Metal mania: from basic ag panels to shakes, shingles, and tiles, there's a metal roofing for every taste and budget - Exterior Products

Building Products, July-August, 2003 by Patrick McCombe

The range of metal roofing choices in type of metal, thickness, finish, and method of application is wider than it's ever been and continues to grow. But selecting the right material for a given application requires some thought. The first step is to choose a type of roofing from three general categories: exposed-fastener panels, standing-seam roofing, and modular panels.

EXPOSED-FASTENER PANELS

For many people, the words "metal roofing" means the screw-down or nail-down agricultural material, or "ag panel," used on barns and industrial buildings, as well as some residences. 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.

Most of these types today 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.

Because each screw hole represents a potential leak, it's worth doing 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.

With installed prices starting at about $150 per square, exposed-fastener panels are the least expensive variety of metal roofing, but they 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 an 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.

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."

STANDING-SEAM ROOFING

In traditional standing-seam construction, L-shaped metal cleats nailed to the deck or spaced purlins are crimped between adjoining panels to provide a watertight joint that also locks the roof in place. The absence of exposed fasteners gives it a more finished look than nail- or screw-down panels.

Site-fabricated roofing. In the past, all standing-seam roofing was made from metal plates or sheets formed into panels on the job site. Today, coil stock is used and mechanized metal-forming equipment has made the process faster and easier. Factory-produced panels also are available, and although they can be difficult to ship, some roofers feel that the heavier stationary equipment results in a better-quality product. Jerry Iselin notes that using factory-produced panels also increases the builder's chances of getting satisfaction if there are problems with the material.

Self-locking panels. Traditional standing-seam roofing calls for special tools like brakes and shears, and installers need advanced metalworking skills more often associated with custom duct fabricating than roofing. To streamline the process, many roofers turn to so-called snap lock panels, which can be assembled without crimping.

MODULAR PANELS

Pressed into lightweight copper, steel, or aluminum to resemble shakes, shingles, or tiles, modular panels are the newest and fastest-growing segment. Consumers appreciate their traditional look, and installers familiar with asphalt shingles or tile usually can make an easy transition to metal without investing heavily in equipment or training.

Modular panels are either nailed directly to the roof deck or installed on 2x2 battens spaced approximately 14 inches apart. Installing battens is extra labor but levels out surface irregularities caused by old roofing and allows more elaborate profiles to be stamped into the panel. Modulars installed without battens have folds along the sides and bottom edge that hook on to preceding shingles and are nailed directly to the roof deck, through the old roofing.

On both types, manufacturers specify felt or polyethylene underlayment to keep sheathing and old roofing from abrading the back of the panels and to offer some protection as a secondary water barrier.

Modular panels are the priciest form of metal roofing. Depending on the metal used, the type of finish, and the pattern, installed costs run from $500 to $1,000 per square.

TYPE OF METAL

Once you've settled on a general roofing category--exposed-fastener, standing-seam, or modular panels--the next important choice is type of metal. In many parts of the United States, that means steel. It's usually the lowest-cost option, and it expands and contracts relatively little in response to changes in temperature. As a result, it can be used in continuous lengths of as much as 36 feet without the risk of panel deformation or elongation of fastener holes. And while steel's susceptibility to rust has limited its use for some residential applications, recent developments in protective finishes and rust-resistant alloys have made it more appealing.