Metal Offers Durable, Energy-efficient Roofing Solutions
Learning Objectives - After this course, you should be able to:
- Describe the two major metal roofing types.
- Explain the importance of underlayments.
- Compare the advantages of metal substrates.
- Discuss recent trends in high-performance, energy-efficient finishes.
The roof of a structure is a key element of any building design and one of its most important structural aspects. Value-conscious building owners seeking long-term roofing solutions that combine aesthetics and performance are increasingly turning toward metal roofs. According to the National Roofing Contractors Association, metal roofs have surged in popularity over the past decade.
Metal roofs have shown an ability to stand up to the elements, and provide low life-cycle costs and longevity with minimum maintenance. In addition, manufacturers are expanding their metal roofing options in a wide variety of finishes, profiles and colors that enhance building aesthetics. Advances in coatings and finishes have enabled the formulation of "cool" metal roofing that add to the roof's energy-saving properties and can impact the costs of heating and cooling the building.
In specifying a metal roof numerous questions arise during design development, and with the plethora of metal roofing products on the market, it is important to understand the complexities of the various roofing systems. Only by specifying the proper roofing system, underlayment, substrate and finish will the full potential of a metal roof be realized.
Characteristics of Metal Roofing Systems
There are two basic types of metal roofing systems, which are classified according to the way in which the roof resists water. This is determined by their slope or pitch. The first type, structural roofing, features water-resistant panels and is used on low-slope roofs. Architectural, or non-structural roofing, is designed to shed water quickly, and requires steeper slopes.
The Structural Roof
Slopes of between 1/4:12 and 3:12 require water-tight structural panels that have been designed to resist the passage of water under hydrostatic pressure (standing water).
Typically, in a structural roof, panels are capable of spanning between structural supports, and are attached by clips to the building structure The floating clip system that fastens the structural panels to the roof accommodates thermal expansion and contraction. Because thermal expansion and contraction affects all materials to some extent, structural panels and flashing must be designed to accommodate that same expansion. It is worth noting that steel expands 1 inch per 100 feet, given a 100-degree temperature change. Aluminum will expand 2 inches per 100 linear feet given the same 100-degree temperature change. Panels designed for a structural roofing application should also be UL-90 rated for wind resistance. (See "Code Considerations," next column.)
Structural metal panel roof systems are typically fitted over a roof assembly that includes structural metal framing and purlins. In areas with heavy snowfall, high wind-load areas or when the Owner requires a roof assembly with a high "R" value, panels may be installed over solid substrates, such as metal decking, plywood, oriented-strand board (OSB) in conjunction with ISO board insulation to achieve the necessary structural load of the panel system along with the required "R" value of this roof assembly. Batt insulation is typically part of the assembly and thermal barriers such as wood blockers may be installed between structural framing and the roof panels.