Oil canning is a perceived waviness across the flat areas of sheet metal panels. It is a naturally-occurring phenomenon that is inherent in all light-gage sheet metal. SMACNA’s new “Architectural Sheet Metal Manual,” 6th edition, provides methods from design to installation to mitigate its occurrence.
Oil canning is more apparent under shallow cross lighting so its presence is more discernible during certain seasons or times of day. Also, differing thermal forces can create waviness--either temporary or sustained--as the sun moves across the sky.
Oil canning is an aesthetic issue, not a structural problem or a defect. It is unrealistic to expect any architectural roof or similar wide-metal element to be totally free of some degree of oil canning.
While oil canning cannot be totally eliminated, adherence to industry accepted and recognized methods of design, metal specification, handling, fabrication, and installation can minimize its occurrence. Careful attention to the causes of oil canning within all the phases of design and construction is the most effective way to reduce its occurrence.
Panel gages and widths scheduled must be selected to minimize oil canning with proper installation. SMACNA’s “Architectural Sheet Metal Manual” provides metal gage recommendations based on several factors and those recommendations should be considered minimum gages and maximum widths.
Specifiers should use metal gages and limit panel widths that based on experience, either their own or that of experienced local sheet metal contractors, has shown as appropriate for a particular application and metal. The most current American Society for Testing and Materials (ASTM) standards should also be reviewed to gain insight into the standard tolerances to which various metals are manufactured.
Color and surface finishes also play a role in how oil canning is perceived. Wide, shiny, dark-colored, light-gage sheet metal panels will exhibit a degree of oil canning that is directly proportional to the width and inversely proportional to the thickness. Darker colors simply accentuate any oil canning that is present; the presence of oil canning can be made less obvious by the use of lighter, more neutral colors. Also, reflective surfaces will be more unforgiving in revealing oil canning while the use of non-reflective or textured finishes aids in masking waviness.
Movement of the primary support system or the structure itself can cause waviness that may become permanent or temporary during certain weather conditions. The fastener system should be designed so that the panels can “float” in response to thermal changes. In addition, the perimeter design is especially important. Ultimately, the magnitude of thermal stresses transferred from the structure to metal panels is carried through the fastener system. Stiffening ribs can be specified within wide panels to break up the panel and to reduce and make oil canning less apparent.
Generally, the heavier the gage the less oil canning will be visible. Oil canning can also be reduced by ordering tension-leveled coils and re-squared sheet stock. Tension leveling involves stretching the metal on coils past its yield strength which provides a flatter surface less subject to oil canning. Re-squared metal simply assures that the metal’s shape will be more amenable to roll or brake forming without generating unwanted surface tensions due to warped raw material or metal edges that are not truly parallel.
Proper handling needs to be addressed in every step of the process from production to final installation. For example, panels should not be carried “flat” or lifted by a single corner to remove one panel from a bundle. In some cases, especially with custom finishes, it may be beneficial to use clean gloves to handle and position metal panels. Appropriate shoes should be worn to avoid scuffing the finish.
Slitting panels from a coil releases and creates residual stress within the metal. Typically, slitting from wider coil stock is unavoidable due to the economic benefits of using wider coil stock. Residual stresses are also created by any forming operations required to develop flat metal into the desired shape. Metal forming equipment should be well adjusted, operated within its design limits, and operated by experienced sheet metal crafters to minimize stresses caused by fabrication.
The sheet metal’s foundation--the substrate--is a very important element of any architectural metal system. For non-structural panels the substrate must be flat with any required felt/membrane or slip sheet, closely conforming to the supporting system. For structural panels, the resulting bearing surfaces must be properly aligned with the underlying roofing and one another, or the metal will “telegraph” the location of each support. Otherwise, stresses induced when the metal conforms to any contouring of the bearing system can create oil canning.
Placing panels too closely to one another at the “long” joints will not allow sufficient room for expansion and can generate waviness as daily and seasonally thermal stresses vary. Fasteners that are over driven or are of incorrect height can severely restrict movement--especially for long or wide panels. This rigidity can transfer stress to the panels through the daily and seasonal thermal variations and can create visible deformations.
Allowances must be made for thermal expansion in all directions; rigid retention methods that are too restrictive can cause oil canning and create stress cracks and tears in the metal, especially along the perimeter. The substrate must be of a material, or set of materials, that will not adhere to the underside of the metal and restrict its normal thermally-driven movements.
Oil canning is typically more visible on a new roof before the natural patina of raw metals or the paint weathers to its normal gloss. Using metals that weather to a natural finish as oxidation develops should be considered as one method to reduce the visual effects of oil canning.
To purchase a copy of SMACNA’s “Architectural Sheet Metal Manual” visit www.smacna.org/bookstore/ or call SMACNA’s Publications Department at(703) 803-2989. The member price is $42 for the book, $50 for the CD-ROM and $42 for the PDF download. The IFUS price for the book is $139, $166 for the CD-ROM and $139 for the PDF download.
Architects and engineers may purchase the new publication at the discounted price of $184 for the book, $220 for the CD-ROM and $184 for the PDF download. The list price for the book is $262, $315 for the CD-ROM and $262 for the PDF download.
To order call SMACNA’s Publications Department at (703) 803-2989 or visit www.smacna.org/bookstore/.