Quick facts on oil canning. What is it?
- Oil canning is a moderate deformation or buckling of sheet material, particularly common with flat sheet metal surfaces. It is seen as waviness in the flat areas of the metal roof or wall material.
- This phenomenon will seem very apparent at times and seem to disappear at others due to the angle of the sunlight.
- Oil canning is inherent in all sheet metals and is the result of a combination of factors. There are no industry standards for what is excessive oil canning.
What it looks like?
Why it happens?
- Uneven stresses in the milling process – Production mills use large rollers to form sheets and coils of various metals. There are small gaps that separate these rollers. During the forming process, very high heat is produced when force is used to flatten the sheets. This heat naturally causes thermal expansion and contraction in the metal and the gaps do not allow even distribution of the stress.
- Roof decking is not truly flat – Slight contours can lead to waviness.
- Over fastening the material to the decking can lead to uneven stress.
- Movement of the primary structure
- Differential Solar heating – This means that as parts of the roof may be in direct sunlight and exposed to thermal expansion, other parts may be shaded and not expand at all. This results in uneven stress on the material
How to avoid/prevent oil canning?
- Gauge selection and panel width: Thicker material = less oil canning ; less space between seams = less oil canning
- Fastening and clip/cleat selection: Do not over-fasten any roofing material to the roof. Follow guidelines from your metal manufacturer for use of fixed vs expansion cleats to allow for expansion and contraction. (i.e. Long pan copper panels over 10ft recommend use of expansion clips)
- Proper handling and storage of your metal: The material may be metal, but is still fragile and needs to be transported and handled in an appropriate manner.
- Using ribbed or corrugated panels: Adding ribs to your panels will strengthen them and show significantly less oil canning then flat panels.
All in all…
Oil canning is most often considered unsightly and should be addressed with a homeowner before any material is put on a roof. This entry does not cover all aspects and conditions in entirety of oil canning, but rather touches on the more common features. Understanding this common roofing issue will help lead to better installation methods in the field.
2) Metal Construction Association (MCA) article:
3) Revere Copper Products, Inc. : Copper and Common Sense, 8th Edition