| 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