This is a term often used in the roofing industry but what it means and the causes are often not fully understood.
The appearance of flatness depends on the distribution of the stresses across the surfaces of the metal sheet. These stresses will change as temperature changes. The ability of metal to transfer the effect of these stresses across the surface without buckling or distorting out of plane, will determine the level of “oil canning” that will occur.
The term has its origins in the old style oil cans which dispensed oil from a long narrow nozzle when the can was inverted and thumb pressure was applied to the base of the can. The can base was designed to buckle in and out when pressure was applied, thus displacing a small amount of oil each time the base was pressed inwards.
Therefore “Oil canning” is a metaphorical term and these days is used to describe the tendency of flat surfaces to show variations in reflectivity due to distortions within the flat plane. For example, a curved mirror will show a stretched and distorted image of a person standing in front of it. A metal surface too will distort the reflection of light if minor variations in and out of a level plane exist. The appearance of flatness is very much dependent on surface reflectivity. It is also caused by mill tolerances, variations in the substrate/decking and purlin alignment. Some paint finishes and metals that have high gloss index will exhibit highly apparent distortion. The visual effects of “oil canning” can be exacerbated by different light conditions and orientation. Darker colours visually accentuate oil canning to greater extent than the lighter, more neutral colours.
While oil canning is an inherent characteristic there are several precautions that can be taken to reduce the oil canning effect on roofs. One is to use thicker material, because thicker material tends to oil can less than thinner material. Another is to heed published technical advice about which cladding profiles can be successfully sprung curved for instance, because this process introduces stresses into flat pan areas which can contribute to this phenomenon. That is why products like LYSAGHT SPANDEK® and LYSAGHT CUSTOM ORB® generally perform well in sprung curved roof applications – because they have no wide flat pans which can distort.
Profiles like LYSAGHT KLIP-LOK 700 HIGH STRENGTH® have transverse flutes in the pans which not only add strength but also assist in minimising the oil canning effect. However even this profile can exhibit “oil canning” if installed incorrectly.
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Severe oil canning can occur when any flat pan roof sheet is sprung curved through a tight radius curve. The tighter the radius, the worse the effect. The darker the colour the more noticeable it becomes. Mis-aligned purlins can also be a contributing factor. If in doubt, seek some technical advice from BlueScope Lysaght, www.lysaght.com
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