The development of “green” high-performance buildings that help to regenerate degraded urban environments is essential to building sustainable communities in the 21st century. Rooftops represent an average of 22% of the total land area in major cities. As such, they present a tremendous opportunity for social, environmental and economic change in our cities, and add a critically important dimension to green building design.
The multiple paybacks of a green-roof infrastructure are far-reaching and involve a mix of public and private benefits.
Storm water quantity improvements. This includes benefits to aquatic habitat, erosion control, flooding and water quality. Virtually all green roofs will hold a minimum of 50% of the total annual rainfall.
Air-quality improvements. By replacing traditional hot roofs with green roofs, plant evapotranspiration can result in a reduction in the urban heat island effect. Cooling our cities, even by only 1°C, can reduce peak load energy consumption by as much as 4% in Ontario, for example, and help reduce the smog events by as much as 10%, according to research undertaken by Environment Canada.
Biodiversity preservation. In some countries, such as England and Switzerland, green roofs are designed to provide and enhance habitats for birds, plants and insects. A recent review of a green roof in Toronto by ecologists at the Toronto and Region Conservation Authority identified four regionally rare species present.
Community gardens and urban agriculture. Although urban agriculture will never replace industrial or farm-based agriculture, there are many opportunities to use roof space for high-quality, organic food products while providing incredibly important “social” infrastructure for communities in and around buildings.
Horticultural therapy. The active and passive interaction between humans and plants resulting in improved mental and physical well-being is referred to as horticulture therapy. A number of hospitals have begun to “green” their roofs, including the award-winning Schwab Rehabilitation Center in Chicago.
Energy efficiency gains. The building experiences efficiency gains through the insulative benefit of plants, growing medium and drainage layers in the winter, and the evaporation and transpiration from growing medium and plants in the summer. Six inches of growing media has been found to eliminate up to 95% of building heat gain in ongoing research conducted by the NRC’s Institute for Research in Construction by Dr. Karen Liu.
Increased roof membrane life. This is the result of the mechanical protection of the membrane from damaging blows, and the insulation of the membrane from ultraviolet radiation. The standard expectation in Germany is that a membrane with a green roof above it will last 40 years.
Noise abatement. Near highways, train stations, airports and other sources of noise, green roofs will reduce both high-and low-frequency sound waves from entering the building.
Improved efficiency of photovoltaics. Research conducted in Germany has demonstrated that green roofs can reduce the ambient temperature and therefore improve the efficiency of solar panels, which can become less efficient by as much as 25% when temperatures rise. This is a significant gain that can help reduce the return on investment for solar panels.
Aesthetic benefits. Green roofs can be beautiful year-round.
No two are the same
Green roofs can be above, at or below grade, and the essential components typically involve a system of lightweight growing media (with a high proportion of porous inorganic material), root-repellant system (either imbedded in the membrane or separate to protect waterproofing from unwanted plants), filter cloths (to keep growing media from clogging the drainage system), a drainage system (to drain excess water from the roof and keep plants from drowning) and a variety of plants (species selection depends on a number of factors, such as climate, wind, depth of growing media, irrigation or no irrigation, etc.).
Green-roof infrastructure can be loose laid, with each of the layers laid out one on top of the other, or modular, with a number of layers built into a prefabricated tray. There also are many products that combine different functions as well as “turn-key” systems that may even include the specification of plants for a particular region. The three general types of green roofs are extensive, semi-intensive and intensive. Extensive green roofs must uniformly use 6 in. of growing medium or less, are lightweight, and often use tough, drought-tolerant species from the Sedum family. Semi-intensive green roofs have varying levels of growing medium and are often used when there is insufficient structural loading capacity to hold a full intensive green roof across the entire roof. They also can provide greater plant variability than extensive systems. Intensive green roofs must uniformly use more than 6 in. of growing medium, and in some places may use as much as 3 to 4 ft to support small trees and shrubs.
Intensive systems, also referred to as “rooftop gardens,” are usually accessible, and this enjoyment helps to offset the higher structural requirements, as well as capital and maintenance expenses. No two green roofs are the same because they can be built to deliver multiple cost savings in other areas within the building, such as reduced capital expenditures on heating, ventilation and air conditioning. They also play a key role in onsite storm water and gray water management.
There are numerous opportunities to design green roofs as part of a holistic building design process. Alternatively, green roofs also can be applied to existing buildings, simply for their public benefits and the energy savings they can generate for building owners.
Few technologies provide as wide a range of public and private benefits as green roofs, while at the same time leveraging mostly wasted roof spaces in our cities and regular reroofing expenditures. The initial cost of an extensive green roof is often at least twice as expensive as a conventional, ballasted roof. As a result, many building owners will not implement the technology.
Public incentives are required to help overcome the hurdle of higher upfront capital costs and help the public recognize the tangible, quantifiable economic values green-roof infrastructure can bring to a community. This is perhaps why cities such as Chicago, Washington, Toronto, San Diego, Montreal, Waterloo, Portland, Oakland and others are developing programs that:
- Collect building-specific data through demonstration projects.
- Quantify the widespread implementation of green roof infrastructure, both in terms of the biophysical impacts (gallons of storm water retained) and the economic impact (estimated offset capital costs on storm water storage tank).
- Develop a range of supportive policies.
For example, a study commissioned by the city of Toronto from Hitesh Doshi and colleagues at Ryerson University found that 8% extensive green-roof coverage of the city’s existing buildings would generate a one-time capital cost savings of almost $400 million and annual operational savings in the order of $40 million. These are primarily related to storm water infrastructure and management, direct and indirect energy savings and reduced beach closures due to fewer combined sewer overflow events.
These include the use of grants, demand side management incentives and storm water feebates. There also are regulatory-based initiatives such as faster approval times for projects, density housing and adopting the Canadian Green Building Council’s Leadership in Environmental and Energy Design (LEED) voluntary standard for procurement as the basis for grants.
The city of Chicago continues to lead in this field with the recent completion of a grant program for homes and small buildings, a density bonus, fast-tracking permitting, green-roof procurement on city buildings, storm water fee reductions and grants to reduce the urban heat island. In 2006, Toronto City Council passed a comprehensive set of policies called “Making Green Roofs Happen,” which includes pilot projects, green-roof procurement for city buildings, staff training and more.
Green Roofs for Healthy Cities, a green-roof industry association, provides a Local Market Development Program to help cities develop cost-effective incentives for widespread implementation. The program involves working with the city and other stakeholders in the community to find unique research and development opportunities that address widespread green-roof implementation.
Green-roof infrastructure, if not properly designed, can cause a wide range of problems such as blocking interior drainage, stressing or collapsing the structure, damaging the waterproofing membrane or causing maintenance issues. Green roofs that are improperly designed and implemented also can have massive plant failure, growing medium compaction, improper drainage, excessive weed invasion and more.
In order to develop a standard knowledge base, Green Roofs for Healthy Cities has been developing an Accredited Green Roof Professional Program that will, when complete, provide a wide range of existing building professionals with a solid foundation in the design, project administration, implementation and maintenance of green roofs. Opportunities to generate maximum client benefits at minimum costs through, for example, the use of a team approach are stressed. The program, due to be completed in 2008, also will provide clients with a measure of confidence in knowing the professionals working on their project understand green-roof implementation and can avoid making costly mistakes while maximizing benefits.
High performance, restorative buildings are being implemented and can become mainstream if the right type of research and policy development is conducted. Such buildings can be designed to generate clean air, water and energy while providing beautiful and healthy places for us to live and work. The movement towards these types of buildings is absolutely essential to the development of sustainable, healthier communities. Green-roof infrastructure has a central role to play in this positive movement.