The mention of a “sustainable” or “green” roof conjures images of rooftops covered with photovoltaic panels, highly reflective roof membrane, or heavily planted vegetative roof surfaces intended to reduce environmental impact by reducing building energy loads, reducing urban heat-island effects, and retaining storm-water runoff. While the environmental benefit of such roofs is self-evident, a key component of an ecologically responsible roof – durability – is often overlooked. Durable roofs provide long-term performance, reducing the frequency of roof replacement and quantity of resources used and wasted to manufacture and install a new roof.
Durable roofs are designed and constructed to reliably resist in-service factors such as weathering, wind, water, foot traffic, and other rooftop activities, and to do so with minimal intervention except for planned maintenance. All roofs, not just those deemed “green” or “sustainable,” should be durable. It is not uncommon to see a roof – even a so-called “green” or “sustainable” roof – hauled to a landfill after 10 or 15 years of leaky or unsatisfactory performance. Construction of a durable roof begins with the planning and design of the roof, identification of the environmental stresses and other in-service factors that can affect the roof, and consideration of how best to isolate the roof system from other interrelated building elements (such as exterior walls, fenestration, and mechanical equipment) that could adversely affect the roof in the event of their failure. A current industry goal for long-term performance for a roofing system is 30 years or greater (Hutchinson, 2007). Considerations that promote long-term roofing performance and durability include the following items:
Adequately slope and drain low-slope roofs. Water that collects on a roof surface exploits any weakness or opening in the membrane to enter and damage the roof system. Slope is essential to promptly drain water, protect the roof, and eliminate overlying water reservoirs available to penetrate breaches in the roof membrane. ·
Protect the roof from the indoor environment. Migration of moisture generated within the building into the roof system can be disastrous for the roofing system – particularly in cold northern environments and humidified buildings where the water can condense within the roof. Careful design, detailing, and construction of air and vapor barriers and their transitions to roof penetrations, parapets, roof edges, and walls can be complex but are essential in protecting the roof from this condition.
Protect the roof from adjacent, weather-exposed elements. Many moisture-driven roof failures are caused by interrelated, non-roofing elements that direct water into the roofing system and by overreliance on sealant as the sole means to keep water out of roof terminations. Rising walls, parapets, windows, louvers, and through-roof penetrations should be reliably and continuously flashed to drain water leakage through these components before water enters the roof. Design should avoid excessive reliance on sealant, which has poor reliability with respect to waterproofing, to protect an otherwise durable roofing system designed to last 30 years or more.
Anticipate and protect the roof from foot traffic. Locate equipment and roof access points to minimize walking distance between the two, or better yet, enclose equipment in penthouses accessible from the interior. Always use a hard cover board beneath the roofing membrane to improve membrane puncture resistance and to protect rigid foam roofing insulation, which is easily damaged by foot traffic.
Develop a proactive inspection and maintenance program. The commonly practiced “out of sight, out of mind” type of maintenance program – one that reacts to wear, leakage, and problems after they occur – often leads to premature roofing failure. Consider an electronic leak detection system to detect water penetration before it can cause significant damage.
Carefully consider the impact of sacrificing quality and reliability for short-term cost reduction. Cost-driven “value engineering” processes during design and construction often compromise the roofing system, resulting in unforeseen problems and substantially reduced service life.
Design and install the roofing system and perimeter conditions to adequately resist wind uplift. Damage frequently occurs on poorly detailed roofs even at wind speeds well below design conditions, which can expose the roof system and building interior to water damage.
Use qualified installers and maintain a high level of quality control. High-quality design and materials are unlikely to provide a successful and durable installation unless the design is properly executed and materials are correctly installed. Use of skilled, qualified labor, combined with frequent monitoring of installation, help ensure long-term satisfactory performance.
Use materials with track records of proven performance. The use of unproven materials can be risky. Research the long-term performance of roofing materials under similar project conditions prior to selection. Do not rely on length of warranty to judge the longevity of a product.
The knowledge of how to construct a durable and reliable roofing system has not really changed in the face of the push toward more sustainable construction. Rather, the current emphasis on sustainable construction is increasing the awareness that there is more at stake than we once thought. Designing and constructing a roof and related systems for durability, followed by responsible roof management, can provide a long-lived roof system and reduce the ecological impact on the environment.
Eric Olson, PE, is a senior project manager at the Waltham, Massachusetts, office of the national engineering firm Simpson Gumpertz & Heger Inc.
References:
“Towards Sustainable Roofing,” Report of the CIB W083/RILEM 166-MRS Joint Committee on Roofing Materials and Systems, CIB Publication No. 271, July 2001.
Hutchinson, Tom “Achieving Sustainability of a Roofing System,” RCI Interface, December 2007
