evapotranspiration on green roofs and walls can reduce the volume of total stormwater entering CSOs, therefore reducing the chances of sewage entering waterbodies during storm events, thereby reducing the risk to water quality. Green roofs and walls also absorb some types of airborne pollutants and dust and produce oxygen, improving local air quality. This example of NBS is also beneficial in reducing the urban heat island effect. Additional insulation provided by green roofs and green walls helps to reduce emissions from heating and cooling and the plants provide greenhouse gas sequestration. Wildlife habitat for birds, insects (especially pollinators) can be created through the addition of vegetation to building surfaces. Green roofs and walls can also be used for vegetable and herb production, which enhances local food production and reduces the environmental impacts of long-distance food transportation.
The Community Side
Green roofs and green walls create more attractive views in cities and towns, especially for apartment dwellers and office tower workers who look onto them. This addition of green space in urban and periurban areas (outdoor and indoor in the case of indoor green walls) can increase property values in the area and provide multiple benefits to physical and mental health. In the case of rooftop gardens or food-producing green walls, people also have access to fresh produce, local food security and opportunities for social interaction and community-building.
 Jiayu, L., Zheng, B., Shen, W., Xiang, Y., Chen, X., & Qi, Z. 2019. Cooling and Energy-Saving Performance of Different Green Wall Design: A Simulation Study of a Block. Energies 12, 2912.
 City of Toronto. 2017. Toronto Green Roof Construction Standard. City of Toronto.
 Missios, P., Banting, D., Hitesh, D., Li, J., Au, A., Currie, B., & Verrati, M. 2005. Report on the Environmental Benefits and Costs of Green Roof Technology for the City of Toronto. Ryerson University.
 Lawlor, G., Currie, B. A., Doshi, H., & Wieditz, I. 2006. Green Roofs: A Resource Manual for Municipal Policy Makers. Environment Canada.
 Beaumont, P. 2012. Green Roofs, the way of the future! The Green Dandelion, University of Rochester.
 Cunningham, N. 2001. Rethinking the Urban Epidermis: A Study of the Viability of Extensive Green Roof Systems in the Manitoba Capital with an Emphasis on Regional Case Studies and Stormwater Management. Thesis. Department of Landscape Architecture, University of Manitoba.
American Society of Landscape Architects (ASLA). 2008. ASLA Green Roof.
Beaumont, P. 2012. Green Roofs, the way of the future! The Green Dandelion, University of Rochester.
Cunningham, N. 2001. Rethinking the Urban Epidermis: A Study of the Viability of Extensive Green Roof Systems in the Manitoba Capital with an Emphasis on Regional Case Studies and Stormwater Management. Thesis. Department of Landscape Architecture, University of Manitoba.
Lawlor, G., Currie, B. A., Doshi, H., & Wieditz, I. 2006. Green Roofs: A Resource Manual for Municipal Policy Makers. Environment Canada.
Jiayu, L., Zheng, B., Shen, W., Xiang, Y., Chen, X., & Qi, Z. 2019. Cooling and Energy-Saving Performance of Different Green Wall Design: A Simulation Study of a Block. Energies 12, 2912.
Missios, P., Banting, D., Hitesh, D., Li, J., Au, A., Currie, B., & Verrati, M. 2005. Report on the Environmental Benefits and Costs of Green Roof Technology for the City of Toronto. Ryerson University.
Primeau, S., Bell, M., Riopel, M., Ewaschuk, E., & Doell, D. 2009. Green Communities Guide: Tools to Help Restore Ecological Processes in Alberta’s Built Environments. Land Stewardship Centre of Canada.