COP:fitness-purpose; oil-canning

11.3 Oil Canning 

Distortion of flat metal areas is an aesthetic problem associated with the manufacture of metal roof and wall cladding and flashings. Flat pan architectural metal panels, wide flashings, and profiled metal cladding with wide pan configurations without stiffening ribs are all liable to show distortion in flat metal areas. It is known as oil-canning or panning.

Oil canning can be defined as visible waviness in the flat areas of metal roofing and wall cladding. It can also be referred to as panning, canning, stress wrinkling or elastic buckling, and is caused by differential stresses in the metal. As the metal tries to relieve these stresses in panels with high width to thickness ratios, material buckles out of plane producing the characteristic waviness of oil canning

It has an aesthetic effect and is not a structural or durability issue. Some highly reflective paint finishes and metals or different light conditions can exacerbate the visual effect of oil canning. Some distortion is inevitable in light gauges. It can become an issue of customer acceptance because customer expectations are often unrealistically high.

The degree of waviness can be hard to measure and is highly dependent on viewing angles, the position of the sun, and the reflectivity of the surface. Cladding installations with a high degree of visibility should be designed to minimise oil canning.

Oil canning is more common where the width of unformed sections is large. It can usually be avoided or minimised in normal rib and trough section profiles with a maximum pan width of 300 mm, and flashings that have a maximum unformed width of 300 mm. See 7.2 Flashing Materials

In standing seam roofs with pan widths of more than 300 mm, some oil canning is normally evident. Many designers regard oil canning in such profiles as inherent to the material and treat it as a desired effect accentuating the material's natural characteristics.

Manufacturers and installers should minimise unintentional non-flat conditions, and any visual waviness should be relatively even and regular.

There are various causes for oil canning:

  • material;
  • roll tool design and setting;
  • installation; and
  • expansion allowance.

 

 

11.3.1 Material 

All profiled metal roof and wall products begin in a coil form. Stresses induced during coil production can contribute to oil canning. Examples of these stresses are:

  • Full Centre: The coil is longer in the centre of the strip than near the edges. This creates buckles and ripples in the mid-coil area.
  • Wavy Edge: The coil is longer on one edge of the strip. That causes waviness on the long edge.
  • Camber: The side edge of the coil deviates from a straight line. The normal tolerance for a 1200 mm wide coil is a 2 mm deviation in a 2 m length, but some forming processes and end uses cannot tolerate that variation.
  • Uneven Material Strength: During the forming process material may tend to draw unevenly from the softer areas rather than evenly as designed; it leaves excessive material in the “harder” areas.
  • Slitting: Generally, coil for flashings and narrower products are cut by slitting from a single, wider master coil. Slitting of a master coil can release and redistribute residual forces. It can also mean that different qualities of the master coil are modified or changed in the slit coil, i.e., a full centre in a master coil can become a wavy edge in a slit coil and the slit coil may not retain all the attributes of the master coil or sister coils.

 

11.3.2 Tool Design 

By the nature of the process, many stresses are created during roll forming. These must be minimised and equalised as much as possible during manufacturing. Forming tools must be designed to form the material progressively.

Corrugated and ribbed profiles are most often formed from the centre and moved outward thereby “pushing” the differential stresses to the edges of the sheet. Generally, profiled metal rib and corrugated profiles, flashings, and most trough sections can be expected to provide finishes free of avoidable distortion.

Standing seam profiles typically need more forming on the edges of the feed material and little or none in the centre of the sheet, which tends to trap uneven stresses in the centre of the profile. Often one edge requires more forming than the other, meaning the stresses developed are not even in the sheet.

Some evenly distributed oil canning can normally be expected in standing seam products with a width of more than 300 mm, and it is considered acceptable.

11.3.3 Installation 

 

Oil canning can occur in fixed cladding, even though it does not fit accurately, when fixings are too far apart or when fixings are overdriven. It can also result from an uneven substrate, irregular bearing on the purlins or by the structural framing being out of line.

Thermal expansion can also increase oil canning. Longitudinal expansion should be accommodated by using sliding clips allowing movement. See 14.4.4.3E Expansion Clips. Transverse expansion is usually accommodated in the upstand of the profile, but this can only happen if adjacent pans are not in contact at the base. Wide perimeter flashings must be designed to allow for independent movement of the flashing and the cladding.

A convex curve in the roof structure can cause canning as it puts the pan of the profile under compression. Sometimes this curve is inadvertent. Concave roof cladding and flashings give rise to oil-canning because the pans are in compression. There are limitations on curved radii to avoid oil canning. See 14.1 Curved Roofs.

Commercially designed truss sections and rafters may have camber induced in their manufacture, anticipating deflection under load. The degree of curve that may be accommodated by any profile is largely determined by the width of the pan and is, also, affected by the material thickness and grade.

11.3.4 Minimising Oil Canning 

Good design and installation can minimise oil canning.

Materials:

  • Use thicker material.
  • Use low gloss paints or embossed surfaces.
  • Use natural weathering materials that dull over time.

Flashings:

  • Limit flashing width to 300 mm.
  • Limit the joined length of fixed flashings to 12 m.
  • Attach wide flashings with brackets that allow independent thermal expansion.
  • Manufacture a stiffening swage into flashings that have a face width greater than 200 mm.
  • Do not fix flashings to timber with a moisture content greater than 18%.

Cladding:

  • Limit cladding length.
  • Ensure the purlin alignment avoids convex curving.
  • Inspect for flatness before installation.
  • Avoid thin materials.