Thermal Expansion and Oil Canning on Aluminum Roofs in Oregon

How much does an aluminum roof move between an Oregon August and a January freeze?

Stand on a Salem or Eugene roof on an August afternoon when the valley air is pushing 100°F and the surface of a dark roof is a good deal hotter than that. Come back on a clear January morning after a freeze and the same roof is sitting below 32°F. Every metal roof in Oregon lives through that swing twice a year, and it also lives through the smaller daily version, warm afternoon to cold night, hundreds of times. Aluminum's coefficient of thermal expansion is roughly double that of steel, so a given panel length grows and shrinks measurably across that range. This is a material property, not a weakness. Interlock's 3105-H24 and 3003-H24 alloy will do it for the life of the roof, and the only question is whether the installation lets it happen or fights it. The freeze-thaw side of the equation is what separates Oregon's regions. Marion County averages 39 freeze-thaw days a year, Polk 40, Multnomah and Clackamas about 36, Benton 31. Up the Gorge at Hood River the count rises to 55, with 76 inches of snow on top, and Wasco County east of the Cascades sees 48. On the coast in Clatsop County the roof barely freezes at all, around 10 days a year, but it never stops cycling between cool nights and warm afternoons. Each of those cycles is a small expansion and contraction the roof system has to absorb without complaint.

How does a clip or interlock system let the panels move?

The wrong way to hold aluminum down is to screw it tight to the deck through the face. A face-fastened panel is pinned at every screw, so when it expands on a July afternoon the movement has nowhere to go. It bows between fasteners, it elongates the screw holes, and over enough Oregon summers those holes become the leak points. It is also where the ticking and popping noise people associate with cheap metal roofing comes from. A floating system does the opposite. Standing seam pans are held by concealed clips that grip the seam but let the pan slide lengthwise. Interlock's shingle, shake, slate and tile profiles use a four-way interlock: each panel hooks into its neighbors on all four sides and is fastened through a concealed flange at the top, so the whole field is locked together and yet free to move as a unit with temperature. Nothing is stressed, nothing is loud, and the underlayment beneath acts as a slip layer. That same four-way interlock is why the system holds in the Columbia Gorge. There are 13 roofs on record in Hood River since 1999 and 12 in The Dalles, in wind that runs sustained for days. Because every edge is engaged, there is no free lip for wind to get under, and ICC-ES ESR-1790 documents wind uplift resistance to 177.5 psf. A roof that can float thermally and still resist that kind of uplift is doing both jobs with the same geometry.

Why does oil canning show up under Willamette Valley winter sun?

Oil canning is the gentle ripple or waviness you sometimes see running across the flat part of a metal panel. It comes from residual stress in the coil the panel was formed from, from a deck that is not quite flat underneath, or from the thermal movement described above. Nearly every flat, light-gauge metal panel has some of it. The roof is exactly as watertight and as strong as it would be without it. What changes in Oregon is how often you notice it. From November through February the sun over the Willamette Valley sits low on the southern horizon, and on the rare clear winter day it rakes across a south- or west-facing slope at a shallow angle. Raking light is the single best way to reveal any surface that is not perfectly planar. The same roof under flat grey October cloud, which is most of the year here, reads dead smooth. Color and panel width do the rest. A black or Deep Charcoal standing seam pan 16 inches wide is the worst case: dark finishes absorb more heat and so move more, and a wide flat pan has the most unsupported surface for a ripple to form across. The 22-square black standing seam roof documented in Warren, Columbia County, in May 2025 is a handsome roof, and the homeowner was told before the deposit that some waviness would be visible on a low winter afternoon. That conversation should happen on every dark standing seam job in the state.

Which profile hides it best on an older Portland house?

Textured, small-module profiles show oil canning least, for the plain reason that there is no broad flat area for a ripple to run across. Interlock's slate, cedar shingle, shake and Mediterranean tile panels are broken into individual shadowed modules, and the eye reads texture, not surface. On a 1910 Craftsman bungalow in Southeast Portland or a foursquare in Irvington, the slate or shake profile also happens to be the one that suits the house. The deck under those houses is the second half of the story. Portland's pre-1930 housing stock was framed with rough-sawn rafters that have sagged and twisted over a century, often sheathed with skip sheathing or boards that were later covered with plywood of uneven thickness, and many of these roofs are carrying a second layer of asphalt on top. Metal follows whatever is under it. A standing seam pan laid over a wavy deck will telegraph every hump, while a small-module interlocking panel bridges minor irregularities far more forgivingly. On the 374 Portland roofs on record since 1998, the pattern has been consistent: tear off to the deck, replace or shim any sheathing that is out of plane, and only then lay the roof. Skipping that step to save a day of labor is how a good aluminum product ends up looking like a bad one. If a homeowner insists on a wide, dark standing seam over a 1905 deck, the honest advice is to budget for deck work first or to choose a textured profile.

When is waviness cosmetic and when is it an actual defect?

The metal roofing industry, and Interlock's Guardian Lifetime Limited Material Warranty along with it, treats mild oil canning as an inherent cosmetic characteristic of light-gauge metal and excludes it from coverage. That is not a dodge. Slight waviness in raking light does not affect the 50-plus-year service life, the 120 mph warranted wind resistance, or the Alunar PVDF finish. A homeowner in Corvallis or McMinnville who sees a soft ripple on a clear February afternoon and nothing the rest of the year is looking at a normal roof. A defect looks different, and an installer who has been on Oregon roofs since 1997 should say so plainly. A panel that has buckled into a sharp crease, seams that have split or unhooked, fasteners that have backed out and are visible on the face, or waviness that has grown noticeably worse over a couple of summers are signs the roof was pinned instead of allowed to float, or that the deck was never flattened. Those are workmanship problems, and in Oregon they fall under the installing contractor's CCB-licensed responsibility, not the manufacturer's material warranty. The practical test is time and season. Note what the roof looks like in low winter light in its first year and again in its third. Cosmetic oil canning stays put. A roof that is fighting its own thermal movement gets worse, and the 36 to 55 freeze-thaw cycles a year in most of western Oregon and the Gorge will find the weak fastener long before the finish warranty runs out.

Common questions