Metal roofing has long been popular for agricultural and industrial buildings, but it's also a practical choice for residential use in every part of the country. Homeowners and builders in the snowbelt can benefit from metal's ability to shrug off ice and snow. In wildfire-prone regions of the West, its natural fire resistance is a major selling point, and its resistance to wind makes it an attractive choice in coastal areas. It's virtually immune to the unsightly mildew growth (actually a variety of algae) that often forms on asphalt shingles in the warm, humid southern states. The reflective qualities of metal also enable southern homeowners to trim their utility bills: According to a 2001 study by Florida Power and Light, some types of metal roofing can reduce cooling costs by up to 23% compared to asphalt shingles. On the Great Plains, where hailstorms can destroy a roof in minutes, some metal panels can withstand the impact of even golf ballsized hail without damage, and many insurance companies offer discounts to homeowners for installing them. At 50 to 150 pounds per square, metal has the lowest weight of any residential roofing, making it attractive in seismically active areas, where heavier roof systems can stress framing members during earthquakes. In many cases, its light weight permits installations over an existing roof, eliminating the labor-intensive tear-off process and the cost of disposal.
The installed cost of metal is higher than that of asphalt shingles, but when life expectancy and maintenance costs are factored in, metal compares favorably to less expensive roofing materials, especially in the South, where asphalt shingles last an average of just 17 years. Environment-conscious consumers and trade groups also like to point out that while asphalt roofing ends up in the landfill at the end of its life span, metal roofing can be recycled.
Those benefits are characteristic of all metal roofing, but selecting the right material for a given application requires some thought. The range of choices -- in type of metal, thickness, finish, and method of application -- is wider than it's ever been and continues to grow. For organizational purposes, though, all metal roofing products can be divided into three general categories: exposed-fastener panels, standing-seam roofing, and modular panels.
Exposed-Fastener Panels
For many people, the words "metal roofing" bring to mind the screw-down or nail-down agricultural material, or "ag panel," used on barns and industrial buildings, as well as some residences (see Figure 1). Easily identified by the lengthwise ridges that stiffen the panels and overlap to provide a weathertight seal at the edges, exposed-fastener panels are attached with galvanized screws or nails over a plywood deck or on spaced purlins.
Figure 1.Exposed-fastener metal roofing has long been used for barns and industrial buildings because it's inexpensive, installs quickly, and lasts 30 or 40 years. It also makes a great residential roof in areas with a rural character, especially in hot or snowy regions.
Ridges or flats? The panels are fastened to the deck or purlins with special-purpose nails or screws equipped with neoprene or EPDM washers, which compress beneath the fastener head to seal the puncture left by the shank. Nails -- the fastener of choice in the days before reliable battery-powered screw guns and still used in some areas -- should always be driven through the raised ridges between flats. Most roofs today, however, are fastened with screws, and while many installers also run screws through the ridges, manufacturers advise against it. Screws have lower shear strength than nails, which makes them vulnerable to breakage when they're left sticking out far enough to clear the ribs, and adequately tightening screws without crushing the ridge takes a delicate touch (Figure 2).
Figure 2.Many installers put screws in the ribs even though manufacturers discourage it. The practice is a holdover from when the dominant fasteners were nails. Using screws and installing them in the flat sections of the panel is considered a better practice by manufacturers, because of screws' lower shear strength and better sealing compared to nails.
While it may seem counterintuitive to those used to fastening with nails, screws should be driven into the flats, at the spacing specified by the manufacturer.
Because each screw hole represents a potential leak, it's worth taking the time to do this part of the job right. Frank Farmer of American Roofs in Flushing, Mich., frequently sees problems with screws from improper installation. "Installers run them in at an angle, or they don't tighten them enough so the washers don't seal correctly -- and small leaks can eventually rot the sheathing," he says. Troy Thomas of Fabral, one of North America's largest metal roofing producers, notes that choosing screws with EPDM washers, rather than lower-cost neoprene, will reduce the possibility of leaks at fasteners.
With installed prices starting at about $150 per square, exposed-fastener panels are the least expensive variety of metal roofing, but this approach does have some drawbacks. Its rustic appearance is inappropriate for some applications, and the protruding fasteners tend to catch leaves and other debris as well as inhibiting snow from sliding off the roof. The panel ridges also create voids beneath the panels that provide ideal habitat for some insects and small animals. While manufacturers provide foam rubber closure strips to seal these cavities, skeptics wonder if the closure strips will last as long as the panels (Figure 3). But Jerry Iselin of Metal Roof Specialties in Tacoma, Wash., says, "I've only seen problems with closure strips maybe a half-dozen times, where they shrunk from poor manufacturing or birds pecked them out to use for nesting material." High-quality closure strips are laminations of two types of foam rubber: A softer outer layer for sealing is bonded to a more rigid internal foam that resists compression.
Figure 3.Rubber closure strips seal up the cavities under the ribs of exposed-fastener panels. Inside closure strips go under the panel, sealing eaves; outside closure strips lie on top, sealing under the ridge cap. Adhesive strips (shown on the outside closure) keep strips properly positioned while the roofing or ridge cap is fastened.
Originally made for barns and industrial buildings, screw-down panels frequently use trim accessories that look out of scale on smaller, residential structures. Many installers make their own on a metal brake or have panel fabricators make smaller versions of their standard trim pieces (Figure 4). When ordering exposed-fastener panels for a residential application, ask your supplier what accessories will look best.
Figure 4. Color-matched flashings, drip-edge, and ridge caps like these from Atas mean that installing metal roofing requires little custom fabrication, but flat sheets for custom crickets and trim are available in the manufacturer's palette of colors.
