Metal Roofing Options, continued

Steel

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 U.S., that means steel. Steel has some obvious benefits: 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 appealing to consumers and specifiers who might not have considered a steel roof in the past.

Galvanizing. One time-tested approach to protecting steel from rust, long used for everything from water pipes to roofing nails, is galvanizing, or coating with zinc. The biggest advantage zinc has over other rust-preventive coatings is its ability to heal itself. When galvanized material is exposed to water, tiny amounts of dissolved zinc flow to scratches, cut edges, and other areas where the base metal is exposed, preventing the underlying steel from rusting.

This fluidity eventually exposes the steel, leaving it at the mercy of the elements, but the thicker the coating, the longer the protection will last. The Metal Roof Alliance recommends that a steel roof have a zinc coating rated at G-90 or better, which translates to .9 ounces of zinc per square foot. In most areas, unpainted G-90 galvanized steel should last about 30 years before corrosion becomes visible, according to Fabral's Troy Thomas. But in coastal and industrial areas, 15 to 20 years is a more realistic expectation. Galvanized steel is widely available preformed into a variety of exposed-fastener panels and as coil stock suitable for fabricating into standing-seam roofing.

Let it rust. Oddly enough, some consumers actually prefer the appearance of rusted steel. The rusted look is especially popular in resort areas in the western mountains, where the rustic "mining town" look is in style. Some roofers use locally manufactured exposed-fastener panels made from plain, ungalvanized steel, sometimes washing them with a vinegar solution after installation to speed the development of a uniform coating of rust (Figure 11). Although the unprotected roofing will eventually rust through, advocates of this approach claim that it provides a reasonably long-lasting roof in dry climates.

Figure 11.Using uncoated or Cor-Ten steel roofing helps new buildings fit in with historic surroundings. This exposed-fastener roof uses plain cold-rolled steel, bur Cor-Ten steel, more traditionally used on bridges and guardrails, creates a similar look.

A more conservative approach to rusted roofing is to use Cor-Ten steel, which is a chromium- and nickel-rich alloy often used for bridges and guardrails. When exposed to weather, Cor-Ten quickly develops a surface layer of rust that also serves to protect the metal beneath from damage.

Aluminum and aluminum-zinc coatings. Aluminized coatings formulated for steel are extremely corrosion resistant, generally last longer than galvanizing, and reflect heat well, reducing cooling requirements. Unlike galvanizing, though, aluminized coatings are not self healing. The material must be handled very carefully, because any scratches in the surface will provide a potential foothold for rust. Cut edges are also susceptible to rust.

Galvalume and Zincalume coatings, two similar proprietary products, combine the durability and reflectivity of aluminum coatings with the "flowability" of traditional galvanizing. Both are about 55% aluminum and 45% zinc, and they share a life expectancy of 40 to 50 years under normal atmospheric conditions. They cost about 5% to 10% more than G-90 galvanized roofing.

Roof paints. Not everyone likes the look of bare metal, so the development of improved paints and coatings makes metal roofing an attractive choice to more buyers than ever before (Figure 12). Unlike site-applied paints, most modern factory-applied finishes are tough and flexible enough to withstand the sharp bends required of standing-seam roofing.

Figure 12.Improved paints have boosted the popularity of metal roofing by promising 30-year maintenance-free good looks. Most exposed-fastener and standing-seam panel producers offer 20 or more standard colors. Modular manufacturers offer fewer choices (5 to 10), but makers of both types offer custom colors for an upcharge.

The least expensive coatings are polyester resins, which have a glossy finish to begin with but quickly lose their brilliance, typically within five years. Polyester coatings are widely used on screw-down panels because of their low cost. Jerry Iselin of Metal Roof Specialties says the problem with the less expensive paints is not that they crack or peel but that they fade faster. He advises clients who want a less expensive paint system to choose light colors. "If I pick a bright red roof and it turns pink, I'm going to be more upset than if I have a tan roof and it turns a lighter shade of tan," he says. Another problem of cheaper paint finishes is that they seldom weather uniformly; solar-facing elevations show fading and damage first.

Silicone-modified polymers are polyester resins with silicone additives to improve their durability. These coatings have better fade resistance than unmodified polyester, and their 30-year durability approaches the life span of fluoropolymers, the best-performing paint finishes.

Fluoropolymers. Commonly referred to by their trade names, Kynar 500 and Hylar 5000, fluoropolymers are less shiny than polyester finishes, but they offer excellent protection against fading and chalking. Fluoropolymer finishes also resist the damaging effects of pollution and salt spray better than other paints. Warranties that protect against chipping, cracking, and peeling for 25 years or longer are common, often with prorated guarantees against fading for 10 to 20 years. Warranty protection for fluoropolymers typically limits fading to 5 Hunter units in the first 10 to 20 years (one Hunter unit is the smallest change in color detectable by the average person). A disadvantage to fluoropolymer finishes is that they are softer than siliconized-polyester systems, increasing the likelihood of paint damage during installation, and sometimes frustrating crews who are used to working with polyester-based paint systems.

Stone coating, which originated in New Zealand and Australia, uses an acrylic resin to bond ceramic granules -- much like those on asphalt shingles -- to the metal surface (Figure 13). Stone-coated panels offer good resistance to hail damage and frequently carry UL's highest hail damage resistance rating, class-4. Although hail may still cause dents, manufacturers claim that they're seldom visible from more than 6 feet away and do not affect a roof's ability to shed water. Customers seeking insurance discounts must sign a waiver freeing insurers from claims for "cosmetic damage."

Figure 13.Stone coating uses granules like those found on asphalt roofing to add a wear-resistant surface that won't fade. Products like these from Metro Roof Products also offer excellent hail resistance; homeowners installing them in hail-prone Texas receive mandated insurance discounts.

No matter what paint finish you choose, engineers suggest a neutral-cure silicone or one-part polyurethane for caulking and sealing, because the cheaper acid-cure silicones (which smell like vinegar) damage paint and protective coatings. End laps and side laps should be sealed with butyl sealant or tape because they absorb panel movement best.

Aluminum

While aluminum typically costs more than steel, aluminum roofing panels have a strong following in coastal and southern regions because their natural reflectivity helps reduce cooling costs. Also, aluminum's natural resistance to rust means the corrosion protection can't wear off or be damaged from rough handling like protective coatings on steel. For customers who don't want a bright, reflective finish, aluminum coil stock and exposed-fastener panels are available with the same factory-applied paint finishes found on steel. Aluminum and steel modular panels are typically painted with a Kynar or Hylar finish. Aluminum has the highest coefficient of expansion of any commonly used metal roofing material, so exposed-fastener panels should be limited to 16 feet. Also, aluminum panels will corrode when installed directly on pressure-treated lumber without an underlayment.

Specialty Metals

Although steel and aluminum are by far the most common residential metal roofing materials, other metals are occasionally used for special effect, although the relatively high prices of most of these options limit their use. Most are available as coil stock for standing-seam applications, but some, such as copper, are also available as shingles or modular panels.

Terne. Thanks to its durability, terne-coated steel -- which consists of sheet steel coated with a lead-tin alloy -- has been a highly regarded material for nearly two centuries. Because of health concerns associated with its lead content, original terne roofing is no longer manufactured, but it has recently been replaced by a new zinc-tin formulation called Terne II from Follansbee Steel in Follansbee, W.V.

Terne II still has one substantial drawback of the original terne: It must be painted soon after installation and repainted every eight years thereafter, or rust will bleed through the plating. As a result, it's less popular than prepainted steel or aluminum for most applications, although it's a more historically appropriate alternative for replacing an original terne roof. Follansbee also makes a terne coating on a stainless-steel base, called TCS II, which never needs painting; but the cost is so high that its use is pretty much limited to commercial projects.

Copper. Sheet copper is another material sometimes used for high-end roofing jobs. Copper is traditionally formed in standing-seam panels, but more recently it's been available in modular panels and shingles (Figure 14). It can be soldered for difficult flashings and low-slope applications, but perhaps the greatest advantage of copper is that it doesn't rely on coatings or paint finishes for corrosion resistance. In most areas, it eventually weathers to a green color, although in dry regions it may remain a red-bronze or purplish color.

Figure 14.Copper, used for centuries to make long-lasting standing-seam panels, has more recently lent its longevity and beautiful coloration to shingle-style panels like these from Zappone. The locking hems on these shingles, also found on steel and aluminum styles, give them unsurpassed wind resistance. Drawbacks include its high cost and the possibility that the runoff will leave a green stain on siding or flatwork. Because it's so soft, copper also has a tendency to take on a slightly wavy appearance in flat areas between seams. This is more a characteristic of the material than a defect, however, since it doesn't compromise the integrity of the roofing. A new copper-plated stainless coil stock material called SUSCOP, available from Sure Roofing Systems, is said to be more rigid than pure copper and to cost significantly less.

Stainless steel, perhaps the longest lasting of any metal, can be formed into standing-seam profiles and was recently introduced as a line of modular shingles by Millennium Tiles (Figure 15). Variations in the protective chromium oxide layer permit four different colors, from naturally shiny stainless to more muted tones resembling weathered bronze and slate. The $600-per-square price (material only) may be an obstacle for many, but the roof should last forever.

Figure 15.Offering the greatest corrosion protection possible, recently introduced stainless modular shingles look modern and seem ideal for coastal areas, where salt spray corrodes other metals.