The standard industry disclaimer says this about oil canning:
“Oil canning can be defined as a perceived waviness in the flat areas of [metal] panels. Oil canning is an inherent characteristic of light-gauge, cold-formed metal products with broad, flat areas. It is not grounds for panel rejection.”
Wiktionary.org has a slightly different definition:
“Moderate deformation or buckling of sheet material, particularly common with flat sheet metal surfaces. It is typically caused by uneven stresses at the fastening points.”
Why do steel roofing panels do that? There are several causes, some of which you can influence as a roofing contractor, and others that originate at the mill or panel fabricator. While oil canning typically causes no structural integrity issue with the panel, many owners are not pleased with the appearance.
Let’s look at some ways to prevent or reduce oil-canning as well as ways to repair or mitigate its appearance.
What causes oil canning of metal roofing?
The problem can occur in several areas of manufacture and installation:
- At the mill
- During panel fabrication
- During or after installation
Oil canning can originate with stress in the metal coil from which the metal panel is milled. It can occur due to issues with fabrication or on installation. Oil canning may show up as a symptom to a bigger problem with the substructure of the roof.
Light gauge metal has the highest tendency for oil canning. Zinc and copper, in particular, are prone to it but any sheet metal can show oil canning under the right circumstances. It is much less likely, however, in heavier gauge metal panels.
At the mill, poorly adjusted or worn roll formers can cause a rippling effect, especially if the metal is too wide, too thin, or weak.
If the mill’s camber and leveling tolerances are out of spec, small gaps separate the rollers. As the metal is flattened, high heat is produced that naturally causes thermal expansion and contraction. The gaps do not allow the stress to be evenly distributed and oil canning results.
During fabrication oil canning may occur if the panel must be slit or as it undergoes further processing and forming.
The roof substructure can create a ripple or dent in the metal roofing panels. Any deviation in the deck, bows, ridges, or camber of the underlayment can cause stress on the panel as it is installed and as the roof weathers. For example, screw heads used to connect metal studs can cause a bump in the metal panel over it, although this is more common in wall panels. Non-flat open framing or substrates can cause the same problem.
Panels that are bundled, stored, and handled carelessly can become damaged. Attachment to the roof can create stresses that cause the metal to buckle:
- Twisting a panel during lifting and placement
- Overdriving fasteners
- Over-engaging cladding material
- Movement of the primary structure
- Differing thermal forces induced by the sun, extreme heat, or extreme cold
- Temperature stress along a single panel that is partially shaded
|Panels with a high sheen or in darker colors tend to show oil canning more readily than those with a duller finish. Shallow cross-lighting and unweathered panels make it more visible as well.|
The impact of oil canning
Most of the time, oil canning does not present a danger to structural integrity but it is a good practice to inspect the panel to make sure the distortion is not resulting from extreme stress. If a fastener is overdriven, severe oil canning may result in a tear at the joint and cause leaking as it expands and contracts.
How to prevent or reduce oil canning
As you can probably discern, proper handling and installation go a long way towards preventing the problem in the field but it can also be avoided before that. There are also methods of reducing the visual impact.
Products made of high-quality, tension-leveled steel that is at least 24 gauge or heavier are less likely to suffer oil canning. Selecting lighter colors and a low gloss finish can help diminish the appearance of any slight deformation of the metal panel.
Designing a low slope metal roof also hides evidence of dents and rippling as does the selection of ribbed or striated panels over flat panels.
During manufacturing, oil canning can be prevented through using a panel profile that is +/- 16 inches for a standing seam panel, which is industry standard. The wider the panel, the more likely oil canning is to appear. Narrower panels show it even less but the labor costs begin to increase as the panel width is decreased.
The avoidance of slitting, recutting, and re-squaring sheet stock is another preventive measure the manufacturer can take.
Leveling strips can keep panels from oil canning. Leveling strips have two goals:
- Make the surface of the metal flat and free of ripples
- Neutralize hidden internal stresses that cause distortion in the panel during secondary operations like stamping.
The manufacturer can also employ tension leveling, an additional rolling process that reflattens coil material as it goes through processing.
Fabricators have methods of preventing oil canning by properly maintaining and adjusting equipment, including levelers, roll formers, cutters, shears, and brakes. Stiffening ribs or embossed finishes also mitigate oil canning.
Installers who carefully plan the panel layout and start point of a run can avoid stressors that cause oil canning. Start square and plumb or the workers may try to pull or push a series of panels to make them fit properly. Without the appropriate amount of space between panels, expansion and contraction will create dents and waves in the metal.
In fact, allow for thermal expansion in all directions by using flexing webs at sidelaps. Use the appropriate cleats as well.
Avoid substrate materials that tend to stick to the underside of the metal and restrict movement. You may need a slip sheet between the panel and the underlayment. Install a 3/8 inch foam backer rod along the center of each panel to create a crown to push any oil canning ripples toward the outer edges where they are less visible.
Repairing oil canning
If oil canning is present in a single panel, you can replace it if you won’t damage the adjacent panels. If it’s a design problem, certain adjustments can help. However, once the panels are installed there is not much you can do to fix oil canning. You could try claiming it as a feature or design element, but overall, you are better preventing the problem in the first place through proper design, manufacture, handling, and installation.
Thin gauge metals, panels with a high gloss or dark color, and wide, flat panels are all more prone to develop or show oil canning. The problem can occur as early as milling or can be cause by poor handling, storage, or installation.
Select high-quality, heavier gauge steel roof panels of light color and install them carefully on a low-slope roof. Oil canning will be completely absent or barely visible.