Metal 101: Understanding Best Use Cases for Metal Roofs and Walls
The longevity and performance of metal roof and wall systems depends on an understanding of material properties, code requirements, and best practices.
Years ago, prefabricated metal panel roof and wall systems did not find wide acceptance in high-end commercial applications. While they were cost-effective, aesthetic perceptions relegated them to large storage buildings, manufacturing facilities, and big box stores. However, versatility, reliable performance, and relatively low installed costs have now made these systems attractive in more sophisticated applications. Manufacturers have expanded their product lines with the aim of maximizing product use across a range of building types. But of course no building enclosure system is without design challenges and considerations. From structural and weather-proofing design to energy efficiency, the longevity and performance of metal roof and wall systems depends on an understanding of material properties, code requirements, and best practices.
Materials and finishes
Most metal wall and roofing systems are proprietary, each manufacturer having its own profiles, method of attachment, and performance specifications. However, there are basic similarities in these assemblies.
Metal systems come in a variety of materials, with aluminum being the most common due to its relative durability, corrosion resistance, and light weight. Aluminum panels are available in flat plates approximately 1⁄8-inch thick or composite panels, where foam insulation is sandwiched between two layers of thin sheet aluminum. Other material options include steel, zinc, copper, and stainless steel.
Depending on the material used, any number of finishes and coatings may be selected based on aesthetic and performance requirements. Paints, such as fluoropolymers, enjoy wide use in these applications, come in a range of colors, and are resistant to fading. Other options are galvanized or anodized finishes, and porcelain enamel coatings.
Generally speaking, there are two types of prefabricated metal wall systems available: closed and open. Closed assemblies (also called barrier or face-sealed systems) use the metal wall panels to resist the infiltration of weather (air and water) into the building. The metal wall panel system is the air and moisture barrier of the façade. To achieve this, the joints between the metal panels must be sealed to form a comprehensive barrier.
As the name implies, open systems do not provide the primary protection against air and moisture infiltration. In these applications, a secondary means of protection must be included behind the panels, with consideration given to the way in which moisture that permeates the cladding can be directed back to the exterior. Rain screens are a common form of open system that have become increasingly popular in the last decade.
How these panel systems are installed and attached to the substrate (and ultimately to the building structure) is the next defining element of their design. The panels are most commonly secured to a metal sub-framing system consisting of “z” girts, metal studs, or proprietary configurations. This securement can be achieved in two ways: by use of exposed or concealed fasteners.
With exposed fastener systems, panels are attached to the substrate using fasteners that are screwed through the panel and left exposed for ease of installation and aesthetic effect. While the fasteners are often gasketed to limit water infiltration, this method is most commonly employed in open panel systems.
Systems using concealed fasteners employ clips to secure panel ends to the sub-framing system with bolts. This method can provide a clean, uninterrupted look to the metal panel system in either closed or open systems.
Metal roof systems
Prefabricated metal roof systems are broadly defined as being either hydrokinetic or hydrostatic. Hydrokinetic systems are not necessarily water-tight. These assemblies shed water by moving it across their surface and off the roof as quickly as possible. This requires that hydrokinetic roofs have adequate pitch to effectively move water and limit infiltration beneath panels. Since panel joints are not weather-tight, hydrokinetic systems require a substrate/underlayment system to protect the interior of the building against incidental moisture that finds its way beneath the roof surface.
Hydrostatic systems are water and air barriers that form the primary protection at roof level against weather infiltration into the building.
Metal roofing systems are also classified by their ability to span between structural members. Architectural systems cannot span long distances and require a continuous substrate system for support. As a result, these systems are often hydrokinetic. That’s because the substrate that is required for support can also act as a surface for an underlayment protection system.
Structural roofing systems can span between structural members without the assistance of a continuous substrate. The panels of these systems derive their strength in a variety of ways, including the use of stronger metals, thicker panels, raised panel ends, intermediate ribs, or corrugation in the panel profile. Because structural systems require no substrate, they are often hydrostatic.
Regardless of the system selected, the metal roof panels are invariably interlocked, or seamed, together. Examples of interlocking methods include flat seams, standing seams, and battens. In hydrostatic applications, seams may also be sealed using sealant or solder for additional weather integrity.
5 Keys To Good Metal Roof and Wall System Design
The following fundamental design considerations must be taken into account for a successful metal roofing or wall project.
Whether it’s a prefabricated metal wall system, roof system, or both, certain fundamental design considerations must be addressed during design.
1. Structural integrity. The initial consideration for building enclosure assemblies is their structural integrity. Façade and roof cladding systems must have the ability to resist a variety of forces to which they may be subjected including gravity, seismic load, wind load, deflection, and inter-story drift. It’s important to remember that every component of the system, from the substrate to the fasteners to the panels themselves, must be considered when designing the cladding to meet the necessary structural requirements.
2. Weather integrity. The ability of the roof and façade assemblies to resist the infiltration of air and moisture is also essential. As noted earlier, these systems can be either closed (hydrostatic) or open (hydrokinetic).
In prefabricated metal façades, closed or face-sealed barrier systems rely on a primary seal between joints — often seam tape or an elastomeric sealant system — to provide weather integrity. These primary seals between panels have a shorter life expectancy than the panels themselves and are therefore the first part of the system to fail. Consequently, they present an ongoing maintenance concern.
In open façade systems, the substrate behind the panels (often exterior grade sheathing with a water-resistant coating or cladding over metal studs) acts as the primary protection against air and water infiltration. In such cases, water that finds its way behind the metal panels must be channeled to the exterior of the building through a system of flashing and weeping. Even with such protection, the volume of water that can infiltrate behind the panels should be limited.
Hydrostatic roof systems rely on the pitch of the roof and the integrity of the panel seams (often enhanced with seam tape, sealant, or solder) to keep water from infiltrating the building. Hydrokinetic systems also limit water infiltration through roof pitch and the integrity of seams, but do not eliminate it. In such systems, an underlayment consisting of, for instance, 30 pound 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 perimeters extending at least 24 inches onto the building must also be considered to limit the deleterious effects of ice.
Steep-sloped roofs must include adequately sized and configured gutters, leaders, and downspouts to remove water that collects on the surface or substrate. Gutters can be external or concealed.
Lastly, weather integrity includes not just resistance to moisture, but to air. Most governing authorities now have specific requirements limiting the flow of air through the building enclosure. As with moisture protection, closed systems rely on the metal panel system to resist such air flow, while open systems depend on a substrate barrier for such protection.
3. Thermal performance. Building enclosures must meet increasingly stringent requirements regarding energy and thermal performance. Metal systems benefit from having a low thermal mass; however, they are not very good insulators. Therefore, the use of insulation within the assembly must be considered.
Energy performance requirements vary depending on climate zone and the model building code adopted in a region. In New York City, for instance, roof assemblies must include at least R-30 of insulation. Metal framed walls must include R-7.5 of continuous insulation plus an additional R-13 of insulation between the sub-framing studs.
Linear and point transmittances (thermal shorts that transfer energy from the interior to the exterior of the building) should be considered and eliminated to the greatest extent possible. Such measures also help reduce the potential for condensation within the assemblies.
4. Expansion and thermal movement. In general, metals have a high coefficient of expansion, meaning that they will expand and contract significantly in accordance with temperature changes. While some metals, such as aluminum and zinc, have a higher rate of expansion than others, all metals will expand and contract in reaction to changes in surface temperature. Accommodation for such movement must be built into the design to avoid issues such as buckling and deterioration of system integrity.
5. Combustibility. The use of certain building materials within a prefabricated metal panel enclosure system (such as foam plastics as insulation or a core material in composite metal panels) may present issues relative to combustibility. The design professional should reference the applicable code for requirements governing the combustibility of such assemblies, and keep in mind that it is not just the assembly itself that must conform to these requirements. Detailing of the transition to adjacent building systems (such as between floors, at windows and doors, and between walls and roofs) must be considered to ensure that the required protection is maintained.
Russell M. Sanders (r.sanders@ hoffarch.com), AIA, is executive vice president, senior director, technical services, and chief operating officer in charge of technical personnel with Hoffmann Architects. Craig A. Hargrove (email@example.com), AIA, LEED AP, is senior vice president and director, architecture.