Performance Characteristics of Metal Roofing

Metal roofing performance can be looked at in its own right for the specific characteristics provided by the material. In addition, it can be assessed by comparing it to the traditional roofing materials that can be replaced by metal alternatives. We will look at both in the following sections.

Metal Roofing General Performance

When reviewing metal roofing, there are some overall performance characteristics that are applicable regardless of the type or appearance style selected.

Weight: One of the more notable traits of metal roofing, along with its durability, is its lighter weight when compared to almost all other roofing materials. Even heavy-duty panels typically weigh less than traditional roofing materials, including asphalt shingles, slate, and tile. Less weight means less need to beef up the roof structure whether in a new or existing building.

Weather resistance: Virtually all metal roofing provides tested and proven resistance to natural and man-made conditions. In high-wind situations for instance, other roofing materials can be vulnerable to blow-off or other failures, which exposes the rest of the building to potential damage. Metal roofing has been independently rated to stay in place in wind conditions up to 130 miles per hour. Similarly, damage from impact due to hail, debris, or other items is a concern for all roofing. The industry standard for testing in this regard is UL 2218: Standard for Impact Resistance of Prepared Roof Covering Materials. Following the procedures in this testing standard, metal roofing is able to achieve up to a Class 4 rating—the highest possible under the test. Metal is also inherently algae resistant in most cases, with stone-coated steel about the only condition where that is not readily achievable.

Fire classification: For code and insurance purposes, a fire classification is important for all roofing materials. Class A is the most desirable in this regard and is typical for metal roofing due to the noncombustible nature of metal. That often means that code compliance is simplified, fire concerns are reduced, and insurance premium discounts may apply compared to other roofing materials.

Finish options: Metal roofing of all types is commonly available in a range of metal finishes from bare galvanized metal to fade-resistant fluoropolymer coatings (PVDF) that are rated for the life of the roof. There are also solar-reflective coatings available for metal roofing that meet the “cool roof” standards of national programs such as LEED and ENERGY STAR and state requirements such as CRRC/2016 California Title 24 Part 6 cool roof requirements.

Durability: This is the trait that metal roofing is probably best known for since it commonly outlasts most other traditional roofing materials and in some cases has even outlasted the building it is applied to. This longevity comes from the fact that galvanized steel is used as the base metal, which is known for resisting rust and corrosion. Adding a factory-applied paint layer, plus other protective finish layers, over the steel usually means that the roofing simply holds up well with little or no annual maintenance required (except for stone-coated steel that needs washing to remove algae growth). This proven durability is the reason that manufacturers offer warranties for 50 years or more up to the lifetime of the building.

All of these traits of metal roofing are inherent in all metal roofing products regardless of the final profile or appearance.

Metal Roofing Compared to Traditional Materials

When considering the use of clay tile, slate shingles, or wood shake roofing for a project, a comparison of performance attributes of metal are worth noting. Looking specifically at metal roofing materials that provide the appearance of these traditional roofing materials, there are some direct comparisons that can be made.

Clay tile comparison: Both clay tile and metal tile can provide the roofing look that is still prominent throughout the Mediterranean and other parts of Europe and has become a part of the history and tradition of a number of communities throughout the United States. However, the weight of clay tile compared to metal is significant—clay tile can weigh 700–1,100 pounds per square compared to a dramatically less 98 pounds per square for metal with a clay tile appearance. That means clay tile may require extra structural support in the roofing system, while metal has no such extra structural support needs. This contributes to metal being installed quickly and cost effectively, while clay tile is considered costly to install.

From a finished appearance viewpoint, clay tile has surface pigments that fade, while metal tiles are typically finished with fade-resistant fluoropolymer (PVDF) coatings. The appearance of clay tile can also be affected over time since it is susceptible to cracking and chipping from being walked upon for maintenance or from hail, falling tree limbs, etc. By contrast, metal tiles can be specified with the highest available rating for impact resistance and remain quite unblemished. Clay tiles can require a lot of energy to produce and transport and can have lot of embedded energy as a result. Clay tiles also do not meet LEED, ENERGY STAR, and similar sustainability ratings for solar reflectance while metal tiles can.

Slate roofing comparison: Slate stone was found by many American settlers to provide a hefty roof while creating an endearing aesthetic that has become well known in a number of regions of the country. While not necessarily as heavy as clay tile, slate roofing can still come in at a hefty 600–800 pounds per square compared to a notably less 96 pounds per square for metal in a slate shingle appearance. Still, slate shingles may require extra structural support to install, particularly on retrofit projects, while metal has no such extra structural support needs. Similar to metal tile, metal slate panels also install quickly and cost effectively, while individual slate shingles are more costly to purchase and install.

Slate shingles may not fade the way that clay tile can, but it is still susceptible to cracking, chipping, or other damage from falling tree limbs or walking upon it for maintenance. By contrast, metal panels with a slate look can be specified with the highest available rating for impact resistance. Like clay, slate shingles can have higher embedded energy than metal due to their heavier weight in transportation, and slate does not meet ENERGY STAR, LEED, and similar sustainability ratings for solar reflectance. Metal panels with a slate appearance excel in both reduced embedded energy and meeting solar reflectance criteria. Perhaps most notable for building owners, slate roofing shingles require periodic repair and maintenance over the life of the building, while metal panels with a slate appearance typically do not.

Wood shakes comparison: The abundance of trees in Colonial America spurred the use of wood shakes and shingles in residential construction. While lighter and easier to work with than tile or slate, the weight of wood shakes can vary based on the moisture content and other features of the wood. Metal shakes by contrast are a predictable 96 pounds per square with no extra structural support required and no shrinkage concern due to drying. Metal panels with a wood shake appearance also install more quickly and cost effectively than individual wood shakes.

Wood exposed to the weather, even with some treatment, has some limitations. Commercially available wood shakes generally have a 20- to 25-year average lifespan, meaning they would need to be replaced at least one or more times on a building compared to metal shakes with a lifespan in excess of 50 years. After each installation, wood shakes typically require regular repair and maintenance due to rot, splits, and cracks, while metal shake roofing is virtually maintenance free. In terms of fire classification, many wood shakes are either unrated for fire safety or treated with toxic chemicals to achieve either a Class B or C fire classification. By contrast, metal shake roofing carries a Class A fire rating, the highest available. Metal shake roofing is also available to meet LEED, ENERGY STAR, and similar sustainability ratings for solar reflectance and recyclability, while wood shakes are not.

In all three of these comparisons, it is easy to see how the inherent qualities of metal roofing provide superior performance while being more economical and more sustainable than their traditional material counterparts.

Dispelling Some Myths about Metal Roofing

Any building material that has been in common use for a while often generates a few stories or tales that get passed from person to person without being validated by facts. The same is true for metal roofing with three things in particular that seem to be misunderstood.

Noise from rain: It is sometimes speculated that rain falling onto a metal roof will create noise that will resonate inside the building. While some people report that they enjoy hearing the sound of rain on a porch roof in summer, others point to metal-roofed barns where sound from rain may be unwanted. In fact, the only way to really hear the sound of rain falling on a roof is when the metal is the only thing between people on the inside and rain on the outside. However, all residential and commercial buildings suitable for occupancy have more than just the metal roofing in the roof and ceiling system. At the very least, there is continuous sheathing under the metal that helps to absorb and dampen any sound of rain. In addition, the insulation required by energy codes in roof and attic spaces provide acoustical benefits as well as thermal benefits. Hence, any sound occurring outside on the roofing surface is likely not perceptible inside the building except in the most torrential of rain storms.

Taking a closer look at this misunderstood aspect of metal roofing, studies performed by The Acoustic Group at the University of Luleå in Sweden determined that rain on a metal roof produces a sound intensity of only 52 decibels.¹ This is equivalent to the sound of a quiet conversation at home, which registers an average sound intensity of 50 decibels.² In comparison, sound is not interpreted as annoying by most people until it reaches the upper 70 decibels, which is actually four times louder than rain on a metal roof.² Hence, in terms of the sound of rain falling on a home’s roof, metal roofs are no different and transmit virtually the same sound intensity as other roofing materials.

Rusting: The myth of rust developing on metal roofing seems to be based on older, less fully finished roofs or cases where an older, inferior finish was damaged. In modern-day metal roofing, multiple coatings are applied that serve as the first and most effective line of defense against rust and corrosion. However, even if coatings are compromised for some reason, the steel itself is galvanized and resists rust and deterioration by the galvanic action. For appearance sake, if a coating is damaged for any reason, making a repair with spot coating on-site is possible if desired or needed.

The durability and resistant characteristics of traditional-appearing metal roofing are found in the multiple finish layers that are applied over galvanized steel.

Lightning: Some people have a perception that a metal roof will attract lightning more readily than other roofing materials. That simply hasn’t been borne out by the facts, including for roofs made of exposed copper without a painted finish. A painted metal roof is no more prone to a lightning strike than a metal car. Tall trees (i.e., wood) and building spires are actually more likely to be struck by lightning, mostly because they protrude up into the sky higher than things around it. Therefore, instead of questioning the materials used on the roof, it is more advantageous to consider an appropriate, grounded, lightning-protection system as determined by design professionals. This is particularly true for high points of a building and buildings exposed in open areas prone to lightning storms, regardless of the materials used.

Understanding the facts around each of these three misperceptions as they relate to modern metal roofing products will help design professionals and building owners make better, more informed decisions on how to address any concerns.