Vegetated roofs, technology long accepted in Europe, still are a relatively new addition to the green-building scene in North America. Despite technical challenges and some trial and error, the concept is catching on in cities across the continent. The reasons are rooted in storm-water management and mitigating the urban heat-island effect as much as they are inspired by the romance of having a garden on one’s roof.

Green Roofs for Healthy Cities, Toronto, is a nonprofit industry association whose mission is to develop the green-roof and -wall industries in North America. More broadly speaking, the organization seeks to develop the living architecture industry, which focuses on the living components of a building. Recently, eco-structure had the opportunity to speak with Steven W. Peck, founder and president of GRHC. He shared his thoughts about the benefits of vegetated roofs, as well as the importance of understanding how they work.

What role do you see vegetated roofs playing within the greater issues of climate change and sustainable construction?

SP: The big-picture perspective for the building industry is that we have to move from a position of something that inherently damages our ability to sustain ourselves to a position that’s restorative or healing in nature.

Some ecologists argue we shouldn’t be building new buildings because of how resource-intensive they are; we should only be retrofitting existing buildings. I believe there is a happy medium, which is to design new buildings so during their life cycles—40, 50, 75 years or more—they actually give back more than they take. They can give back in terms of biodiversity, clean water and renewable energy. They can help cities adapt to the impacts of climate change, manage storm water and cleanse the air that passes over them. They can provide fantastic living environments for people to live, work and play. Green-roof technology and other forms of living architecture are able to do these things by using rooftops and walls, which are underutilized areas within our cities.

Do you feel the trend is taking hold in North America?

According to survey data GRHC compiles based on square footage installed by our members, the vegetated-roof industry is growing by 30 percent per year; this is a conservative number. I think we’re going to see explosive growth in the green-roof industry during the next five to 10 years because more designers, owners and jurisdictions are recognizing the public and private benefits of vegetated roofs. An increasing number of jurisdictions are, for example, providing regulatory requirements; financial incentives; or other incentives, like floor-area-ratio bonuses, storm-water fee-bates and green-space allocation requirements in support of green roofs. This includes state governments, like Maryland; federal departments, like the [Washington, D.C.-based] U.S. General Services Administration; and cities, such as Chicago, New York, Philadelphia, Portland and Toronto. I see the U.S. federal government stepping up to support the greening of cities and infrastructure because of the green-job potential and because these efforts provide a multitude of benefits, such as helping to clean the air, managing storm water, cooling our cities and making them more livable.

Will these kinds of incentives help mitigate some of the hesitation that might arise from the economic downturn?

Absolutely. One of the challenges behind most technologies that deliver restorative, healing benefits during the longer term is that they have higher up-front capital costs. A classic role for government is to recognize there are longer-term public and private benefits and to minimize these up-front costs through a variety of incentives or create an even playing field through regulations. The state of New York has implemented a tax-abatement program for green roofs, which is designed to reduce the upfront costs by about 25 percent and deliver urban cooling and storm-water management benefits to New Yorkers. It’s a nice public/private partnership that is going to leverage the interest of private building owners in creating usable green spaces on top of their roofs while providing tangible environmental and economic benefits to the city.

What is the Green Roof Professional, or GRP, program?

The GRP program is aimed primarily at architects and landscape architects but is applicable to a wide variety of professionals who have been taking GRHC’s courses. In 2003, GRHC pulled together some of the leading subject-matter experts from different disciplines and created an introductory course, Green Roofs Design 101. Subject-matter experts included roofing contractors, roofing consultants, engineers, architects, landscape architects, manufacturers, horticulturalists and irrigation consultants. The 101 course covers the principles, costs and benefits of green-roof design.

GRHC then developed three full-day courses covering the spectrum of initial pre-design through installation; contracting and managing of trades; and completion and maintenance, which is a vital and often-neglected area. This body of knowledge has been developed through multidisciplinary, peer-reviewed efforts that have involved more than 100 experts.

The GRP accreditation was created because GRHC needed a way to recognize the people who were taking the time and effort to really understand how green-roof systems can deliver the maximum client benefits and how to avoid common problems associated with their design, installation and maintenance. We like to joke that we’re learning about the black arts, which involve the non-living components, and the green arts, which involve the living components. The black arts really have to come together with the green arts in terms of design, installation and maintenance if you want a successful green-roof project.

GRHC’s goal is that an accredited GRP will be someone who is able to speak the language of all the trades and disciplines, manage a project effectively and understand the best practices we have worked to establish during the past six years. People who pass the GRP exam will be able to better serve the needs of their clients and identify and deal with problems while they’re still minor. It is not possible for us to say that someone who becomes a GRP is competent as an installer or designer, but we can say he or she has passed a multidisciplinary exam developed by the green-roof industry that shows an understanding of the fundamentals and best practices.

Are there any common mistakes that are made with vegetated-roof design?

There are quite a few ways to have a failed green roof. Three fundamental things must be considered all the time. First, you must protect the structural integrity of the roof throughout the process. That holds true during the design of the roof and installation and maintenance. Second, it’s very important to protect the waterproof integrity of the roof. It’s critical the roof is protected, the waterproofing and assembly are properly specified and, most importantly, the entire system is properly installed and inspected. Third, you have to develop and implement a program with the long-term health of the plants in mind. Ultimately, if the roof doesn’t fail structurally and the integrity of the waterproofing isn’t compromised, the client is going to look at the plants and see this as a key performance indicator. The plants are the pièce de résistance of the green roof. They’re what captures people’s attention. You must put in place a basis for long-term plant survival, which involves many design, installation and maintenance considerations. Maintenance neglect is a problem for many projects. You can’t plant a green roof and expect it’s going to look fantastic two or three years later without any maintenance. There are no maintenance-free green roofs.

Are you seeing any notable research or technological innovations in North America?

There is a trend to more holistic approaches to green-roof design and research. This year, the GRHC Awards of Excellence Program has had about 40 percent more entries than last year. We’re seeing a lot of innovation in the different ways a green roof can be designed to provide additional value.

Then there is innovation taking place in determining how to measure the “soft” values of green roofs, like noise abatement and property-value increases. For example, there has been research conducted recently by Dr. Ray Tomalty et al. for GRHC; "The Monetary Value of the Soft Benefits of Green Roofs" will be released this year and looks at noise attenuation from an extensive green roof. The research indicates a reduction of approximately 5 decibels within a building from an extensive green roof.

Our Integrated Building Water Management Committee has been exploring research opportunities associated with how to design green roofs as a component of an integrated-water-management system. Such a system involves multiple aspects of the building and surrounding site. Through holistic design, it is possible to store, cleanse and reuse water, as well as maintain vegetation. Many states are facing severe water shortages, which promise to worsen in the coming decade.

A green roof cools a building through evapotranspiration, a process in which incoming solar radiation is used to evaporate water from the growing media and transpire moisture from the plants. The result is that plants and growing media are able to generate a net-cooling benefit for surrounding buildings. This process results in a cool layer of ambient air above the rooftop that also can be used to pre-cool air-conditioning-unit air. You can draw the cooler air from the roof, along the zone of 6 to 12 inches [152 to 305 mm] above the roof, which can reduce your air-conditioning demand load anywhere from 10 to 18 percent.

Research by Paul Mankiewicz of the Gaia Institute, New York, has shown that if you reduce demand load for energy through evapotranspiration on a building, you actually can reduce water consumption. Depending on what kind of electricity source you’re using and where you are in the country, there are different amounts of water required to supply the energy used to power your air-conditioning unit.

Urban agriculture is another example of green roofs that brings different aspects of the building into the value proposition. The energy and greenhouse-gas emissions associated with food production and transportation can be partially offset by growing food in cities. Urban rooftop agriculture also can help feed hungry people and build a sense of community.

You also can use green roofs in combination with photovoltaic panels. The green roof assembly can be designed to help anchor the panels through its drainage layers, growing medium and plants. More importantly, your roof will not get excessively hot so you can increase the efficiency of the panels’ electricity production. Additional research in this area related to systems optimization is needed.

Have green roofs been shown to benefit the local climate in cities where they are used?

We do know that green roofs can help reduce the urban-heat-island effect. There have been a number of studies by Columbia University, New York; Environment Canada, Gatineau, Quebec, Canada; Lawrence Berkeley National Laboratory, Berkeley, Calif.; and Ryerson University, Toronto, that show that if you implement green roofs on wide scale, you actually can cool the entire city and save tens of millions of dollars through reduced air-conditioning loads.

Earlier this year, GRHC was awarded a grant from the Washington-based U.S. Green Building Council to develop an energy calculator that will provide an estimate of the green-roof energy-saving potential for different buildings in different cities. The work is being led by Dr. David Sailor and his team at Portland State University, Ore., and Dr. Brad Bass at Environment Canada. The goal is to provide industry professionals with an easy-to-use tool that will allow them to estimate energy and related cost, as well as greenhouse-gas-emission reductions resulting from green-roof installation. Our goal is to have the energy calculator completed by 2010.

What are the common misconceptions about vegetated roofs?

The message I receive when I teach the Green Roofs Design 101 introductory course is that people don’t realize some of the complexities involved in green-roof projects. They’re not all extremely complicated, but they need careful design and execution to fully deliver for a client.

In addition, there are real economic benefits. Green building has taught us that when you get into holistic, integrated design, you’re able to move value around from one aspect of the building to another to get the best possible outcome for the client. Green roofs are a microcosm of green buildings. They are multidisciplinary in their nature and require a lot more thought and care than many people think. The new GRP accreditation likely will prove to be a cornerstone in helping to ensure the expansive growth and success of green roofing and the practice of living, restorative architecture.