Essentials

Oil canning of metal panel roof systems can be a controversial issue because an owner's or designer's aesthetic expectations often are not realized when confronted with the appearance of oil canning.

Some building owners believe a metal roof system never should oil can, but oil canning is an inherent characteristic of metal. You can attempt to reduce it or mask it, but it is virtually impossible to stop. A contractor who informs his customers about the potential for oil canning and provides options for minimizing it can reduce the chance of controversy. A contractor also must make it clear to customers that oil canning will not affect the performance of a metal panel roof system.

Uneven stresses

Oil canning occurs when there are uneven stresses within metal. Uneven internal stresses can develop when poor quality metal coils with side camber or nonstraight edges pass through a roll-forming machine. NRCA suggests tension leveling be specified and contractors specifically order tension-leveled metal.

Uneven stresses also can occur when coil stock is slit during manufacture to the width of a metal panel, stretching the metal if slitting blades are dull or misaligned. Ensuring alignment of slitting blades and keeping them sharp can reduce the possibility of oil canning.

Uneven stresses can be induced by roll-forming equipment. Rolling a panel from a piece of flat stock metal certainly introduces stress into the metal. If roll-forming equipment is misaligned or a panel is formed in a short distance, the resulting stresses could be unevenly distributed. Ensuring alignment of the panel rollers and preferably using more rolling stations can help reduce the possibility of oil canning.

Uneven/nonuniform decks and temperature changes also can introduce uneven stresses into metal panels. A deck should be even and uniform before installing metal panels and verified by a general contractor or installing contractor. Also, be aware of large temperature differences during installation. These differences can introduce uneven stresses.

Trying to "stretch" metal panels to gain fractions of an inch over a roof's width can introduce uneven stresses within metal roof panels. Pulling a panel taught, installing the clips and letting the panel relax also will introduce uneven stresses, specifically at clip locations.

Solutions

Specifying and using heavy gauge metal—metal that is not stretched so easily—for panels will reduce the possibility of oil canning.

Heavy gauge metals also will help eliminate oil canning that comes from welding, break forming, hoisting operations and other procedures that either heat, bend or stretch metal and introduce uneven stresses.

Installing metal panels that have striations or stiffening ribs will lessen the effects of oil canning. These modifications not only stiffen the panel and reduce oil canning but also help mask any oil canning that occurs.

Using panels with low-gloss paints or mill finishes or using naturally weathering metals like copper, which develop a matte finish, will help mask the appearance of oil canning by reflecting less light, therefore reducing the visual effects of oil canning.

One of the best ways to eliminate the post-installation debate is to explain this phenomena before a job starts. Tell the owner some amount of oil canning is likely to occur, and explain all the precautionary measures to minimize or mask the inherent problem. Also, building a mock-up of the metal panel roof system can establish the baseline for an acceptable amount of oil canning. Be upfront with the inherent problem. This not only can mitigate the debate, but it will make the building owner feel a part of the decision-making process.

Some in the metal roofing industry are discussing establishing a standard about oil canning. The metal roofing industry needs to promote the minimization of oil canning and its effects, as well as establish benchmarks for acceptable amounts of oil canning on a finished metal panel roof system.

Although it may be difficult to provide written descriptions of acceptable oil-canning limits, it is viable to accomplish this with photographic descriptions of varying levels of oil canning. Defining levels by visual standardization would be a positive step toward preventing controversy when an owner's aesthetic expectations are not met. This also would provide contractors with information for owners to establish what they expect aesthetically from a metal panel roof system.

For more information about oil canning, see "How to minimize oil canning's effects," June 1999 issue, page 58.

James R. Kirby is an NRCA senior technical director.