What You Need to Know About Oil Canning in Sheet Metal
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.
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.
Sources:
1) http://en.wiktionary.org/wiki/oil-canning
2) Metal Construction Association (MCA) Article:
3) Revere Copper Products, Inc. : Copper and Common Sense, 8th Edition
My home is new construction and not complete yet, entrance only 10x 10 is ribbed metal and showing oil canning, how do I address with my builder? Should it be redone?
This can be from coil that is slit and not 100% parallel. It can by pan forming machinery being out of proper adjustment or alignment. If your roof deck is uneven or humped in places. If the installer clamped the panels together instead of letting them lie stress free. Angling the fasteners toward the panel to keep clip against rib will cause it. But a 10×10 shouldn’t have issues. The smaller the area the less likely the problem. Sounds like either product or installation quality issues. I have 40 years experience with 3rd largest roofing company in the USA.
Can you clarify what you mean by “over-fastening”. I understood it to mean “fastening too tightly to the deck by overtightening the fastener” but another person has understood it to mean “spacing fasteners too closely to each other”. We would be using clip-style concealed brackets on a standing seam. Thanks
Overfastening can be achieved by either placing mechanical fasteners too closely together or by over tightening the fasteners.
i’ve just built a log home with a red metal roof.It is showing significant oilcanning.How should i address this with my builder and how if anything can be done.This beautiful roof looks like shite.
does strapping a sheeted roof contribute to oil canning?
Oil Canning in automotive metal bodies is not acceptable and has been extensively studied. As Tom states above it comes about due to stress differentials… this could be due to heat during rolling into sheet but most thin metals are cold rolled and annealed so there is very little thermal stress. If you have soft sheet steel and you roll form a ridge where you bend the metal will become work hardened and there will be springback and residual stress. If there is an area such as a pocket for a door handle on your car door where there is lots of stretch next to an area where there is no stretch, the stress will cause distortion of the unstrained area and cause “rabbit ear”. Auto companies prevent rabbit ears and other forms of oil canning by ensuring the whole door panel has a minimum strain. Over seating nails will cause an indentation (strain) and this will produce a ring of distortion in the surrounding metal.