Aluminum vs Steel Roofing: Weight, Corrosion and Salt Exposure
Aluminum will not rust and weighs a third of steel. Steel is three times stiffer and moves half as much. Here is how the two metals genuinely differ and where each belongs on an Ontario roof.
Steel dominates the metal roofing market in Ontario, and for most inland residential roofs it is the correct choice. Aluminum is the better answer in a narrower set of conditions, and when those conditions apply, the difference is not marginal. The two metals differ in four ways that matter: density, stiffness, thermal movement, and how they corrode.
Weight
Steel has a density of about 7.85 g/cm³. Aluminum is about 2.70 g/cm³ — roughly a third. A 0.032 in. aluminum panel and a 24 gauge steel panel are close in thickness but the aluminum weighs well under half as much, and both are light compared with the roof coverings they replace.
In practice, this rarely decides a residential re-roof. Ontario houses framed for asphalt shingles have ample capacity for either metal, because both are far lighter per square than the shingles coming off. Weight becomes a live issue on a heritage structure with a marginal frame, on a recover over an existing assembly where you are adding rather than replacing load, or on a long carry up ladders where crew handling and panel damage during handling are the real constraints.
Stiffness and Thermal Movement
Here the ranking reverses. Steel's elastic modulus is about 200 GPa; aluminum's is about 69 GPa, so aluminum is roughly one third as stiff for the same section. At equal thickness an aluminum pan deflects more, oil cans more readily, and dents more easily under a knee or a dropped tool. Aluminum roofing therefore tends to be specified thicker than the steel it replaces, and aluminum systems lean harder on rib geometry and striations to stay flat.
Aluminum also expands about twice as much as steel — roughly 23 µm per metre per degree Celsius against roughly 12 for steel. A long panel on a Southern Ontario roof sees an enormous swing: a dark surface can reach well above 60°C on a July afternoon and drop below −25°C in January, so a 12 m run of aluminum moves several times more than most people expect. That movement has to go somewhere. In a properly designed system it goes into floating clips, slotted fastener holes, and expansion allowance at the panel ends. In a badly designed one it goes into elongated fastener holes, distorted seams and noise.
This is why exposed-fastener aluminum roofing is uncommon and rarely a good idea on long runs. A screw through the panel face fixes a point that needs to move. Aluminum belongs in concealed-clip systems that were engineered around its expansion.
How Each Metal Corrodes
This is the real dividing line. Steel roofing does not resist corrosion on its own at all — it survives entirely on its coatings. The metallic coating (Galvalume or galvanized) and the paint system above it are the protection. Where those are breached and eventually consumed, the steel oxidizes into red rust, and rust occupies more volume than the metal it came from, so it lifts and undercuts the surrounding coating. Corrosion in steel is self-propagating in that sense.
Aluminum protects itself. Exposed to air it forms an aluminum oxide film a few nanometres thick, almost instantly, and that film is dense, adherent and self-repairing. Scratch it and it re-forms. Aluminum does corrode, but it corrodes differently: instead of general rusting it pits locally, and the corrosion product is a white or grey powder that is roughly the same volume as the metal, so it does not undercut coatings the way rust does. Aluminum roofing does not develop rust streaks, and a scratched aluminum panel does not start a spreading failure.
Chloride: The Condition That Changes the Answer
The oxide film that makes aluminum durable has one main enemy: the chloride ion. Chloride is small enough to penetrate the passive film and initiate pitting, which is why aluminum is not automatically the winner in every salt environment. But aluminum's pitting is localized and slow, while chloride on breached steel coating drives fast, general, undercutting corrosion. In genuine salt exposure, aluminum still wins decisively — which is why aluminum is the standard specification for oceanfront roofing.
Ontario has no ocean coast, and that matters. A roof in Smithville, Grimsby or Hamilton is not seeing marine salt aerosol. What Southern Ontario does have is road salt, and road salt behaves like a coastal exposure at ground level and for a limited height above it. Relevant conditions here are:
- ✓Properties directly on a heavily salted arterial road or highway, where winter spray carries as an aerosol well beyond the pavement edge.
- ✓Low roof elements near grade — porch roofs, canopy roofs, skirt flashings, bay roofs over a driveway — that catch splash rather than just airborne spray.
- ✓Eavestrough, downspout outlets and splash zones where salted meltwater from a driveway or walk ends up.
- ✓Industrial neighbourhoods with sulphur-bearing emissions, where acid deposition consumes zinc-based coatings faster.
- ✓Buildings with heavy chlorinated or ammoniacal exhaust nearby — pool enclosures, some agricultural and food-processing exhaust.
For a typical two-storey roof plane, none of these apply strongly, and painted Galvalume steel is the sensible, well-proven choice. For eavestrough on a salted driveway edge and for low elements in the splash zone, aluminum is worth specifying even when the main roof is steel.
Fabrication Differences That Affect the Job
Steel and aluminum do not behave the same in a shop or on a roof, and that shapes what details are available.
- ✓Soldering. Tin-coated and terne-coated steels solder well, which allows fully sealed flat-lock and gutter-lining details. Aluminum cannot practically be soldered in the field, so aluminum assemblies rely on mechanical seams, sealants and welded shop fabrication.
- ✓Welding. Aluminum welding needs different equipment and destroys the temper of heat-treated alloys in the heat-affected zone. Field-welding aluminum roof components is not routine work.
- ✓Forming. Aluminum is softer and forms easily, which is a genuine advantage on complex trims and curved work, but it also creases and kinks more readily during handling.
- ✓Cutting. Both should be sheared, nibbled or cut with a fine-tooth metal blade. Abrasive saws throw hot filings that embed and — on a steel roof — rust in place.
- ✓Fasteners. Both metals need fastener compatibility checked. Aluminum in particular must not be paired with copper or bare graphite, and stainless fasteners in aluminum are acceptable mainly because the fastener is small relative to the panel.
Cost and Availability, Honestly
Aluminum coil generally carries a higher material cost than equivalent coated steel, and aluminum pricing is more volatile because it tracks a commodity market closely. Steel roofing is also more widely stocked in Ontario in more profiles, colours and gauges, so lead times and colour matching for future repairs are usually easier with steel. Neither of these is a reason to specify the wrong metal for a genuinely aggressive exposure, but both are real project considerations.
A Simple Decision Rule
Specify painted Galvalume steel for the main roof planes of an ordinary Southern Ontario house, and choose the gauge and paint system deliberately. Specify aluminum where chloride exposure is continuous — the splash zone near salted pavement, eavestrough on a driveway edge, low canopy and porch roofs on a busy road — and where a complex geometry benefits from a softer, more formable metal. Where you mix the two on one building, plan the drainage path so the two metals are not in direct contact and no runoff carries dissolved metal from a more noble component onto a less noble one.
Frequently Asked Questions
Does aluminum roofing dent more than steel?
Yes, at equivalent thickness. Aluminum is softer and less stiff, so it takes an impression more readily from hail, a ladder foot or a dropped tool. Aluminum systems compensate with heavier thickness and stiffer rib geometry. The offsetting advantage is that a dent in aluminum never becomes a rust site.
Is aluminum worth it if I live a few hundred metres from a salted highway?
For the main roof, usually not — airborne road salt drops off quickly with distance and height, and a good painted Galvalume system with well-detailed edges handles that exposure. Aluminum is more clearly justified for eavestrough, downspouts and any roof element within the splash zone of salted pavement.
Which metal is noisier in the rain?
Neither, in a properly built assembly. Rain noise on a metal roof is a function of what is underneath: a solid deck with underlayment and an insulated attic below is quiet regardless of metal. Noise complaints almost always trace back to panels installed on open purlins with nothing behind them.
Can aluminum and steel be used on the same building?
Yes, and it is common — for example steel roof panels with aluminum eavestrough. The requirement is that they not be in direct contact where an electrolyte can bridge them, and that the drainage sequence does not run water off a more noble metal onto a less noble one. Isolation with butyl tape, a non-conductive washer or a factory-coated separation handles the contact points.
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