What causes oil canning on a metal roof?
Short answer. Oil canning is visible waviness in the flat areas of metal panels, caused by stress locked into the steel coil, roll forming, thermal movement that fasteners or clips do not allow, and an uneven deck underneath. It is mostly cosmetic, does not usually affect weather tightness, and many panel makers treat it as a normal characteristic rather than a defect.
The main causes
- Coil stress. Steel is rolled and slit at the mill, and slight differences in length across the sheet, called edge wave or center buckle, are locked in before the panel is ever formed.
- Roll forming. Machine settings, worn rollers or a panel run too fast can introduce stress into the flat pan.
- Restrained expansion. Metal grows in the sun and shrinks at night. On a Georgia summer day a dark panel can swing widely in temperature. Overdriven screws or clips that do not let the panel slide force that movement into waves.
- Uneven substrate. Humps in the deck, proud nail heads, or overlapping underlayment telegraph through a smooth panel.
- Handling. Long panels lifted from the middle or twisted during carrying can bend permanently.
How it is reduced
- Striations, pencil ribs or a slightly textured pan break up the flat surface so waves are less visible.
- Narrower panels and heavier gauges are stiffer.
- Floating clips on standing seam let panels move lengthwise.
- A flat, clean deck and a slip sheet under the panels help.
- Low-gloss finishes reflect less of the distortion.
Can it be fixed?
Not really once it is installed. The waviness often looks worst in low morning or evening sun and nearly disappears at midday. If a specific panel is badly distorted, the fix is replacing that panel. Before signing, ask whether the contract or warranty addresses oil canning, since many exclude it.
For more on panels and decks, read metal roofing in Georgia and OSB vs plywood roof decking.