Oil Canning

Oil canning, also called stress wrinkling or elastic buckling, is the rippled or wavy look that can appear in the flat parts of metal roof and wall panels. It occurs in steel, aluminium, copper and zinc. It is most noticeable on wide flat pans, thinner metal, glossy or dark finishes, and when low-angle sunlight strikes the roof. The waves may appear and fade through the day as the panels heat and cool.

Causes

The Metal Construction Association and panel manufacturers in North America and Australia list similar causes:

• Coil stresses: uneven strain left from steel rolling, such as a longer centre ("full centre"), wavy edges or camber.
• Slitting and roll-forming: cutting coil to width and forming edges can redistribute or trap stresses in the flat.
• Supports: decks or purlins that are out of plane, structural movement, or cambered framing.
• Installation: over-engaged seams, over-driven or angled fasteners, clips that are too tight or loose, and misaligned panels.
• Restrained thermal movement: panels pinned at both ends, or bound by trims and flashings, buckle when they expand.
• Handling and storage: twisting panels or carrying them flat.

Reducing it

• Use thicker metal or tension-levelled coil.
• Choose narrower panels, profiles with stiffening ribs or striations, or formed shingle, shake and tile profiles, which have little flat area.
• Specify low-gloss, textured or embossed finishes and lighter colours.
• Install on a flat, in-plane substrate, use clips that allow movement and avoid overtightening.
• Handle and store panels carefully so they are not twisted or distorted.

Is it a defect?

Industry guidance treats oil canning as an aesthetic condition that normally does not affect structural performance or weathertightness. Unless the contract sets specific flatness criteria, it is generally not accepted as sole grounds for rejecting panels, and paint or panel warranties usually exclude it. Severe distortion can, however, point to structural movement or distressed connections and should be investigated.