In this next installment in our series about roof coverings and common problems, we’ll be checking out some of the things your home inspector will be looking for when assessing a metal roof—an option that is enjoying greater popularity in neighborhoods throughout the U.S., although it still has a long way to go to catch up with asphalt shingles.
The growing interest in attractive aluminum, stainless or galvanized steel, copper, or zinc shingles and panels can be linked to several factors. These include price, the availability of a veritable artist’s palette of colors, versatility in a range of roof slopes, and their ability to effectively shed precipitation. Add to this is a greater resistance to impact damage and non-combustibility and it’s no wonder why metal roofs are finding their way onto more and more residential homes.
A note about the inspection: If slippery conditions are present or if the stability of the structure is in question, your inspector may choose to assess metal shingles or panels by means other than walking upon them. Your inspector will not get up on a roof composed of formed metal shingles since these can be easily damaged when stepped on.
Different types of metal roofs are known to last longer than others, such as corrosion-resistant copper, which has a lifespan of up to a century but is more expensive than other options While boasting several advantages, bear in mind that metal roofs—both structural and architectural types— have their share of problems as well. Here’s a brief checklist of issues that the professionals at A-Pro Home Inspection have found when evaluating metal roofs over the last 27 years.
Corrosion: For obvious reasons, corrosion tops our list of metal roof defects. Some exposed metal surfaces are vulnerable to this natural process that can gradually destroy the material through electromechanical and/or chemical reactions to the environment, such as moisture in the air attacking defects and penetrations on a steel shingle or panel (e.g., scratches and holes). Cracks and crevices can trap stationary water and hold it against the surface, such as under improperly driven fasteners, beneath certain types of fastener gaskets, or within minuscule fissures that were formed during manufacturing. As with any type of corrosion, your inspector will check to determine the extent of the damage and if there’s evidence of leakage.
Cosmetic-only-type corrosion is typically reported in climates with extreme humidity. Like copper, aluminum roofs are corrosion-resistant but are likely candidates for galvanic corrosion if in contact with other types of metal. Aluminum or zinc coatings on steel roofing panels (as well as applications of barrier-style coatings) can help protect the underlying material from corrosion, with thicker coatings being more effective in preventing damage and extending the life of the roof.
Fastener Concerns: Your inspector will check fasteners—most often screw/washer combinations driven through the metal or the use of clips. The report may include conditions such as missing, protruding, or incorrectly spaced fasteners; overdriven or angle-driven fasteners; use of a fastener not appropriate for a metal roof; and screws that are loose or have backed out—all conditions that make the roof susceptible to wind uplift and moisture penetration. Unlike screws, for example, clip-type fasteners are hidden from view and allow panels to expand and contract, which may help prevent a defect known as “oil-canning.”
Oil-Canning: This is defined as the appearance of waves or distortions in light-gauge metal roofs, regardless of whether the material is steel, aluminum, copper, or zinc. The severity of how the waviness is perceived can be influenced by a number of factors, such as roof color and from which angle the roof is viewed. While often a defect associated with stressors from the production process, rough handling, or improper storage, installation mistakes may also be at fault, such as panel misalignment, overdriving fasteners, insufficient panel spacing that does not allow for expansion/contraction, laying panels or shingles over a roof deck with leveling inconsistencies, and poorly attached clips.
While oil-canning is a cosmetic issue that homeowners may find annoyingly unsightly, it does not affect the roof’s performance. However, it may call attention to other details that will be looked at by the inspector. Additionally, your inspector may note hail damage to metal panels and shingles, which, if severe enough, can lead to moisture penetration. Evidence that the roof has been damaged by being walked upon may also be indicated.