by sadia_badhon | October 22, 2020 1:55 pm
by Tracy Myers, AIA, RRO, LEED AP, NCARB
A high-performance roof system is durable, resilient, and allows a building to be occupied after a disaster event and continues performing during a loss of power. Designing beyond code minimums could help a roof stay intact with minimal damage during a storm.
A green roof could be used to mitigate the urban heat island effect. A blue roof could be used to temporarily retain water on the roof to help manage stormwater. A cool, reflective roof could improve a building’s energy efficiency by reducing the amount of heat gain and associated energy demand for air-conditioning units. Utilizing skylights to provide natural light could help with post-disaster use of a building when the power grid has gone down.
Buildings benefiting from a high-performance roof include essential structures, such as hospitals and power infrastructure, for obvious reasons. Less obvious beneficiaries include grocery stores and cold-storage facilities where it is expensive to replace inventory. These outlets also are valuable to communities after a disaster. Keeping a roof intact and dry allows buildings to continue functioning as well as mitigates the risk of the loss of valuable goods.
There are several reasons for using a high-performance roof. An owner may request a roof requiring minimal maintenance. Local laws or regulations may demand design and performance requirements specific to their jurisdiction. The risk of future litigation may also be a driver for an owner or developer to request a roof designed and constructed to exceed minimum code requirements.
To design a high-performance roof, it is essential to consider building science, the fundamentals of heat, air, and moisture, as well as understand the local climate zone and the different pressures acting on the building enclosure. It is advisable to employ more conservative detailing strategies at the roof edge and attachment points as well as consider the value of a ‘belt-and-suspenders’ approach to the overall design. The selection of durable materials and system configurations play a role in the design of high-performance roofs.
Building science matters
The roof is an essential component of the building enclosure. It acts as a ‘barrier’ or ‘separator’ between the exterior and interior conditions and controls the flow of mass and energy (read Roofs, Design, Application and Maintenance by Maxwell Baker, National Research Council Canada, 1980).
It is important to review the second law of thermodynamics, as it relates to buildings, to better understand how building science affects roofing design. According to the law:
Another way to think about it is that moisture, heat, and air are regularly seeking balance.
The most impactful form of moisture is liquid (bulk) water, and it can move in many directions, not just down—think wind-driven rain. Next, there is capillary action, air-transported moisture follows, and, finally, vapor diffusion. The designer and builder should also consider construction-related moisture and moisture within building materials.
Several types of pressures act on the building enclosure. Wind produces positive pressure on the windward side and negative pressure on the leeward, including the roof. Negative pressures tend to be greatest at the roof because wind speed increases with height. When air pressures below the roofing system are greater than those above the roofing system, the pressure differentials can cause roofing system failures due to uplift forces.
The stack effect produces positive pressure at the roof and inward pressure at the base of the building. Mechanical ventilation creates positive pressure (air supply rate greater than exhaust) or negative pressure (air exhaust rate greater than supply). These differences in pressures contribute to air and moisture movement throughout the roof system.
The impact of wind on the roof can be significant. Corner and perimeter loads are higher than in the field. Roof edges are vulnerable and initial failures often originate there. For this reason, building codes include the American National Standards Institute/Single-ply Roofing Industry (ANSI/SPRI) ES-1, Wind Design Standard for Edge Systems Used with Low-Slope Roofing Systems, low-slope edge securement requirements. Parapets can lower edge suctions at the roof, depending on the height of the parapet. The building’s configuration, classification (open, enclosed, etc.), and exposure can affect the calculation of the uplift forces acting on the roof. Wind-related pressures can lead to expensive and potentially dangerous roof failures.
Air barriers are systems comprising various materials and components. They are required by the International Energy Conservation Code (IECC) and work to control airflow and associated moisture. IECC also provides multiple compliance options utilizing a material’s or an assembly’s approach. Although not required by code, designers can specify compliance with whole building testing, which can holistically predict the in-situ air barrier system performance. Since they are restricting the movement of air, air barrier systems must be designed to withstand negative and positive pressures.
Vapor retarders manage vapor diffusion. They are not required by code in roofs, but designers may choose to specify them under certain circumstances, such as concrete roof decks where moisture can be an issue (read “Structural Concrete Decks, Vapor Retarders, and Moisture – Rethinking What We Know” by Helene Hardy Pierce and Joan P. Crowe). There are three classes of vapor retarders with different levels of permeance. A designer can calculate temperature gradients through an assembly to determine placement and need. Vapor retarders can act as air barriers and can be included in the air barrier system if detailed correctly. A good rule of thumb is warm air condenses on cold surfaces and vapor retarders are typically located on the warm side of an assembly. The challenge can be to determine the direction of vapor drive, especially in mixed climates. Understanding the project’s climate zone is one factor in determining whether a vapor retarder is recommended and where it should be installed.
The International Code Council (ICC) publishes several codes applying to roof design (for purposes of this article, the 2015 and 2018 International Code Council [ICC] volumes are referenced. Each state and/or local municipality may amend the ICC and/or adopt their own codes. As such, this article is not intended to provide specific project code interpretation). The International Building Code (IBC), Chapter 1, “Scope and Application” notes the code “provides the minimum requirements to provide a reasonable level of safety, public health, and general welfare” through “safety to life and property”. IBC does not offer specific guidance on how buildings should perform after a hazard event beyond life-safety considerations. Chapter 13 of IBC says, “Buildings shall be designed and constructed per the International Energy Conservation Code (IECC). Chapter 1 of IECC provides requirements that construction documents be prepared by a registered design professional and that the insulation materials, R-values, air sealing details, and the building’s thermal envelope be included within the construction documents.”
Chapter 15 contains the majority of the roof-related requirements, including ANSI/SPRI ES-1 low-slope edge securement testing and compliance. Chapter 15 of the code also requires “roof coverings shall be designed and installed per code and manufacturer’s instructions. Parapet walls shall be properly coped, not less than the thickness of the parapet wall. Flashings shall prevent moisture/water from entering. Non-ballasted roofs, mechanically attached or adhered, shall be designed to resist wind load pressures for components and cladding.”
Roof decks and coverings must be designed for wind loads per Chapter 16 and Sections 1504.2 to 1504.4. Roof coverings shall resist impact damage per ASTM D3746, Standard Test Method for Impact Resistance of Bituminous Roofing Systems, D4272, Standard Test Method for Total Energy Impact of Plastic Films by Dart Drop, or Section 5.5 of Factory Mutual (FM) 4470, Single-Ply, Polymer-modified Bitumen Sheet, Built-up Roof (BUR) and Liquid Applied Roof Assemblies for Use in Class 1 and Noncombustible Roof Deck Construction. When designing a roof to better withstand impact hazards, consider the roof as a system and not just the attributes of individual components.
Chapter 16 of IBC provides the structural requirements for roofing systems, including the American Society of Civil Engineers (ASCE) 7, Minimum Design Loads for Buildings and Other Structures, wind pressure design needs. Wind design data shown on the construction documents includes risk category, wind exposure, internal pressure coefficient, and design wind pressures used for exterior cladding materials. There are opportunities for design professionals to create more durable roofing systems by making conservative selections (e.g. a more conservative risk category or wind exposure factor).
Building functionality after a natural hazard event depends on the ability of the structure and the supporting infrastructure to withstand the disaster. In other words, even if a building remains intact after a storm, it may be difficult to occupy if the electrical grid is not functional. Current codes generally do not address continued functionality after a hazard event. However, ICC is a member of the Federal Emergency Management Agency (FEMA) Resilient National Partnership Network, a founding member of the U.S. Resiliency Council, and a signatory to the National Institute of Building Sciences (NIBS) Industry Statement on Resilience. An argument can be made to update codes to address emerging issues, such as extreme weather events, pre-disaster planning and post-disaster building functionality.
In their 2019 Natural Hazard Mitigation Saves report, NIBS found a monetary benefit is associated with communities adopting up-to-date building codes and designing beyond code minimums. According to the 2019 NIBS report:
However, front-end parties responsible for funding and construction appear to receive less benefit than end-users. This may be due to the economic investment required of front-end parties to implement resilience strategies. The report also documents non-monetary mitigation benefits, such as a reduction in deaths, nonfatal injuries, and post-traumatic stress disorder.
Resilience and durability
Resilience and durability refer to the ability to recover from difficulties and to withstand wear, pressure, or damage. If a roof can stay intact and functional during and after a storm event, it becomes resilient in the sense the building can remain or get back into service quicker, which is beneficial to the local community.
Weather patterns are changing and, in many instances, becoming more severe. Designers need to consider not only past incidents, but anticipate future events to design a high-performance roof system.
The United States has sustained 250 weather and climate disasters since 1980, where overall damages and costs reached or exceeded $1 billion (including consumer price index [CPI] adjustment to 2019). The total cost of these 250 events had exceeded $1.7 trillion (adjusted for inflation). Also, four out of five of the most massive hurricanes in the United States have occurred in the last 15 years.
Costly hailstorms are increasing in the United States, with the average year now accumulating between $8 and $14 billion in hail-related insurance losses. In 2014, Verisk Insurance Solutions reported insurers paid claims totaling more than $54 billion from 2000 to 2013, with 70 percent of those losses occurring in the latter half of that time period. Hailstorms are not only becoming more severe and frequent, but also the entire country is subject to these storms, as represented by the warmer colors in Figure 1.
Increasing urbanization and rising temperatures contribute to urban heat island effects (i.e. urban areas experience warmer temperatures than nearby rural areas because the surfaces in urban areas typically absorb and retain more heat.) This is because many urban areas are dense with many dark, absorptive, and impervious surfaces.
Along with climate change, many cities across the nation are dealing with aging infrastructure that is not designed to service growing populations. This has become a big issue with combined sewer systems collecting rainwater runoff, domestic sewage, and industrial wastewater. Under normal conditions, the piping transports collected wastewater to a sewage treatment plant for treatment, and then discharged to a water body. During storms, when the volume of stormwater grows, it can exceed the capacity of the treatment system, and untreated stormwater and wastewater discharge directly into nearby waterways. These combined systems can be found in over 850 municipalities.
The changing climate affects roofing in many ways by causing:
High-performance detailing and specifications
Air can carry a significant amount of moisture. Therefore, it is critical to limit the amount of air infiltrating a roof system. Installing an air barrier—and vapor retarder when it is appropriate—and two layers of insulation with staggered joints helps mitigate the pressures acting on the roof. Locating fasteners below the insulation layers reduces thermal bridging, thereby improving the thermal performance of the roof system and increasing energy efficiency.
While the selection of individual components is a part of roof system design, it is preferred to consider the roof as a system instead of individual elements. Creation of a durable roof system design will take into consideration the characteristics of the roof covering as well as the underlying assembly.
Single-ply membranes represent more than 50 percent of the United States roofing market (read “Ice Ball Impact Resistance of Heat-Aged TPO Roofing Membranes” by Sarang Bhawalkar, Tammy Yang, and Thomas J. Taylor). Commonly used ‘cool-roof’ single-ply membranes include thermoplastic polyolefin (TPO), polyvinyl chloride (PVC), polyvinyl chloride ketone ethylene ester (PVC-KEE), and KEE. Each of these membranes offers the benefit of welded seams and multiple membrane thickness options. Ethylene propylene diene monomer (EPDM) is a thermoset synthetic rubber roofing membrane that also comes in white with multiple thickness options.
Understanding the location and use of the building helps inform the roof membrane selection. Some membranes, such as PVC-KEE or KEE, may be an appropriate choice for roofs subject to chemical exposures. TPO membranes are inherently flexible, while PVCs require the addition of liquid and solid plasticizers for flexibility. Migration of liquid plasticizers is one of the modes of failure for PVC membranes. As the plasticizers migrate, the membranes become more brittle and prone to damage. Specifying an inherently flexible, high-performance TPO membrane or a PVC-KEE membrane with solid plasticizers may increase the roof’s longevity. Dark-colored EPDM membranes have a successful history of field performance, but as a thermoset membrane, it relies on adhesives for its seams, which can fail over time. Some manufacturers are offering single-source hybrid roofing systems incorporating a multi-layer bituminous roofing system, covered with a single-ply ‘cool-roof’ membrane, thereby achieving multi-ply redundancy as well as the energy savings of a reflective roof. When specifying a reinforced single-ply membrane, review the thickness over scrim, which is the thickness of the roofing membrane layer over the internal reinforcing. As a general rule, a thicker membrane layer over scrim equates to a more durable membrane.
Studies have been performed to evaluate the puncture resistance of single-ply thermoplastic membranes as well as ice ball impact testing of heat-aged and non-heat-aged TPO membranes by Sarang Bhawalkar, Tammy Yang, and Thomas J. Taylor, who work for a roofing manufacturer (read “Puncture Resistance of Thermoplastic Single-Ply Roofing Membranes” by Sarang Bhawalkar and Thomas J. Taylor; “Understanding the Ice Ball Resistance of TPO Membranes;” and “Ice Ball Impact Resistance of Heat-Aged TPO Roofing Membranes” by Sarang Bhawalkar, Tammy Yang, and Thomas J. Taylor). While impact-testing may not indicate a product’s in-situ performance when subject to hail impact, it does provide performance testing information that can be considered by the roof designer (Consult “Concerns with Impact Testing” by Mark Graham). These studies have found:
Protected membrane roofing (PMR) assemblies, also referred to as inverted roof membrane assemblies (IRMA), are also an option for increased durability since the roofing membrane is covered and protected from damage. Green and blue roofs can be considered PMR or IRMA roofs.
As mentioned earlier, the roof edge is one of the most vulnerable points on the roof. Section 1504.5 of IBC requires low-slope edge securement to be “designed and installed for wind loads per Chapter 16 and tested for resistance per test methods RE-1, RE-2, and RE-3 of ANSI/SPRI ES-1, except basic design wind speed, V, shall be determined from Figures 1609.3(1) through 1609.3(8) as applicable.”
The ANSI/SPRI/FM 4435 ES-1 2017, Test Standard for Edge Systems Used with Low Slope Roofing Systems, is the latest version of the standard adopted as part of 2018 IBC. It provides the “basic requirements only for resistance testing for roof edge systems under simulated wind load conditions.” The standard “applies to low-slope roof systems, with low-slope defined here as roofs having a slope less than or equal to 9.5 degrees (2:12).”
There are a variety of sources for ES-1-tested products. Pre-manufactured systems may have been pre-tested for compliance. The National Roofing Contractors Association (NRCA) and the Sheet Metal and Air Conditioning Contractors National Association, Inc. (SMACNA) include roof edge details that have been tested for conformance with the standard (see the Sheet Metal and Air Conditioning Contractors’ National Association (SMACNA) Technical Resources Bulletin on “STRI Wind Uplift Testing”). To increase the durability of the roof edge, specify a continuous cleat and increase the thickness/gauge of the cleat. Make sure to attach the coping and blocking to the underlying substrate securely.
Green roofs can help to mitigate climate change effects on urban environments by reducing and slowing runoff into aging combined sewer systems as well as lowering the roof temperature, thus helping to minimize the urban heat island effect. The vegetative overburden in these systems also protects the underlying membrane. Green roofs could be a good option for locations with impact risks such as hail.
Blue roofs also control the amount of water into local sewer systems through the delayed discharge of water. They reduce the peak stormwater impact on wastewater treatment plants. Vegetative components can be integrated into these as well. Similar to green roofs, the membrane is protected.
IECC provides roof solar reflectance (how much solar energy is reflected by the membrane) and emittance requirements (how much absorbed heat is radiated back into the atmosphere) for low-slope roofs over conditioned spaces in Climate Zones 1 to 3. Studies have also shown cool roofs provide energy savings in all climates when the cost of electrical demand charges are taken into account (read “Cool Roof Use in Commercial Buildings in the United States: An Energy Cost Analysis” by Thomas J. Taylor and Christian Hartwig).
Cool roofs are not the solution in every case, as each project and its location must be considered individually. Existing wood deck roof systems can pose certain risks when converting to a highly reflective membrane, and there are elevated sources of interior moisture (read the article “Practical Application of Hygrothermal Modeling of West Coast Wood Deck Systems” by A.O. Desjarlais, H. Hardy Pierce, W. Woodring, and S. Pallin). Due to the lower roof temperature on a cool roof, the self-drying aspects of dark roof membranes no longer apply. Additionally, there is a risk of condensation.
Photovoltaics on the roof
In the case of photovoltaic (PV) installations, it is crucial to consider the useful life of the equipment relative to the roof system and to carefully review all attachment points for both structural integrity as well as adequate waterproof detailing. Utilizing energy storage in conjunction with power generation can provide backup power during grid interruption.
Portland-based Lever Architecture designed the renovation and expansion of the Nature Conservancy’s Oregon headquarters. As sustainability was integral to the project design, the team decided to maintain and renovate the original 1970’s building.
The roof of the Oregon Conservation Center building comprises two systems. The first is composed of an 80-mil TPO membrane, a 13-mm (1/2-in.) thick glass mat-faced, moisture-resistant gypsum cover board, 100 mm (4 in.) of closed-cell polyisocyanurate (ISO) foam, a vapor retarder, and roof sheathing on the existing, sloped roof structure of the three-story portion of the building. This particular roofing type was selected for its high albedo, longevity, and durability. It is designed as a cool roof system featuring a grid-tied PV system that offsets 25 percent of the building’s annual energy usage. The PV system is also designed for connection to a battery storage system to provide off-grid power during outages.
The second roof system is located on the single-story building addition, and features an intensive green roof as well as a roof deck built with Forest Stewardship Council (FSC)-certified cedar decking sourced from the Nature Conservancy’s Ellsworth Creek Preserve in Willapa Bay, Washington. The roof structure for the addition is made from five-ply and seven-ply FSC-certified cross-laminated timber (CLT). The green roof features soil depths varying from 150 mm (6 in.) to 1 m (3 ft), and includes vegetation and plantings native to the Rowena Plateau in Oregon’s Columbia Gorge. The waterproofing system below both the green roof and the roof deck is a reinforced, hot-rubberized asphalt (HRA) membrane with an integrated leak-detection system. The roof assembly includes 100 mm of extruded polystyrene (XPS) board insulation above the HRA membrane system. The cedar roof deck is supported on adjustable pedestals that bear on the rigid insulation. The HRA membrane system was selected for its high performance and integrated leak detection system.
The project achieved Leadership in Energy and Environmental Design (LEED) Gold certification and incorporates abundant daylighting and operable windows, which lessens its dependence on surrounding infrastructure.
Design best practices
Design and detail all control layer components and use the red pen test (using a pen of any color to trace the control layer(s) on the drawings without lifting it off the paper) to ensure continuity. For continuous control layers:
Specification best practices
Determine whether a prescriptive, performance, or combination specification is most appropriate for the project and be consistent throughout the document. Select a basis of design where applicable and coordinate with related sections, describe all system components with applicable standards and tests, understand single-source requirements for warranty and performance, and clearly define quality control, installation procedures, and owner, designer, and constructor responsibilities.
Value engineering can happen even with diligent and conscientious designs. Common examples of value engineering include:
Value-engineering decisions need to be carefully considered by the design team. There is often a push to agree to cost-saving measures. Licensed professionals are well-served to consider the applicable standard of care as well as relevant industry standards when evaluating value-engineering suggestions. Risks include:
Mechanically attaching the roof membrane can lead to increased air infiltration and risk of condensation. Membrane billowing occurs as a result of wind-induced suctions lifting the membrane between attachment points Billowing may not be a defective condition, but it can be disconcerting to observe and increases the load on roof fasteners and the membrane. Mechanically attached systems can also lead to an increase in thermal bridging (areas of higher thermal conductivity), which lowers energy efficiency and poses possible condensation risks at the fasteners.
Building codes are minimum standards and generally do not consider functional recovery after disaster events. IECC requires the depiction of thermal envelope and air sealing details on construction documents. Vapor retarders at the roof are not required by code. It is best to consider climate zone and building use, and calculate temperature gradients to determine need and placement of vapor retarders. Improperly located vapor retarders can cause damage to an assembly.
Use the red pen test to ensure all control layers—moisture, air, vapor, and thermal—are designed and detailed correctly.
Resilience is about the building and the roof as well as the infrastructure serving the facility. It is no longer adequate to consider past events when designing for future hazards and storms.
Pay close attention to the roof edge design, which is often the point of initial failures. Many types of pressures act on the building and the roof, such as wind, stack effect, and mechanical ventilation.
Consider blue, green, and cool roofs to mitigate the effects of climate change. When detailing, limit airflow and associated moisture into the roof system. Locate fasteners below insulation and adhere top layers to reduce thermal bridges.
Consider the roof as a system rather than as individual components to better understand expected performance.
Tracy Myers, AIA, RRO, LEED AP, NCARB, is the founder of Myers Consulting. Her Southern California-based practice focuses on issues related to building enclosure performance, forensic architecture, and expert witness services. Myers has over 28 years of experience in traditional and forensic architecture, building enclosure investigation, peer review, quality assurance, building codes, and roofing design and investigation. She is a member of multiple ASTM committees on roofing, waterproofing, thermal insulation, and building performance. Myers can be reached via e-mail at firstname.lastname@example.org.
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