Metal Roofing Materials

In the discussion of metal roofing (see Metal Roofing: A Buying Guide), the word "metal" covers a lot of ground. Several metals are used for roofing materials: steel, stainless steel, aluminum, copper and zinc alloys. Each has different properties that affect durability, price, and appearance.

Steel, used for most metal roofing, is heavier and sturdier than aluminum. Manufacturers have settled on a number of durable coatings and finishes that protect steel from rust and corrosion. It's usually zinc-coated for corrosion protection, then sealed. A coating of epoxy primer offers adhesion and a baked-on acrylic top coating adds color and protection. Because sheet systems are designed for commercial applications, they generally are given highly durable paint finishes. One popular flurocarbon coating used on many products is called Kynar(r). Stainless steel--a very expensive roofing material--won't rust or corrode. Terne coating (see below) can give it a natural, matt-gray finish.

Aluminum, extremely lightweight, is the material used in much residential metal roofing. It won't rust, but must be painted or coated for appearance. Coatings are similar to those used on steel. Environmentalists express concerns about using this precious resource for purposes such as roofing. Copper metal shingles make a visually striking roof that may last as long as the house.

Copper, rooted in centuries of use, will not rust, has no "finish" to scratch or peel, is soft enough to easily tool, and weathers naturally to a beautiful verdigris patina. Unfortunately, it's very expensive. Alloys are formulated for strength, graceful weathering and durability. Cost depends on the specific material but, as a group, they are pricey.

The Benefits of Metal Roofing

Metal beats out conventional roofing materials on a number of counts:

Expected life. Properly installed, a metal roof should last as long as the house, completely sealing out water, surviving high winds and easily shedding snow. Metal is resistant to fire, mildew, insects and rot. Warranties vary widely but most companies strongly back their products for from 20 to 50 years.

Weight. Compared to tile at 750 pounds per square (an area equal to 100 square feet) or concrete tile at 900 pounds per square, metal roofing is lightweight. Most are from 50 to 150 pounds per square. Some types of metal shingle systems may be applied over one or two existing roofs without the need for tear-off or adding structural support. In fact, if you're building a house or an addition, you can often downsize or reduce the number of roof support members.

Speed and ease of metal roofing installation. Most metal roofing materials come in multi-shingle sections or in sheets. An accomplished contractor can install these quickly. If your roof is stripped off and a storm is on the way, shortening the process by a day or two may prove to be a critical advantage. Note: You can also buy single metal shingles that are relatively easy--but time intensive--to install. (Unlike three-tab asphalt shingles, each piece must be individually placed and nailed.)

Fire resistance. Because metal roofs are noncombustible, they're given a Class A fire rating (the most resistant). Part of a roof's classification depends on materials beneath the surface that could ignite in intense heat, so some metal roofs applied over an old combustible roof--such as wood shingles-- may be rated lower.

Heat conduction. Metal reflects radiant heat from the sun, minimizing midday heat gain. Though the material itself is low in insulation R-value, many systems utilize a dead-air space between the metal and roof deck to increase energy efficiency.

Minimal roof pitch. Most metal roofing materials can be installed on gently pitched roofs without danger of leaking. Typical minimum roof pitch is 3-in-12 (the roof rises 3 inches for each horizontal foot).

Metal Roofing Drawbacks

Though metal roofing offers many pluses, there are a few drawbacks or concerns worthy of consideration. For the most part, roofing manufacturers have faced these concerns and improved their products to address or solve many of them.

Cost. The biggest drawback is initial cost. Metal roofing is equivalent to other premium materials: from about $150 to $600 per square (100 square feet). The secret is that you get it back if you stay in the house for a long time. Of course, if you plan to move in a couple of years, you probably won't get the cost returned in value. Figure you can also save on engineering the supporting structure and maintenance.

Noise. For some, the sound of rain tapping on the roof is romantic and homey; for others, it's like living inside a drum. In a rainstorm or hailstorm, living beneath thin sheets of metal is bound to be noisier than beneath thick slate or tile. Noise can be controlled, however, both by using materials that have structural barriers to minimize the drum effect and by applying them over sound-deadening insulation and solid plywood sheathing.

Denting. Just as your car will dent if a golf ball hits it, a metal roof may dent if large hailstones fall on it. Aluminum and copper, much softer than steel, are more prone to denting. Some are guaranteed not to.

Though you shouldn't have to walk on a roof that doesn't leak, there may be occasions when a plumber needs to snake out a vent pipe or a chimney sweep will need access to the chimney flue. You can walk on some metal roofs, but not all--depending on how the particular product is made and the type of construction supporting it. Also, metal is slippery when wet.

Marring. Some painted finishes can peel, chip, fade, scratch or chalk, although nearly all are guaranteed for many years. Walking on some types--particularly those with a granulated-stone surface--may show wear. Installers must be careful not to scratch or dent roofing during installation--panels must be treated with care. Unlike conventional roofing, some metal shingle systems are installed from the top down, eliminating the need to walk on them. Once installed, it may be necessary to hose-off roofing now and then to keep it looking good.

Expansion and contraction. Because metal expands and contracts as it warms and cools, most new products have fastening systems that account for movement; otherwise, fasteners that secure roofing tend to work loose.

Modification. Roofing materials installed in large panels are more difficult to replace if damaged than individual shingles. Also, if you remodel or add-on to your home 10 or 20 years from now, it may be difficult to match the material.