By Laura M. Cavanaugh
By now, we’ve all heard about green roofs. Sustainability is a growing concern and roofs can potentially make buildings more environmentally friendly. Although numerous types of roof systems advertise environmental benefits, not all can be considered sustainable or green. Evaluating various roof options in terms of sustainability first requires consideration of how we define “green.” A “green roof” most commonly refers to a vegetative roof system that contains live plants atop the roof membrane. While this system offers many environmental benefits, the discussion of sustainability in roofing should not be limited exclusively to vegetation. By adopting a big-picture definition for green roofs, understanding current benchmarks for evaluating a roof’s environmental impact, and considering overall life-cycle impact, one can better understand roofing in terms of achieving sustainability goals.
Most generally, a roof can be considered “green” if it is environmentally friendly. The International Council for Research and Innovation in Building and Construction, an international group of experts in roofing material and design, formerly knows as the “Conseil International du Bâtiment” (CIB), recognizes a roof as sustainable if it adheres to the following tenets:
- minimizes the burden on the environment by using the earth’s resources responsibly
- conserves energy by improving the thermal efficiency of roofs
- extends roof life span by improving long-term performance (CIB 2001)
Using a definition of “green” that considers the sustainable qualities of a roof rather than its particular components, a vegetated system is not the only option worthy of the green roof designation. To evaluate and compare the relative “green-ness” of roofs, the characteristics of each system must be examined in terms of overall environmental impact throughout the roof’s entire life-cycle. All too often, when the environmental effects of a roof are investigated, the focus is on active in-service life while other key life-cycle issues that affect sustainability are ignored. To correct for this, a truly green roof should be identified as:
” a roofing system that is designed, constructed, maintained, rehabilitated, and demolished with an emphasis throughout its life-cycle on using natural resources efficiently and preserving the global environment.” (Oak Ridge National Laboratory, 1996)
By considering this new definition and incorporating the three tenets listed above, the environmental impact of both vegetative and non-vegetative systems over their life-cycles can be evaluated to reveal how green they actually are.
The variety of available roof systems, characteristics, and performance, in addition to the difficulty of analyzing a roof’s potential in terms of life-cycle, makes evaluating a roof by the true green roof definition rather challenging. A manageable approach consists of two primary components: 1) identifying limitations of the established measurement standards to avoid applying them incorrectly, and 2) considering overall roof system durability.
While highly reflective roofs can significantly decrease energy required for air conditioning in certain situations, true energy savings vary based on building design, operation, cooling and heating equipment, and local weather. Often promises of energy savings for reflective roofs overstate what is actually achieved. In some cases, reflective roofs lead to increased energy use in cooler regions where additional heating is required to compensate for the reflection of sunlight (Hutchinson 2007). Light colored roofs soil and can potentially lose of 35-50% of reflectivity during the first three years without regular cleaning (Hutchinson 2007). Additionally, proper insulation is often more important for energy efficiency than the nature of the roof surface. In addition to material properties, life span, and other characteristics, it is necessary to consider the environmental effects of a particular roof system based on behavior in its actual location.
While many aspects of the environmental effects of roof systems are either difficult to predict or vary widely, durability is a well-studied attribute that is fundamental to sustainability. Systems with long service lives need to be replaced less frequently, thus requiring fewer raw materials for production and creating less waste. This is significant because six to nine million tons of discarded roofing materials are sent to landfills in the United States each year (Athena 2007). Although the service life of a roof greatly depends on the quality of the design and installation, varying durability between popular systems ought to be considered.
Properly installed vegetative roofs offer respectable durability; many manufacturers claim their products have service lives of 20 years or longer. This performance, in combination with numerous other benefits, allows vegetative roofs to be considered green in terms of the longevity definition. Singly-ply membrane reflective roofs, such as PVC and TPO membranes, however, may suffer from a relative lack of durability compared to other systems. Depending on the membrane, single-ply reflective roofs can be expected to last about five years less than a built-up, asphalt-based alternative (Ducker 2004). This shorter life span and the varying energy benefits go against the true green roof definition. Therefore, reflective roofs may not be truly sustainable options in many cases when considering the three tenets of sustainability. Although traditional roofs are generally not thought of as sustainable, much less “green,” their solid performance in terms of durability deserves some recognition. By potentially lasting longer than a single-ply reflective system, a built-up asphalt roof compares favorably in terms of life-cycle environmental impact.
The many options available for roofing design can make choosing a system complicated, especially when aiming to maximize building sustainability. Vegetative, reflective, and various traditional roof systems each offer different benefits and limitations under different conditions. There is certainly no “one-size-fits-all” green roof option. However, by considering all three tenets presented by the CIB (minimizing environmental burden, conserving energy, and extending life span), the definition of a green roof can evolve to better reflect overall sustainability. When this definition is applied in common roofing design and the environmental effects of a roof over its entire life-cycle are considered, buildings will be more sustainable, no matter what color the roof may be.
References
Athena Sustainable Materials Institute. (2007) “Enhanced Recovery of Roofing Materials”.
Ducker Worldwide (2004). “Low Slope Roofing Life Cycle Cost Analysis.” Troy, MI.
Hutchinson, Thomas. (2007) “Cool Roofing: A 10-Year Retrospective.” Buildings. Feb., 2007.
International Council for Research and Innovation in Building and Construction (CIB). (2001). “Toward Sustainable Roofing.” Publication 271. W083 / RILEM 166-MRS Joint Committee on Roofing Materials and Systems.
Oak Ridge National Laboratory (1996). Sustainable Low-Slope Roofing Workshop. Oak Ridge, Tenn.
Laura M. Cavanaugh is a Staff Engineer with Facility Engineering Associates, P.C. Laura is an active member of FEA’s Sustainability Task Force and participates in the company’s roofing consulting projects.
Facility Engineering Associates, P.C.
11001 Lee Highway, Suite D
Fairfax, VA 22030-5018
e-mail: laura.cavanaugh@feapc.com
phone: (703) 591-4855
fax: (703) 591-4857
