Essentials

Metal roof systems often are misunderstood, especially by building owners and consumers. For instance, many building owners are unaware that there actually are two types of metal panel roof systems. Also, building owners may not realize metal roof systems are not limited to steep slopes—they can be installed for low-slope applications, as well. But once a building owner chooses to install a metal roof system, he must be well-informed about the special design considerations that are unique to metal roofing.

System types

Metal roof systems traditionally are divided into two categories: architectural and structural.

Architectural metal panel roof systems typically are hydrokinetic, water-shedding systems. These systems are designed to shed water rapidly over panels' surfaces, so the panels' seams are not necessarily watertight. For panels to shed water quickly, a roof must have adequate slope. NRCA recommends a roof slope of 3-in-12 (14 degrees) or greater for architectural metal panel systems.

Architectural metal panel roof systems historically have been composed of copper, terne metal, light-gauge aluminum, light-gauge stainless steel, zinc, tin, light-gauge galvanized steel, lead and lead-coated copper sheets. These materials are considered nonstructural and, consequently, require a continuous or closely spaced roof deck for support. Underlayment, slip-sheet and ice-dam protection, if applicable, are recommended as secondary barriers against water penetration.

Structural metal panel roof systems typically are hydrostatic, water-barrier systems. The joints, laps and junctures in these systems are designed to resist the passage of water under hydrostatic pressure. NRCA recommends a structural metal panel roof's minimum slope be 1/2-in-12 (2.5 degrees).

Structural metal panel roof systems have the strength and capability to span structural members (joists or purlins) without being supported by a roof deck. Structural metal panels derive their strength from using heavier gauges of metal or being profiled with high side ribs with stiffening ribs or intermediate ribs located in the pan.

Structural metal panels also can be used as architectural metal panels though the reverse is not true.

Stainless steel, galvanized steel and aluminum commonly are used to fabricate structural metal panel roof systems. If metal panels are installed over intermittent supports, an underlayment typically is not used. However, if there is a solid substrate, NRCA recommends using an underlayment.

Design considerations

Once the decision has been made to install metal roofing materials and an appropriate system has been chosen, a roof system designer must consider the following issues, which are unique to metal roof systems:

• Moisture control. Expect condensation to occur on metal panels' undersides. To control potential condensation problems, a roof system designer must consider the overall building design and how it integrates with the roof assembly, as well as specifically address vapor retarder, insulation and ventilation issues.
  
  
• Panel attachment. It is critical fasteners and clips be designed to meet wind-uplift requirements. Wind-uplift requirements are dictated by the governing building code.
  
  
• Thermal movement. Metal roof systems must be able to accommodate thermal movement. Typical metal roof systems fix panel expansion and contraction in a predetermined direction.
  
  
• Oil canning. This aesthetic problem is related to distortions in the flatness of metal panels. NRCA suggests building owners and designers address this issue before beginning a project rather than after completion.
  
  
• Snow guards. Because metal panels have smooth surfaces and transfer heat quickly, snow and ice tend to loosen and slide suddenly from panels. Roof system designers should evaluate whether they need to incorporate snow guards into their designs.
  
  

The items listed are just a brief overview of metal roof system design issues. For more information, roof system designers are encouraged to reference the Architectural Sheet Metal and Metal Roofing section of The NRCA Roofing and Waterproofing Manual, Fifth Edition, or contact NRCA's Technical Services Section at (847) 299-9070; fax (847) 299-1183; or access NRCA's Web site, www.nrca.net.

Joan P. Crowe is an NRCA manager of technical services.