Oil Canning in Metal Roofing: What Causes It and How Installers Minimize It
Oil canning is waviness in the flat part of a metal panel. It has several separate causes, most of them controllable, and it is worth understanding before you sign a contract.
Oil canning is the visible waviness that appears in the flat areas of metal roof and wall panels. It shows up most strongly in raking light, on large panels, on dark glossy colours, and when you look along the roof rather than square at it. It is the single most common source of disappointment with an otherwise well-built metal roof, and most of what causes it is decided before the first panel goes on.
What It Is and What It Is Not
Oil canning is an aesthetic condition. The panel is still watertight, still structurally sound, and still performing. Most manufacturers state explicitly in their literature that oil canning is an inherent characteristic of light-gauge formed metal and is not grounds for rejection or a warranty claim. That is an industry-wide position, not one manufacturer being difficult, and it means the time to address it is during specification rather than after installation.
Ask any contractor quoting a smooth-pan roof directly: what is your position on oil canning, and what are you doing to minimize it? A contractor who says it will not happen is either inexperienced or telling you what you want to hear. A contractor who explains gauge, coil, deck prep and panel width is one who has thought about it.
Cause One: Stress Already in the Coil
Steel arrives at the roll-former as coil, and coil carries residual stress from the rolling mill. The common defects have names in the trade: coil set, where the sheet wants to curl in the direction it was wound; crossbow, where it curls across its width; edge wave, where the edges are longer than the centre; and centre buckle, the opposite. None of these are visible in the tightly wound coil, and all of them are released when the sheet is unwound and formed.
The mitigation is tension levelling, a mill process where the coil is stretched past its yield point to even out the internal stress. Tension-levelled coil costs more and is not what every supplier stocks. On a project where a smooth pan matters, it is worth specifying and worth asking whether the panel supplier is providing it.
Cause Two: The Deck
A metal panel is thin enough to follow whatever it is laid over. If a sheathing joint is proud, if a rafter has crowned, if a truss chord is bowed, if there is an old high nail or a lump of dried adhesive, the panel telegraphs it. On a re-roof this is common, because the existing deck may have been under shingles for decades and no one has looked at it flat.
- ✓The deck should be inspected plane by plane, not just for rot but for flatness, before any underlayment goes down.
- ✓High spots get sanded or the fastener reset. Low spots get shimmed. Damaged or delaminated sheathing gets replaced rather than covered.
- ✓Underlayment matters too. Wrinkles, overlapping laps and proud staples all print through. Self-adhered membrane should be rolled flat and laps kept to the specified overlap rather than doubled up randomly.
- ✓Installing over an existing shingle layer without a suitable substrate is one of the most reliable ways to produce a wavy metal roof, because the shingle courses themselves are a repeating ridge.
Cause Three: Restrained Thermal Movement
Metal expands and contracts with temperature, and Southern Ontario gives it a wide range to work through. Steel moves roughly 12 micrometres per metre for every degree Celsius. Over a 60 degree swing from a cold January morning to a hot July afternoon on a dark roof, a 12 metre panel changes length by roughly 9 mm. Aluminum moves close to twice that.
If the panel cannot move that distance freely, it has to go somewhere, and it goes out of plane. The system is designed around this: each panel has exactly one fixed point, everything else is on clips that let it slide, and longer runs use two-piece floating clips. Pinning a panel at both ends, over-driving the clip fasteners so the clip cannot move, or fastening through the panel face at a flashing all defeat that design and produce buckling near the restraint.
Cause Four: Handling and Installation
Panels are easy to damage in ways that do not show until they are on the roof and the sun hits them.
- ✓Bundles stored on uneven ground, or stacked too high, take a set that never fully comes back out.
- ✓Long panels lifted by two people at the ends will flex in the middle. Panels need to be carried on edge, with enough hands or a proper lifting arrangement for the length.
- ✓Forcing a panel into alignment rather than resetting it locks stress into the pan.
- ✓Over-tightening a seam, or running a seamer with worn or wrongly set rollers, pulls the pan edges and distorts the flat.
- ✓Walking on the pan rather than near the seams during installation, especially on wide panels in warm weather, leaves permanent deflection.
What Actually Reduces It
There is no single fix. Reducing visible oil canning is a matter of stacking several partial improvements, and every one of them is a decision that gets made before installation starts.
- ✓Roll texture into the pan. Striations, pencil ribs or a stiffening bead are the most effective single measure, because they both stiffen the pan and break up the reflection.
- ✓Use a heavier gauge. 24 gauge steel is meaningfully stiffer than 26 gauge.
- ✓Use narrower panels. Less unsupported flat area between seams.
- ✓Specify tension-levelled coil.
- ✓Choose a matte or low-gloss finish, and understand that dark glossy colours reveal the most.
- ✓Prepare the deck properly and do not skip the flatness check.
- ✓Use the correct clip type for the panel length and set the fixed point deliberately.
- ✓Give the crew the time and the manpower to handle long panels correctly.
Viewing Conditions Matter More Than People Expect
The same roof can look flawless at noon and wavy at six in the evening. Low-angle sun across the plane is the worst case, and it is why a west-facing roof often looks worse than a north-facing one on the same house. Before judging a finished roof, look at it at several times of day and from the angles it is normally seen from, rather than from the one position that shows it most harshly.
Frequently Asked Questions
Is oil canning covered by a warranty?
Generally not. Most manufacturers state in their published literature that it is an inherent characteristic of formed metal and is excluded. Read the specific warranty document for the system being installed and ask the contractor what their own position is before you sign.
Does oil canning get worse over time?
It varies with temperature, so a roof looks different on a hot afternoon than on a cold morning, and that variation continues for the life of the roof. It does not generally progress in the sense of steadily worsening, unless something is actively restraining panel movement, in which case the cause should be found and corrected.
Can oil canning be fixed after installation?
Rarely in any satisfying way. If it is caused by a specific restraint, a pinned fastener or a deck high spot, that can sometimes be corrected locally. Waviness from coil stress or panel geometry cannot be pressed out. This is why it is a specification issue rather than a repair issue.
Do standing seam roofs oil can more than exposed-fastener panels?
Corrugated and ribbed exposed-fastener profiles have closely spaced corrugations across the whole sheet, so they have very little flat area and rarely show oil canning. Standing seam has a wide flat pan by design, which is exactly where the condition appears. It is a consequence of the flat aesthetic, not a defect in the system.
Is aluminum worse than steel for oil canning?
Aluminum is softer at the same thickness and expands roughly twice as much per degree, so it is generally more prone to visible waviness in wide flat pans. It is chosen for other reasons, mainly corrosion resistance in coastal or aggressive environments, and it is usually specified in a heavier thickness or with a textured pan to compensate.
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