What is oil canning?
Oil canning can be defined as the perceived waviness of a metal panel and is an inherent characteristic of light-gauge, cold-rolled flat metal products. This waviness generally depends on the continuous width of the flat, and metal profiles which have wide flat surfaces, or more commonly referred to as “architectural” roofing and siding panels are more prone to exhibit oil-canning. Oil canning can be more apparent during certain times of the day and in certain seasons based upon the angle at which sunlight hits the roof or wall, and based upon temperature differential.
What causes oil canning?
Production process stresses
Oil canning can sometimes occur during the early stages of the flattening and coiling production. According to the Metal Construction Association (MCA), there are three separate stresses that can be inflicted on the metal during production that could lead to the development of oil canning:
- Full center: When the sheet or coil is longer in the middle section, which creates ripples or buckles along the center.
- Wavy edge: When the edge of the sheet or coil is longer along the edge of the strip.
- Camber: When the coil or sheet deviates from a straight line when laid flat
Metal can also be subject to oil canning during the roll-forming and slitting process.
- Slitting: The slitting of a master coil can release and redistribute residual forces. The coil’s response can create or increase oil canning.
- Roll-forming: Architectural panel profiles typically require more forming along the edges than in the middle of the sheet. They often require more forming along one side than the other. This requires that forming begins along the sides. The sequential “working” of the sheet will have a tendency to “trap” uneven metal contained within the central areas.
Color and gloss type
Darker colors and high-gloss finishes are more likely to show the effects of oil canning than lighter colors and low-gloss finishes. Please note: Any color or gloss type has the same chance of developing oil-canning though visibility of said oil canning may be more present in darker colors and high gloss finishes.
Panel installation, handling, and storage
It is important to remember that metal coils and sheets, both before and after rollforming or slitting, require specific care and support. Improper can and support can happen during panel installation, handling, and storage.
- Misalignment of the support system: Structural supports that are produced, fabricated, and installed within allowable tolerances can create a “non-planer” or bearing surface. Stresses induced while panels conform to this surface can contribute to oil canning.
- Over engagement of panels: Most panels accommodate transverse thermal expansion by flexing of webs and by “take-up” at side joints. When panels are over engaged, those relief features are hindered or eliminated. In the extreme case, the over engagement process itself can generate waviness. Either cause can contribute to oil canning.
- Over driving fasteners: This operation creates stresses in the panel and provides a “reading line” along the fastener alignment.
- Longitudinal expansion: The surface temperature of exposed panels cycles throughout the year and even fluctuates daily. The temperature and the cycle depend on many variables (e.g., project location and building orientation, cloud cover, panel configuration, surface finish or color, system thermal insulation characteristics). Under temperature fluctuation, the panels expand or contract. If panels are restrained by fasteners, clips or perimeter details, they accommodate thermal forces through several mechanisms. These include “slotting” around fasteners, out-of-plane “bowing”, and local distortion of flat areas. The magnitude of thermal force depends on the restraint provided, on the base materials’ physical properties and on the temperature differential between the support structure and external skin. Waviness can be amplified when there is uneven fastener restraint along the panel. Such restraint is common on “concealed fastener” systems having fasteners along one edge and an interlock along the other. Waviness caused by thermal forces differs from the other forms of oil canning in that waves can appear and disappear daily as the sun moves across the sky.
- Movement of the primary structure: Excessive differential deflection, racking, drift, or settlement within the primary structure can cause noticeable waviness within panel flats. This distortion can be temporary or sustained.
- Handling: Carrying panels in the flat orientation or twisting panels can induce a wavy appearance to a previously flat panel. Twisting can occur if one corner of a panel is used to lift a panel or to remove the panel from a bundle.
How to minimize oil canning?
Coil producers and panel manufacturers attempt to minimize these conditions and produce quality products. Research continues on improved production methods. Regardless, all of the above factors contribute to oil canning in architectural roofing or siding products. While a number of factors are a function of the panel design, there are steps that the designer, panel manufacturer, and erector can take to reduce the chances of oil canning.
- Coil: Tension leveling or stretcher leveling, processes whereby the metal sheet is “stretched” in coil from beyond its yield point, will provide a flatter surface less prone to oil canning. In general, the heavier the thickness the less likely a product is to oil can. The possibility of oil canning can be reduced by ordering tension leveled and resquared material. Tension leveling should be done only after the coils are slit.
- Design: The use of fasteners and clips that allow panels to float without causing thermal stresses is a means of controlling oil canning on roof panels. The addition of stiffening ribs in the panel profile “break-up” the flat surface or a surface painted with a low gloss paint system will also help reduce visible waviness in the metal.
- Installation: More stringent specifications regarding the alignment of the supporting structures will focus attention on this critical aspect. Instructions to the erection forces regarding proper handling, spacing, and fastening should be part of the manufacturer’s delivery packet.
- Handling: Proper handling is one of the most critical considerations and should be addressed in production, transportation and installation activities.
Conclusion
Sadly, there is no cure or remedy that will eliminate oil canning due to metal’s nature to revert back to its original form. It is important to stay proactive and recognize oil canning before it’s installed. If you follow the recommendations discussed in this article, you will have a better chance of reducing or even eliminating this problem before it begins.
