Thermal Expansion and Oil Canning in Aluminum Roofs: A New England Guide

Does aluminum expand and contract on a roof?

Aluminum expands as it heats and contracts as it cools, and it moves more than steel for the same temperature change. Its coefficient of thermal expansion is roughly twice that of steel, which is basic, well-documented physics rather than a quirk of any one product. On a roof, that movement is continuous. A dark panel in direct sun heats well above the air temperature, then cools quickly after sunset or when a cloud passes. Each cycle makes the metal grow and shrink slightly. The movement in a single panel is small, but it adds up across a long run of metal and repeats thousands of times over the life of a roof. A roofing system that ignores it will eventually show the stress, either as loosened fasteners, distorted panels or noise. A system designed for it lets the metal move freely.

Why does thermal movement matter more in New England?

Thermal movement matters more in New England because roofs here see a very wide temperature range over the year. A winter night in Vermont, New Hampshire or inland Maine can drop below zero, while a sunny July afternoon can push a roof surface well past 90°F, even in towns where the air is milder. The swings are not only seasonal. A bright winter morning after a cold night can warm the sunny side of a roof quickly while the shaded side stays frozen. Spring and fall bring large day-to-night changes as well. That range means a roof in New England has to accommodate more total movement than a roof in a mild climate. Systems built with rigid face-fastened sheets feel it at every screw. Systems built to float handle it with much less stress.

How does an interlocking roof allow aluminum to move?

An interlocking roof allows aluminum to move because each panel is held by its locks and concealed fasteners rather than pinned through the face. Interlock panels connect on all four sides to the panels around them, and the fasteners sit beneath the interlock, where they hold the panel down without locking it rigidly in place. As the metal warms, the panel can shift a tiny amount within its interlock. As it cools, it shifts back. The movement is distributed across many small panels instead of concentrated along a long sheet, which keeps it small at any one joint. This floating attachment also protects the weather surface. Nothing is drilled through the exposed face, so there is no rubber washer being worked back and forth by expansion. That is one reason concealed-fastener systems hold up better in climates with big temperature swings.

What is oil canning?

Oil canning is a visible waviness or rippling in flat areas of a metal panel, most noticeable in low-angle sunlight. It gets its name from the way old oil cans flexed when pressed. It appears mainly on wide, flat panels such as some standing seam and flat-pan products. Several things contribute to it: stresses left in the metal during manufacturing, the way the panel is fastened, an uneven deck, and thermal expansion pushing against restraint. The effect can come and go during the day as the metal warms and cools. Oil canning is considered a cosmetic condition across the metal roofing industry. It does not mean the panel is weakened or leaking. On a historic Colonial or a home in a design-conscious neighborhood, though, it can bother an owner, so it is worth understanding before you choose a profile.

Which roof profiles show oil canning least?

Textured, shingle-style profiles show oil canning least, because they have little flat surface for waviness to appear on. Interlock's Slate, Cedar Shingle, Shake and Mediterranean Tile profiles are stamped with depth, edges and texture that stiffen each panel and break up reflected light. Those profiles also look at home on many New England houses: slate on a Victorian or Colonial, cedar-look panels on a Cape or shingle-style coastal house, and shake on a farmhouse or ski-country home in New Hampshire or Vermont. Flat standing seam is a clean, modern look that suits barns, contemporary homes and some farmhouses. It is more likely to show some waviness in raking light. Owners who choose it should know that minor oil canning is a normal characteristic of flat metal rather than a defect.

Can thermal movement cause noises or leaks?

Thermal movement can cause occasional ticking or popping sounds on some metal roofs, but on a properly designed and installed system it should not cause leaks. Sounds usually come from metal sliding against a fastener or another surface as it warms or cools quickly, such as when sun hits a cold roof in the morning. Interlocking panels with concealed fasteners tend to be quiet because the movement is spread across many small joints. A solid deck and underlayment also dampen sound. Leaks related to movement are mostly a problem on exposed-fastener sheets, where repeated expansion works screw holes and washers loose. If you hear unusual noises or see a leak on any metal roof, have it checked by a crew trained on that system. An in-home roof consultation is also a good time to ask how a system handles movement on your house.

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