Aluminium vs Steel Pergola: Which Actually Lasts Longer Outdoors?
Both materials can give you twenty years or more outdoors. What differs is how they fail — and which failure you can live with depends on where the pergola stands and who maintains it. This page compares the two on the things that decide lifespan, not on the marketing claims.
The short answer
In coastal, humid or chemically exposed locations, aluminium wins because there is no iron in it to rust. For very long unsupported spans or heavy snow, steel wins because it is roughly three times stiffer. For a typical home or restaurant terrace, aluminium is the default — not because it is stronger, but because the maintenance burden over twenty years is far lower.
What actually decides the lifespan
Material choice sets the failure mode. These five things set the number of years:
- Alloy and temper — 6063-T5 or T6, stated explicitly rather than “aluminium”.
- Wall thickness in millimetres, specified separately for column, beam and blade.
- Coating system and film thickness in micrometres.
- Fastener material — stainless A2/A4, never plain steel against aluminium.
- Drainage and ventilation detail, so water cannot stand inside a closed section.
A cheap aluminium pergola with 1.2 mm walls and a thin coat starts blistering within a few years. The same material at 2.0 mm with a proper coating film is still sound a decade later. “Aluminium vs steel” matters far less than “thin vs properly specified”.
Corrosion — where the two diverge most
Steel contains iron. Without protection it rusts, and it starts at cut edges and on the inside of closed sections where you cannot see it until it is structural. Hot-dip galvanising followed by a topcoat is the only genuinely durable protection, and both steps add cost and lead time.
Aluminium does not rust; its oxide layer passivates. But it is not immune. In salt air, and at unprotected cut edges, it can pit. Where aluminium touches steel — fasteners, base plates, a steel subframe — galvanic corrosion eats the aluminium, because aluminium is the less noble metal in that couple. This is the most common design mistake in mixed-material pergolas.
- 6063-T5/T6 with a powder coat film of at least 80 µm for outdoor exposure.
- Seal or cap every cut edge and drilled hole.
- Stainless A2/A4 fasteners with isolating washers at any aluminium-to-steel interface.
Strength, span and weight
| Property | Aluminium 6063-T5 | Structural steel S235 |
|---|---|---|
| Density | ≈ 2.7 g/cm³ | ≈ 7.85 g/cm³ |
| Elastic modulus | ≈ 69 GPa | ≈ 200–210 GPa |
| Thermal expansion | ≈ 23 × 10⁻⁶ /°C | ≈ 12 × 10⁻⁶ /°C |
| Section needed for equal stiffness | Larger / deeper | Smaller |
| Self weight for equal duty | Roughly one third | Baseline |
Read it this way: aluminium’s modulus is about a third of steel’s, so an aluminium profile has to be deeper or thicker to reach the same stiffness. But its density is also about a third, so the finished aluminium structure usually still comes out lighter for the same duty. That matters for freight, for foundation load, and for whether two people can lift a beam on site.
The exception is span. Beyond roughly eight metres unsupported, or under high snow load, steel’s stiffness becomes decisive: aluminium either needs a very deep beam — expensive and visually heavy — or an intermediate column you did not want.
Thermal movement — the failure nobody plans for
Aluminium expands about twice as much as steel per degree. Over a 6 m beam and a 40 °C swing, aluminium moves roughly 5.5 mm while steel moves about 2.9 mm. That will not snap anything by itself, but if both ends are rigidly fixed it becomes stress at the joints: sealant fails, fixings loosen, and after a few seasons the connection starts to open.
The detail that prevents it is simple and almost always omitted from quotations: fix one end, and let the other end slide — slotted holes, a sliding shoe, or a movement joint. Ask for it in writing, because it is invisible once the roof is up.
Maintenance over twenty years
| Aluminium, powder coated | Steel, galvanised + topcoat | Steel, painted only | |
|---|---|---|---|
| First intervention | Essentially none | Topcoat at 5–8 years | Repaint at 2–3 years |
| Typical interventions in 20 yrs | 0–1 | 2–3 | 6–10 |
| Visible failure | Chalking, blistering, pitting in salt air | Zinc consumed, then rust from inside | Rust blooms, film peeling |
| Repairability | Clean and touch in locally | Re-galvanising in situ is impractical | Sand and repaint |
The real point is not strength but executability. Re-galvanising a steel beam that is already installed effectively means taking the structure down. Aluminium maintenance is washing and local touch-up, which an owner can do.
Cost per decade, not cost per kilogram
Steel’s raw material price is lower than aluminium’s, but a finished-structure comparison has to include four things the material price hides: galvanising and topcoat, the heavier lifting and transport, the more frequent maintenance, and — in commercial work — the cost of closing the terrace to do it. On most residential and hospitality pergola projects, aluminium comes out lower over ten years. Only at very long spans or heavy load does steel’s structural efficiency outweigh that.
When steel really is the better choice
- Single spans beyond about 8 m with no intermediate column allowed.
- High snow load regions where deflection governs the design.
- An exposed industrial aesthetic is actually wanted.
- Budget is tight and periodic repainting is explicitly accepted.
We supply steel bracketry and subframes for exactly these cases. If your project is one of them, saying so is more useful than being sold aluminium.
What to write into the specification
- Alloy and temper: 6063-T5/T6 — not simply “aluminium alloy”.
- Wall thickness in mm, item by item: column, main beam, blade.
- Coating type and film thickness in µm.
- Fastener grade, plus the galvanic isolation detail at mixed-material joints.
- Thermal movement provision at the beam ends.
- Drainage path and how closed sections are ventilated.
Frequently asked questions
Does aluminium rust?
No — aluminium contains no iron, so it does not rust. It does corrode in specific conditions: pitting in salt air where the coating is damaged, and galvanic corrosion where it is bolted directly to steel. Both are prevented by coating integrity and by isolating dissimilar metals.
Will a steel pergola actually rust?
Yes, unless it is hot-dip galvanised and top-coated. It starts at cut edges and on the inside of closed sections, so by the time it is visible it has usually been progressing for years. Paint alone is a maintenance cycle, not a protection system.
Which material is stronger?
For the same cross-section, steel is about three times stiffer because its elastic modulus is far higher. But whether a structure is strong enough depends on span and load, not on the material label — most residential pergola profiles already carry far more than the actual roof load.
Which one is lighter?
Aluminium, at roughly one third of steel's density. For the same duty the finished aluminium structure is usually lighter as well, which reduces freight cost and foundation load.
Can aluminium and steel be combined?
Yes, and it is common — steel subframes, aluminium roofs. But every contact point needs galvanic isolation: isolating washers or tape, and stainless fasteners. Skip that and the aluminium corrodes at the interface, which is the single most common mixed-material failure.
How long will each last outdoors?
With correct specification and coating, both can exceed twenty years. The difference is the failure mode: steel fails by corrosion, which is hard and expensive to reverse; aluminium fails by coating ageing, which can be locally repaired. Which of those you can live with is the real decision.