Oil Canning in Metal Roofing
Oiling canning is defined as “stress wrinkling” or a visible wavy, ripple appearance in the flat of any metal panel. Oil canning is subjective and is normally an aesthetic concern only; it does not affect the roof’s structural integrity. The industry has universally noted in their specifications and proposals that the presence of oil canning is not a cause of rejection. However, it has not stopped complaints, especially if the roof slope is greater than 1:12.
Experts in the field of metal forming will tell you that the only way to avoid oil canning is to form a corrugation along the flat with only 3” of flat between corrugations. This is not possible with the Architectural requirements of today’s designers. The best we can do is to form small striations in the flat of the panel to hide the oil canning or minimize its appearance
Oil canning is caused by internal stresses within thin gauge metals. These stresses are introduced during field installation, mill production of the coil or during fabrication such as slitting and forming. Field installation conditions, techniques and tolerances can also impact oil canning.
Causes of oil canning:
1. Luff in the coil at the time of initial rolling at the mill. This is usually caught at the time of roll forming, in which case, the coil will not be used.
2. Forming induced luff as the panel profile is roll formed. Sometimes if the roll former is not adjusted correctly, small wrinkles can be induced in the panel as it is formed. This can usually be prevented with a small adjustment of the roll former before the panel is rolled.
3. Installation induced oil canning due to roof traffic of installers. Excessive installation traffic on the panel between purlins or joists can cause oil canning.
4. Secondary structures that are not in plane at the time the roof is installed.
5. Thick insulation without foam spacer blocks or the use of short clips with thick insulation.
6. Temperature induced oil canning. As the sun hits the roof, the metal expands and some angles will heat part of the roof before the rest, which can cause a distortion of the panels causing the oil canning. This situation is present in all metal roofs and is unavoidable
There are some design conditions that will minimize oil canning. When designing or quoting a roof with a slope of more than 2:12, you should be aware of the possibility of owner’s complaining of oil canning. The cause of oil canning listed in in 3 thru 5 above, can be minimized by placing the panels over a rigid deck. This will add additional expense to the project but in high profile projects such as churches and public buildings it is well worth the additional cost.
If the project limitations prevent the use of a deck under the roof, the installation of a backer rod centered in the center of the panel flat will slightly raise the flat and hide much of the oil canning. A backer rod is a foam closure used in precast walls to fill the joints before sealant is added. The backer rod can be found at most construction supply houses, it is supplied in long lengths.
There are other methods to reduce the probability, severity and visual impact of oil canning. It is thought the material gauge and tension leveling, as well as the panel design will minimize oil canning. We suggest never to use a total flat pan, striations will help make oil canning less apparent. The finish of the panel can contribute as well. Light colored, lower gloss panels will reflect less light making it less noticeable.
There is no sure fire way to avoid oil canning, but understanding some of the factors that contribute to it is the first step to reduce the occurrences. There are too many uncontrollable factors that play a role and no manufacturer, fabricator, or installer can assure total prevention.