Steel or aluminum: which metal roof suits Ontario?

For almost every home and cottage in Ontario, coated steel is the right metal roof: Galvalume or galvanized steel in 24 or 26 gauge is stiffer, more dent resistant and carries snow load better than aluminum, and it usually costs less. Aluminum’s big advantage is that it does not rust, which matters most in salt air on an ocean coast; Ontario’s lakes are fresh water. Aluminum earns its place here only in specific exposures, mainly heavy road salt spray and some industrial settings, and even then it needs a roof designed around how much it moves.

The standing seam we roll-form on site is 24 and 26 gauge steel, so read this as a steel roofer’s view. We have tried to be fair to aluminum where it deserves it.

What you are actually comparing

Coated steel

Roofing steel is never bare. It carries a metallic coating, and then a paint system on top.

• Galvalume is steel coated with an aluminum-zinc alloy, roughly 55% aluminum. It is what much of the painted residential roofing steel in Ontario uses now. The aluminum in the coating gives long barrier protection; the zinc protects scratches and cut edges by corroding sacrificially in place of the steel.
• Galvanized steel is coated in zinc alone. G90 is a common designation for roofing and trim. Zinc gives strong sacrificial protection at cut edges, while in most open-air roof exposures Galvalume’s barrier protection generally lasts longer.

Thickness is given in gauge, where the lower number is thicker. 24 gauge is about 0.024”, 26 gauge about 0.018”. Our 24 vs 26 gauge guide covers when each makes sense.

Aluminum

Aluminum roofing is sold by thickness in thousandths of an inch rather than gauge. For residential standing seam, .032” and .040” are commonly used, with .040” or heavier more usual for wider panels, longer runs and commercial work. It forms its own oxide layer, so it does not rust and needs no metallic coating, though it is almost always painted.

Note that the numbers are not directly comparable. Aluminum is a much softer, less stiff metal than steel, so .032” aluminum is thicker than 24 gauge steel on paper and still less rigid.

Corrosion: the argument for aluminum, and where it applies

This is the reason people ask about aluminum, so it deserves a straight answer.

Salt water is the real case for aluminum. On an ocean coast, airborne salt chloride lands on the roof constantly and attacks steel coatings, especially at cut edges and scratches. That is where aluminum roofing earns its reputation. Ontario’s Great Lakes and inland lakes are fresh water. A cottage on Georgian Bay or Lake Muskoka is not a seaside exposure, and a properly coated, properly painted steel roof does very well there.

Road salt spray is the exposure that matters in Ontario. A house tight to a heavily salted highway, a building beside a municipal salt yard, or a garage where salt slush is thrown up against low eaves sees chloride every winter. Mist from passing traffic carries further than people expect. That is where steel coatings work hardest, and where we would want to know about it before specifying anything. Our metal roof lifespan article flags the same thing: if you are close to a busy salted road, mention it.

Some industrial and agricultural settings are aggressive too: buildings near certain processing plants, or barns where ammonia from livestock reaches the underside of the roof. These are judgement calls, and the coil manufacturer’s warranty terms usually spell out which environments they exclude.

For ordinary Ontario homes and cottages, Galvalume steel with a good paint system is not where the roof fails. Details, flashings and sealants fail first, long before the steel does.

Aluminum has its own corrosion rules

Aluminum does not rust, but it corrodes when it touches the wrong things:

• Dissimilar metals. In contact with copper, or with plain steel fasteners, aluminum corrodes galvanically. Fasteners and clips must be compatible, usually stainless steel or aluminum.
• Water running off copper. Copper flashings or a copper cap upstream will etch aluminum below, and the same is true for Galvalume.
• Treated lumber. Modern copper-based wood treatments are aggressive to aluminum. Separation is needed where panels or trim touch treated wood.
• Wet masonry and mortar. Alkaline materials attack aluminum, so wall flashings need detailing.

None of this makes aluminum a poor roof. The installer just has to know the material.

Hail and denting

Thickness for thickness, steel is the harder, stronger metal and dents less. Aluminum roofing is made thicker partly to make up for that, so at common roofing thicknesses the gap narrows, and dent resistance also depends on the alloy and temper of the particular coil. Even so, aluminum is generally the more dent-prone of the two on a smooth flat pan. Ontario hail rarely marks 24 gauge steel cosmetically. On aluminum, heavier thickness and bead ribs in the pan are the main defences.

Foot traffic follows the same pattern: a careful boot in a 24 gauge steel pan is routine, and aluminum needs more care.

Thermal movement and why clip design matters

Every metal roof grows and shrinks with temperature, and Ontario swings from below -25°C to above +35°C on a sunny roof.

Aluminum moves roughly twice as much as steel for the same temperature change. On a 30-foot steel panel, the movement over that range is in the order of a quarter inch. The same length in aluminum moves around half an inch.

On standing seam, that movement is handled by the clips. Each panel is pinned at one point and the rest slides. With aluminum, the clips need longer slots or two-piece sliding designs that genuinely allow the extra travel, and the trim at the ridge, eave and walls has to be detailed to let the panel move under it. Get that wrong and the panel buckles in the heat, drags on its fasteners, or tears at a pinned detail. On long cottage runs with dark colours in full sun, this is where an aluminum roof is won or lost.

Steel moves too; it just gives the installer more margin.

Strength under snow load

As a material, steel is roughly three times stiffer than aluminum. On a roof, that shows up in how a panel spans between clips and how it handles a deep, wet Ontario snowpack.

Clip spacing for any standing seam roof should be set from the manufacturer’s tables for the site’s ground snow load and wind exposure, not left to a default. With aluminum, those tables generally call for closer clip spacing, heavier material, or both, to carry the same load as steel. In Muskoka, Huntsville or the Collingwood snow belt, that difference is real.

We size clip spacing to the specified ground snow load on every steel roof we install, and it is one of the questions in our guide to choosing a metal roofer.

Finish options

Both metals are available in PVDF (Kynar 500) and SMP paint systems, and the paint choice matters more than the metal for how the roof looks in 20 years. PVDF holds colour and gloss far longer and is worth it on dark colours and sunny elevations. More on that in metal roof colours that hold up in Ontario.

The colour range depends on the coil supplier rather than the metal. Steel roofing coil is widely stocked in Ontario in a broad range of colours; aluminum roofing coil can be more limited, with longer lead times for some colours. Ask what is actually in stock before you fall in love with a chip.

What drives the cost

We do not publish prices, but the drivers are clear:

• The metal. Aluminum coil usually costs more than painted steel coil for the same roof area, and its price follows commodity markets.
• Thickness. Heavier aluminum to get closer to steel’s stiffness adds cost.
• Clips and fasteners. Stainless or aluminum fasteners and sliding clips cost more than standard steel components.
• Clip spacing. Closer spacing on aluminum means more clips and more labour.
• Detailing. Separating aluminum from treated wood, masonry and copper adds time at transitions.

On a steel roof, the drivers are gauge, finish, seam type and roof complexity. Our Ontario cost guide explains them, and the online range tool gives you a whole-project range for your address.

Side by side

	Galvalume or galvanized steel	Aluminum
Common thickness	24 ga (~0.024”) or 26 ga (~0.018”)	Commonly .032” or .040”
Rust	Protected by metallic coating and paint	Does not rust
Best corrosion case	Inland, freshwater and normal Ontario sites	Salt air, heavy salt spray, some industrial
Dent resistance	Better, thickness for thickness	Softer, generally dents more easily
Thermal movement	Baseline	Roughly double
Stiffness under snow	About three times stiffer as a material	Needs closer clips or heavier material
Fastener compatibility	Standard coated fasteners	Stainless or aluminum only
Relative cost	Lower	Usually higher

Where each one makes sense

Choose steel for nearly every Ontario home, cottage, farm building and commercial roof. It handles snow, hail and foot traffic better, it costs less, and with a PVDF finish it holds its colour for decades.

Consider aluminum where corrosion exposure is genuinely severe: a building directly beside a heavily salted highway or salt storage, certain industrial sites, or a property with an unusual exposure that the steel coil’s warranty excludes. In those cases, also look at heavier coated steel with a PVDF finish and good edge detailing, which is often the more practical answer.

If you have a site like that, tell us at the site visit. If we think your exposure genuinely calls for aluminum, we will say so rather than sell you a steel roof that does not suit it.

Questions people ask

Is aluminum better for a cottage on Georgian Bay?

Usually not. Georgian Bay is fresh water, so the salt air case for aluminum does not apply. Wind exposure and snow load are the real design questions on the Bay, and stiffer steel with a double-lock seam and correctly spaced clips handles both well. Use PVDF paint on elevations facing the water, where reflected sunlight is hard on colour.

Will a steel roof rust in Ontario?

Not in any way that matters on a normal site over the life of the roof. Galvalume and galvanized coatings protect the steel underneath, and the paint protects the coating. Where you see rust on metal roofs, it is usually at scratched or cut edges left untreated, under debris that holds moisture in valleys, or on old unpainted barn roofing well past its life.

Is aluminum lighter, and does that matter?

Yes, aluminum is roughly a third the weight of steel for the same volume. On a house roof it rarely matters, because standing seam in either metal is far lighter than slate, tile or two layers of asphalt. Weight occasionally matters on an old structure being assessed for load, and that is a question for the site visit.

Does a thicker aluminum panel dent less than steel?

It can. Dent resistance depends on both thickness and the strength of the metal, so a heavy aluminum panel can hold up comparably to thinner steel, and the specific alloy matters. The simple comparison of the number on the spec sheet does not settle it. If hail resistance is a priority, 24 gauge steel with bead ribs is a strong, well-proven choice, and for aluminum ask the supplier for the alloy, temper and thickness in writing.

Can I mix steel and aluminum on the same house?

Yes, it is common: aluminum eavestrough, soffit and fascia with a steel roof is the normal Ontario combination. The rule is to keep the metals from sitting wet against each other and to use compatible fasteners where they meet.

If you are on a site with unusual exposure, or just want to know which metal and gauge your roof needs, send us an address and a photo, or call 705-816-5282. You can also get in touch online.