There are a number of practical advantages to choosing a metal roof for your next steel building project.
Tile roof systems are a smart choice for homeowners, architects, builders and roofers, because they are durable, are available in different styles and colors, and provide energy efficiency.
But compared to tile roof systems, metal roofs provide even more advantages. These systems are fire resistant, lightweight, low maintenance, and adept at shedding rain and snow. In addition, metal is a durable material that will not deteriorate like asphalt shingles when exposed to the elements or crack when walked across, like clay barrel tiles. If designed properly, a metal roof can withstand extreme wind speeds, making it ideal for climate zones that experience hurricanes and even tornadoes of moderate strength (F2 on the Fujita Scale, or 120 mph winds). Metal is also a good choice for the environment, as the average metal roof system contains 25% reclaimed content and is 100% recyclable at the end of its life cycle, which can exceed 50 years. This means a significant reduction in the amount of landfill waste associated with a typical roof tear off. Moving beyond the general benefits, let us explore one of the most popular designs in metal roofing today, the standing-seam.
The standing-seam roof system is so called because the seams are raised and joined together above the center of the panel, or flats, creating clean vertical lines and a channel for rain and snow to be swept away by. Here is an example of two different standing seam panels, the trapezoidal and the vertical leg/flat pan:
Vertical Leg/Flat Pan
Trapezoidal Pan
These panels, or pans, come preformed and are cut in standard lengths of 8, 10, or 12 feet, and are most often made from steel. The thickness of the steel used commonly ranges from 24 to 29 gauge (the higher the number, the lesser the thickness), with the higher grade systems tending, as one would assume, toward the lower end of the scale. Once the section is fastened to the support via a flange or clip, which does not penetrate the exterior and is concealed by the next panel, the ridge seams are then crimped together using a machine - typically an electrical seaming device. The seamer is on rollers, so an operator simply places it down on one end of the joint and then removes it once it has reached the other end, leaving a finished seam in place. Since the seam itself essentially holds the panels to the fasteners, preventing the sections from unzipping, their design is all important.
One seam of interest that has proven itself in testing laboratories and real world applications is the 360 + 90 degree seam. This design provides additional load bearing bends between the seam and the stresses of panel deflection (when the panel is pushed sideways). This is a seam geometry that also isolates the seam sealant, which is applied at the edge of the seam to prevent water intrusion. Here are two forms of this seam type that also correspond to the panels seen above.
Triple-Lok / Quad-Lok (Flat Panel)
EZ-Lok / Triple-Lok / Quad-Lok (Trapezoidal Panel)
Certifications and listings credited to the 360 + 90 degree (Triple-Lok) seams:
- Underwriters Laboratories UL-90 Classification Construction No. 556 or No. 506 (vertical leg)
- Factory Mutual Class I-90 listing
- Corps of Engineers CEGS 07416 Uplift Test
- ASTM E 1592 Uplift Test
- ASTM E 1680 Air Infiltration
- ASTM E 1645 Water leakage
In addition to seams, the proper sealants must be used in order to ensure that a metal roof will shed water properly. The recommended sealant to employ is butyl tape. Tape sealants are the preferred choice for an important reason: they contain a significant percentage of solids content (97% or so) and thus will not shrink as much as the gun-grade alternative, which usually contains about 70% solids and 30% solvents. Shrinkage occurs when the solvents evaporate, potentially compromising a seal. The butyl polymer tapes (1/2" wide is most often the practical size for panel seams) may last 30 years or more if not exposed to UV rays. This is accomplished, as can be seen in the panel diagrams above, by always applying the sealant inside the seam. It is worthwhile to note that silicone must never be used as a sealant on metal roofs, as many of these products contain acids, which can potentially harm metallic coatings. Further, silicone does not tend to adhere well to metal.
One of the secrets behind a metal roofs' long life are the metallic coatings that have been developed to shield them from the elements. There are three types of plating used to protect steel from the primary enemy of most metals - corrosion. Stemming from the Latin corrodere - meaning literally to "gnaw to pieces" - metal can react with oxygen, water, or pollutants in the air (chemical corrosion) to form oxides on its surface. This oxidation takes the form of red rust on steel and is prevented by the first form of plating: pure zinc. Commonly referred to as galvanized steel, the pure zinc coating functions as a physical boundary that shelters the steel core. Since it is more reactive, or breaks down faster than steel, zinc essentially provides a sacrificial layer of molecules. Pure aluminum is also used to coat steel and according to the editors of the Journal of Light Construction, provides significantly better protection than galvanizing, due to the truly inert (non-reactive) nature of aluminum itself. The third form of plating is actually a combination of the first two. Known as galvalume, the weight of zinc and aluminum is generally AZ-50 or AZ-55, meaning a composite of either 50 % aluminum / 50 % zinc or 55 % / 50 % respectively.
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