| THE BUILDING ENVELOPE | By Russell M. Sanders, AIA, and Craig A. Hargrove, AIA, LEED AP, Hoffmann Architects

DESIGN HOFFMANN ARCHITECTS STRATEGIES FOR METAL WALL AND ROOF SYSTEMS

Russell M. Sanders, AIA, is President of Hoffmann Architects, an archi- LEARNING OBJECTIVES tecture and engineering firm specializing in building exteriors. In addition After reading this article, you should be able to: to building enclosure evaluation for existing structures, he specializes in design detail assessment and constructability review for new construc- + DESCRIBE the materials and finishes typical of metal wall and roof tion. He can be reached at: [email protected]. assemblies, including the benefits and shortcomings of each, and the Craig A. Hargrove, AIA, LEED AP, Senior Vice President and Director of applications to which they are best suited. Architecture with Hoffmann Architects, oversees demanding high-perfor- mance building envelope projects. Manager of the firm’s New York office, + DISTINGUISH between “open” and “closed” metal wall systems, defining he leads project teams in developing design details that meet weather each in terms of its waterproofing strategy, as well as its compatibility integrity, energy efficiency, and aesthetic goals with cost-effective solu- tions. He can be reached at: [email protected]. with exposed or concealed fastener designs.

+ CONTRAST “hydrokinetic” and “hydrostatic” metal roof assemblies and ears ago, prefabricated metal panel wall and explain the relationship between these water protection approaches and roof systems did not find wide acceptance architectural or structural metal roof systems. in high-end commercial applications. While + EXPLAIN how best to address design considerations, such as expansion, these systems were cost-effective, aesthetic differential movement, structural integrity, weather protection, thermal perceptions relegated them to large storage performance, and condensation, for typical metal wall and roof assemblies. buildings, manufacturing facilities, and big box stores. However, versatility, reliable per- formance, and relatively low installed-costs have now made these systems attractive in Sponsored by more sophisticated applications. Today, owners and Yarchitects often collaborate to use these products in distinctive and aesthetically innovative design

42 | BUILDING DESIGN+CONSTRUCTION | January/February 2021 | BUILDING ENVELOPE | HOFFMANN ARCHITECTS

solutions. Manufacturers, recognizing sion. This can also occur in areas that see an opportunity, have expanded their significant amounts of water accumulation,

product lines with the aim of amplifying OF HOFFMANN ARCHITECTS PHOTOS COURTESY such as gutters. When using zinc panels, product use across a range of building protective coatings applied to the at-risk types. Of course, no building enclosure surfaces should be considered. system is without design challenges Copper is an extremely durable mate- and considerations. rial option that generally does not require From structural and weatherproofing a protective coating. Although, as with to energy efficiency, the longevity and zinc, the use of such coatings at the un- performance of metal wall and roof sys- derside of panels can be advisable under tems depends on an understanding of certain conditions. Uncoated copper also material properties, code requirements, tends to stain adjacent surfaces and and best practices. materials over time. Stainless steel never requires a protec- MATERIALS AND FINISHES tive coating and provides a bright appear- Most metal wall and roof systems are ance. An aesthetic alternative is terne proprietary, each manufacturer having plate, stainless steel coated with an alloy their own profiles, method of attach- that is over 99% tin. Terne plate provides a ment, and performance specifications. weathered appearance similar to zinc but, However, there are basic similarities in like zinc, requires ventilation behind the these assemblies. panels to prevent pitting and corrosion. Metal systems come in a variety Depending on the material used, any of materials, with aluminum being number of finishes and coatings may be the most common due to its relative selected based on aesthetic and perfor- durability, corrosion resistance, and mance requirements. Paints, such as light weight. Aluminum panels are fluoropolymers, enjoy wide use in metal available in flat plates—solid plate wall applications, as they come in a range approximately one-eighth inch thick—or of colors, and are resistant to fading. composite panels, where foam insula- Porcelain enamel coatings fuse tion is sandwiched between two layers powdered glass to the metal surface at of thin sheet aluminum. a high temperature. Like fluoropolymers, Steel is another commonly used ma- they are durable and offer designers a terial. Strong and providing superior im- number of appearance options. pact resistance, steel is often selected Many finishes have been developed for façades in high-traffic areas and for more for their protective properties than roofs that must span longer distances for their appearance options. Galvaniz- between supports. However, steel’s ing, for instance, is a sacrificial protec- strength means it is heavier than other tive coating of zinc applied to steel that materials, necessitating more underly- has a silver-gray appearance and is used ing structure to support the load. Steel to prevent corrosion of the underlying also requires a protective coating to steel. Anodizing, an electrolytic process prevent deterioration due to corrosion. used on aluminum to increase its cor- Zinc panels are also becoming more rosion resistance, is only available in a popular because of the distinctive ap- limited range of colors (typically black, pearance and durability afforded by the bronze, and silver). surface patina acquired during weather- It’s important to remember that not all ing. Patina formation occurs naturally in finishes are compatible with all materi- the presence of moisture and carbon di- als. Even when an appropriate coating or oxide. Care should be taken where mois- paint is selected, application timing can ture may be present without sufficient significantly impact longevity. Typically, it carbon dioxide, such as at the underside Common problems in metal panel systems include: is best to apply coatings and paints after 1) condensation, 2) corrosion, 3) movement and of roof panels, where condensation and water intrusion, 4) peeling coatings, and the metal has been formed, to avoid other incidental moisture can cause corro- 5) discoloration and staining. hairline breaks in the material at bends.

44 | BUILDING DESIGN+CONSTRUCTION | January/February 2021 HOFFMANN ARCHITECTS

PREFABRICATED METAL WALL SYSTEMS This securement can be achieved in two ways: by With metal panel systems Generally speaking, there are two types of metal use of “exposed” or “concealed” fasteners. offering an expanded range wall systems available: “closed” and “open.” With exposed fastener systems, panels are of features and options, examples of metal-clad Closed assemblies (also called “barrier” or “face- attached to the substrate using fasteners that are structures can be found in sealed” systems) use the metal wall panels to re- screwed through the panel and left exposed for nearly all building types, sist the infiltration of weather (air and water) into the ease of installation and aesthetic effect. While the including this luxury apartment tower. building. The metal wall panel system is the air and fasteners are often gasketed to limit water infiltra- moisture barrier of the façade. To achieve weather tion, this method is most commonly employed in protection, the joints between the metal panels open panel systems. must be sealed to form a comprehensive barrier. Systems using concealed fasteners employ Open systems, as the name implies, do not clips to secure panel ends to the sub-framing sys- provide the primary protection against air and mois- tem with bolts. This method can provide a clean, ture infiltration. In these applications, a second- uninterrupted look to the metal panel system in ary means of protection must be included behind either closed or open assemblies. the panels, with consideration given to the way in which moisture that permeates the cladding can PREFABRICATED METAL ROOF SYSTEMS be directed back to the exterior. Rainscreens are Prefabricated metal roof systems are broadly de- a common form of open system that have become fined as being either “hydrokinetic” or “hydrostatic.” increasingly popular in the last decade. Hydrokinetic systems are not necessarily “wa- How metal wall panel systems are installed and ter-tight.” These assemblies shed water by moving attached to the substrate (and ultimately to the it across their surface and off the roof as quickly building structure) is another defining element of as possible. This requires that hydrokinetic roofs their design. The panels are most commonly se- have adequate pitch to effectively move water and cured to a metal sub-framing system consisting of limit infiltration beneath panels. Since panel joints “z” girts, metal studs, or proprietary configurations. are not weathertight, hydrokinetic systems require

BDCuniversity.com | BUILDING DESIGN+CONSTRUCTION | 45 | BUILDING ENVELOPE |

using sealant or solder for additional weather integrity.

DESIGN CONSIDERATIONS

HOFFMANN ARCHITECTS Whether designing a prefabricated metal wall system, roof system, or both, the architect or engineer must address certain fundamental design considerations. While manufacturers can be instrumental in this effort, a design professional with knowledge of the building enclosure is often indispensable. The following is a brief list of some of the issues to be addressed during design. • Structure. One of the primary con- siderations for building enclosure as- semblies is their structural integrity. Façade and roof cladding systems must have the ability to resist a va- riety of forces to which they may be subjected, including gravity, seismic load, wind load, deflection, and inter- story drift. It’s important to remem- ber that every component of the system, from the substrate to the fasteners to the panels themselves, a substrate/underlayment system to protect the must be considered when designing the cladding Once considered budget interior of the building against incidental moisture to meet the necessary structural requirements. cladding confined to that finds its way beneath the roof surface. • Expansion. Metals will increase or decrease warehouses and industrial facilities, today’s metal Hydrostatic systems, on the other hand, are in size in response to a change in temperature. panel systems are viable water and air barriers that form the primary protec- A metal’s tendency to be affected in this way is options even for high-end tion at roof level against weather infiltration into defined as itsthermal expansion coefficient. projects. the building. The higher the coefficient, the more a material Metal roof systems are also classified by their will expand or contract because of a change in ability to span between structural members. Archi- temperature. Zinc and aluminum, for instance, tectural systems cannot span long distances and re- have relatively high coefficients (19.0 and 13.1, quire a continuous substrate for support. As a result, respectively), while that of steel is relatively low these assemblies are often hydrokinetic, since the (7.0). This means that zinc panels will expand substrate that is required for support can also act as nearly three times as much, and aluminum nearly a surface for an underlayment protection system. twice as much, as steel. Structural systems can span between structural Accommodation for such movement must be members without the assistance of a continuous built into the design to avoid issues such as buck- substrate. The panels of these systems derive ling, fastener failure, and deterioration of system their strength in a variety of ways, including the use integrity. Movement tolerance can be achieved of stronger metals, thicker panels, raised panel in a variety of ways, including slotting of fastener ends, intermediate ribs, and corrugation in the openings, limiting the length of panel sections, and panel profile. Since structural systems require no incorporating expansion joints. substrate, they are often hydrostatic. • Weather Integrity—the ability of roof and façade Regardless of the system selected, metal roof assemblies to resist the infiltration of air and panels are typically interlocked, or “seamed,” moisture—is essential. As discussed earlier, these together. Examples of interlocking methods include systems can be either “closed” (hydrostatic) or flat seams, standing seams,and battens. In hy- “open” (hydrokinetic). In either case, consideration drostatic applications, seams may also be sealed of weather integrity and accommodation for mois-

46 | BUILDING DESIGN+CONSTRUCTION | January/February 2021 ture are necessary design challenges. underlying substrate, and is comprised of mineral In prefabricated metal façades, “closed” or wool or foam plastics such as extruded polystyrene face-sealed barrier systems rely on a primary seal or polyisocyanurate. between joints—often seam tape or an elasto- Energy performance requirements vary depend- meric sealant system—to provide weather integrity. ing on climate zone and the model building code These primary seals between panels have a adopted in a region. In New York City, for instance, shorter life expectancy than the panels themselves roof assemblies must include insulation with a and are therefore the first part of the system to thermal resistance of at least R-30. Metal-framed fail. Consequently, they present an ongoing mainte- walls must include R-7.5 of continuous insulation, nance concern. plus an additional R-13 of insulation between the In “open” façade systems, the substrate behind sub-framing studs. the panels (often exterior-grade sheathing with As much of prefabricated metal panel wall and a water-resistant coating or cladding over metal roof systems are, by definition, metal, linear and studs) acts as the primary protection against air point transmittances (thermal shorts that transfer and water infiltration. In such cases, water that will energy between the interior to the exterior of the inevitably find its way behind the metal panels must building) should be considered and eliminated to be channeled to the exterior of the building through the greatest extent possible. Such measures also a system of flashing and weeping. Even with such help reduce the potential for condensation within protection in place, the volume of water that can the assemblies. infiltrate behind the panels should be limited. Because of the need to attach the panels to the Hydrostatic roof systems rely on the pitch of the roof and the integrity of the panel seams (often en- hanced with seam tape, sealant, or solder) to keep water from infiltrating the building. Hydrokinetic sys- tems also limit water infiltration through roof-pitch and the integrity of seams, but do not eliminate it. HOFFMANN ARCHITECTS In such systems, an underlayment consisting of, for instance, #30 felt paper and a slip sheet (building paper), must be applied to the substrate to protect the building interior. In certain climates, membrane protection at the roof perimeter extending at least 24 inches onto the building must also be consid- ered to limit the deleterious effects of ice. Steep-sloped roofs must include adequately sized and configured gutters, leaders, and down- spouts to remove water that collects on the surface or substrate. Gutters can be external or “built-in” (concealed). Lastly, “weather integrity” includes not just resistance to moisture, but to air. Most governing authorities now have specific requirements limiting substrate, often by means of metal girts, insula- the flow of air through the building enclosure. As tion within the assembly is frequently interrupted, This aluminum standing- with moisture protection, closed systems rely on causing thermal shorts and reducing the overall seam roof system is installed over ice dam the metal panel system to resist such air flow, efficiency of the wall or roof. In such cases,isola - membrane, as well as while open systems depend on a substrate barrier tors, typically in the form of lightweight, highly stiffening ribs at the flat of for preventing air infiltration. porous coatings called aerogels, can be applied to each panel to minimize oil canning and telegraphing • Thermal Performance. Building enclosures must the sub-framing system to reduce the transfer of of structural members. meet increasingly stringent requirements regarding energy along its surface. energy and thermal performance. Metal systems • Condensation. The potential for condensation to benefit from having a low thermal mass; however, develop within the façade or roof system must be they are not very good insulators. Therefore, the considered during design and measures taken to use of insulation within the assembly must be a limit the deleterious effects of such moisture. While consideration during design. Typically, the insula- providing adequate ventilation behind or beneath the tion is installed between the metal panels and the panels is sometimes sufficient, a correctly designed

BDCuniversity.com | BUILDING DESIGN+CONSTRUCTION | 47 | THE BUILDING ENVELOPE |

and installed vapor retarder is often necessary. TESTING AND MOCK-UPS OF • Sound Attenuation. Typically, metal panel systems are not very effective at mitigating METAL PANEL SYSTEMS the transfer of sound. During design, the STC (“sound transmission class”) or OITC (“outdoor/ he complexity and performance requirements of enclosure systems, indoor transmission class”) rating for the project in general, and prefabricated metal panel systems, in particular, should be established and the sound attenuation T make mock-ups and testing a worthwhile consideration. The fol- properties of the system identified. Measures lowing is a summary of options available at various stages of design and can then be taken to enhance the properties of construction. the assembly through, for instance, the incorpora- tion of additional insulation or air cavities. MANUFACTURER’S TEST DATA AND TECHNICAL • Combustibility. The use of certain building DOCUMENTATION materials within a prefabricated metal panel At the design phase, the manufacturer’s test data and technical documen- enclosure system (such as foam plastics as tation should be provided. This allows the design team either to confirm insulation or a core material in composite metal that the proposed assembly can meet the various project requirements panels) may present issues with combustibility. or to identify key information that might be missing. Incomplete technical The design professional should reference the documentation from the manufacturer may be an indication that the system applicable code for requirements governing the is not appropriate for the application, or that additional testing is required combustibility of such assemblies. Keep in mind to validate its use. that it is not just the assembly itself that must conform to these requirements; detailing of the VISUAL MOCK-UP transition to adjacent building systems (such (VMU) as between floors and between walls and roofs) During pre-construction,

HOFFMANN ARCHITECTS must be considered to achieve the required fire a visual mock-up (VMU) protection across the entire exterior envelope. allows all stakeholders • Appearance. While this article focuses on the to observe a represen- technical aspects of metal wall and roof sys- tative sample of the tems, appearance is nonetheless an essential proposed system and design consideration. Technical considerations approve it for aesthetics can affect that overall appearance, and care and design intent. The should be taken to avoid choices that can have a Mock-ups provide a representative sample to VMU should enable the evaluate design and performance. detrimental effect on aesthetics. team to confirm overall Consult with manufacturers regarding the appearance, propor- potential for issues such as telegraphing, where tions, color, and texture, as well as the effectiveness of attempts to limit the outline of underlying stiffeners or fasteners undesirable visual characteristics, such as “telegraphing” and “oil-canning.” appear on the surface of the metal panel, or oil- A VMU can be a project-specific mock-up fabricated on- or off-site, or it canning, a wavy or dimpled appearance on the can be a similar system installed on a previous project, as identified by the surface of the panel. design team and manufacturer. • Snow and Ice Accumulation. In certain cli- mates, the potential for snow and ice to build up PERFORMANCE MOCK-UP (PMU) on roofs is a concern. Such accumulations can A performance mock-up (PMU) is a representative sample of the enclo- damage perimeter assemblies and fall from the sure assembly installed at an off-site testing facility, where approved building, creating a hazard. test methods can ascertain the system’s conformance with performance Improper drainage design can often lead to requirements, including structural integrity, thermal resistance, acoustics, such build-up at roof edges. Gutters and leaders and air and water resistance. PMU testing typically takes place concur- that are too small or configured incorrectly can- rently with construction. not adequately remove runoff before it refreezes, In short, a manufacturer’s published data provides an overview of a resulting in large accumulations of ice at gutters wall or roof assembly’s performance, but it is no substitute for indepen- and eaves that can damage perimeter assem- dent verification testing. Pre-construction VMU testing establishes design blies and infiltrate the building through a process parameters, while in-situ PMU testing during construction, for issues such known as “ice jacking.” During design, calcula- as air and water resistance, provides validation that the system meets tions should be performed to ensure that the performance requirements. roof’s drainage system is adequately sized and that gutters are configured in accordance with

48 | BUILDING DESIGN+CONSTRUCTION | January/February 2021 | THE BUILDING ENVELOPE |

PHOTO CREDIT roof design to limit this possibility. As with a roof’s drainage system, calculations must be performed HOFFMANN ARCHITECTS to affirm that the system is restraining snow and ice in a manner that will promote gradual release of runoff over time and limit accumulation in any one area of the roof. Ideally, such systems should not penetrate the metal roof panels. Proper attic ventilation helps to prevent ice dam formation at eaves, alleviating strain on the snow guard system and reducing the possibility of leaks. • Dissimilar Metals. Different metals that come in contact with each other within the assembly can lead to corrosion, through galvanic action, and to staining, through water runoff. When the poten- recognized standards such as the Sheet Metal and tial for galvanic action exists, dissimilar metals For this military train- Air Conditioning Contractors’ National Association should be separated by a protective coating or ing camp, standing-seam Architectural Sheet Metal Manual. other method. aluminum roofs with a high-performance resin Even with an adequately designed drainage • Sustainability. It’s important to note that many coating met fast-track system, the potential for accumulation of snow and pre-manufactured metal panel systems include project demands, while ice in amounts large enough to be a danger still anywhere from 25% to 95% recycled material. At offering durability in the harsh seaside climate. exists. Consideration should be given to incorpo- the end of their lifespan, most metal panel sys- rating an appropriate snow guard system into the tems can be recycled again.+

LASTING BEAUTY

The sunshine state is no stranger to extreme weather conditions, which is why property owners for residential structures consider using metal when they need to replace a roof. Florida Native Roofing installed Image II™ roof panels from Metal Sales on this single-family Cypress Residence. The panels are coated with Fluropon® from Sherwin-Williams Coil Coat- ings, in the color Taupe. Together, the metal roof and solar reflective coating will help the owner save money on energy costs and will deliver exceptional durability. There’s no place like home when you have a roof that will endure harsh weather conditions for decades. coil.sherwin.com CIRCLE 768

50 | BUILDING DESIGN+CONSTRUCTION | January/February 2021