Thermal Expansion and Contraction: How a Metal Roof Is Designed to Move
A long steel panel can change length by more than half an inch between a January night and a July afternoon. Every good metal roof detail exists to let that happen freely.
Every metal roof on every building is moving right now. Not much, and not quickly, but continuously — lengthening through the morning as the sun comes onto it, shortening through the evening as it radiates heat back to the sky. The difference between a roof that lasts and one that starts leaking in its second decade is almost entirely whether the assembly was built to let that happen freely or to fight it.
How Much Movement Are We Talking About?
The arithmetic is straightforward. Steel has a coefficient of thermal expansion of roughly 0.0000065 inches per inch per degree Fahrenheit. Aluminum is about twice that. Take a 40-foot steel panel — 480 inches — and a realistic Southern Ontario surface temperature swing. A dark panel on a still, sunny July afternoon gets very hot; the same panel on a clear January night sits well below the air temperature because it radiates to the sky. A 190 degree Fahrenheit swing between those two extremes is not exotic.
480 inches times 0.0000065 times 190 gives about 0.59 inches. Nearly five-eighths of an inch of length change on a single panel, between its coldest and hottest states. An aluminum panel of the same length moves roughly twice that. That movement has to go somewhere, and it does not care what your fastening schedule looks like.
The Fixed Point
A standing seam panel is anchored at exactly one location along its length, and free everywhere else. That anchor is the fixed point, and where it goes determines the direction the panel grows.
- ✓Fixed at the ridge, free at the eave: the panel grows downhill. This keeps the top of the panel tight to the ridge detail and lets the eave hem slide on its cleat. It is the more common arrangement on steeper roofs.
- ✓Fixed at the eave, free at the ridge: the panel grows uphill. Used where the eave detail needs to be rigid, and it requires a ridge detail with enough room for the panel to advance into.
- ✓Either way, only one end is fixed. Fastening both ends is the fundamental error.
- ✓Fixed clips near the anchor point and floating clips through the rest of the run is a common way this is implemented; the manufacturer's instructions define the pattern.
How the Details Accommodate the Movement
Floating clips are the main mechanism — a base screwed to the deck, and a tab captured in the seam that slides in that base. But the whole assembly participates. The eave hem hooks over a continuous cleat rather than being screwed down, so it can slide along it. The panel's upper end has an upturned leg that sits in a ridge closure with clearance rather than being pinned to it. Trim laps are made with a slip joint on long runs. Wall flashings hook and hem rather than screwing through the panel. Slotted holes appear where a fastener must pass through something that will move relative to something else.
The most damaging single mistake in metal roofing is fastening a long panel rigidly at both ends. It looks solid on installation day. Over the following seasons the panel has nowhere to grow, so it buckles in the field, the clips elongate their screw holes, seams begin to open, and the eave or ridge detail is progressively pulled apart. By the time it leaks, the damage is distributed along the entire panel.
Panel Run Length Limits
Because movement scales with length, every standing seam system has a run length beyond which the manufacturer requires floating clips, and often a further length beyond which the roof needs to be broken into separate runs with an expansion detail between them. On a long, low-slope run — a commercial building, a large agricultural roof, a long shed dormer — that transition detail is not optional trim, it is a structural provision. The alternative is a panel run so long that even floating clips reach the end of their travel.
Oil Canning: Related but Not the Same
Oil canning is the visible waviness in the flat area of a metal panel. It is not a structural defect and it is not a leak, and it is an inherent characteristic of light-gauge formed metal — every manufacturer says so. But thermal stress is one of its contributors, alongside deck irregularity, residual stress in the coil, over-tight clips and handling during installation.
The practical mitigations are worth knowing because they are decisions made before the panels arrive. Narrower panels oil can less than wide ones. Striations or a pencil rib rolled into the pan break up the flat surface and dramatically reduce the visible effect. Heavier gauge material is stiffer. A genuinely flat deck removes the single biggest contributor. Properly set floating clips let thermal stress relieve itself instead of accumulating in the pan. Low-gloss and matte finishes show far less than a high-gloss finish, because oil canning is only visible through reflection.
The Noise
Homeowners with new metal roofs sometimes call about ticking or popping sounds in the late afternoon and again after sunset. That is thermal movement — panels advancing or retreating against clips and cleats. It is normal, it is not a sign of anything coming loose, and it settles down as the assembly beds in. Where it is pronounced, the usual causes are a slightly tight clip somewhere in the run, or panel-to-deck contact without an effective slip surface. A proper underlayment doubles as a slip sheet and reduces it.
Exposed Fastener Panels and Movement
This is where the difference between the two systems becomes concrete. An exposed-fastener panel is rigidly pinned at every screw, which means it cannot move as a panel — instead, the movement is absorbed at each fastener, working every hole slightly larger, cycle after cycle. Over enough years the holes elongate and the washers can no longer bridge them. That is the mechanism behind the well-known pattern of long exposed-fastener roofs developing leaks at fasteners over time, and it is why panel run lengths on those systems matter too.
Frequently Asked Questions
Why does my metal roof tick in the evening?
The panels are contracting as they cool and moving against their clips and cleats. It is thermal movement working exactly as designed. It is most noticeable in the first year and typically quietens as the assembly settles.
Is oil canning a defect I can have fixed?
Manufacturers universally class it as an aesthetic characteristic rather than a defect, and it cannot be removed from an installed panel. It is managed at the specification stage — narrower panels, striations or a pencil rib, heavier gauge, a matte finish, and above all a flat deck.
Does panel colour affect how much the roof moves?
Yes. A dark panel absorbs more solar radiation and runs hotter, so it reaches a higher peak temperature and therefore a larger total swing than a light one on the same roof. The difference is real but it does not change the design approach — the roof is detailed to move regardless.
How long can a standing seam panel run be?
It depends on the system, the clip type and the material — aluminum moves about twice as much as steel for the same length. The manufacturer publishes maximum run lengths for fixed and floating clip arrangements, and beyond a certain length the roof has to be broken up with an expansion detail.
Get a Free Metal Roofing Quote.
We measure your roof remotely by satellite and deliver a written, itemized quote — same day, no obligation.