Mastering Roof Inspections: Metal Roofs, Part 6
by Kenton Shepard and Nick Gromicko, CMI®
The purpose of the series “Mastering Roof Inspections” is to teach home inspectors, as well as insurance and roofing professionals, how to recognize proper and improper conditions while inspecting steep-slope, residential roofs. This series covers roof framing, roofing materials, the attic, and the conditions that affect the roofing materials and components, including wind and hail.
DAMAGING CONDITIONS
The important properties of coatings for metal roofing products include resistance to:
- color-fading, as UV degrades pigments;
- chalking, as oxidation leaves a chalk-like substance on the surface;
- moisture-related problems;
- abrasion;
- staining;
- corrosion, the rate of which is increased by pollution and salt spray;
- cracking caused by flexing; and
- damage from solvents and acids.
Most of these properties are tested according to ASTM standards during manufacturing, which you are not responsible for confirming.
Surface Defects
Surface defects can be caused by the manufacturing process, and some may result in premature failure. Other causes include the following.
- Blistering is caused by the expansion of gas in or under the paint.
Courtesy of the National Coil-Coating Association
- "Chatter" appears as a series of traverse marks in the paint or metal.
Courtesy of the National Coil-Coating Association
- Catenary or oven scratches show as streaks in the paint parallel to the panel's length.
Courtesy of the National Coil-Coating Association
- Dirt lines appear parallel to the panel's length and are caused by particles trapped between the applicator roll and the strip. They can also be caused by air bubbles trapped in the paint.
Courtesy of the National Coil-Coating Association
- Paint splatter can happen during the manufacturing process.
Courtesy of the National Coil-Coating Association
- Pinholes are usually caused by degassing in the metal coating, resulting in tiny holes that penetrate through the paint to the metal.
Courtesy of the National Coil-Coating Association
- Skipping is an irregular paint application caused by improper contact between the applicator roll and the strip.
Courtesy of the National Coil-Coating Association
- Water staining is a type of white rust that forms on the surface of aluminum and is caused by moisture.
Courtesy of the National Coil-Coating Association
Oil-Canning
“Oil-canning” is a term used to describe ripples in metal panel roofs. It’s seldom seen in metal shingle-type roofs, corrugated or heavily formed roofs, which is a type of roofing that has extreme bends and folds pressed into it as part of its architectural profile.
Oil-canning can have a variety of causes, including problems with the steel-coil manufacturing during panel fabrication, such as when one part of the metal coil (the center or one side) is longer than the other parts. It can be caused by installation. You don’t need to identify the cause. Oil-canning is a cosmetic issue unless it’s caused by structural movement, in which case it may result in leakage at some point. Unless your employer says otherwise, you probably won’t mention it in your report.
**************************************************
Learn how to master a roof inspection from beginning to end by reading the entire InterNACHI series: Mastering Roof Inspections.
Take InterNACHI’s free, online Roofing Inspection Course
Mastering Roof Inspections
Roofing Underlayment Types
Inspecting Underlayment on Roofs
Fall-Arrest Systems
Roofing (consumer-targeted)
More inspection articles like this
