Solar canopy structures are a 25-year commitment. The material you choose for columns, purlins, and secondary framing determines corrosion performance, maintenance burden, shipping cost, and structural efficiency across that entire service life. Three materials dominate the commercial solar carport market today: aluminum alloy, hot-dip galvanized (HDG) steel, and ZAM (Zn-Al-Mg) coated steel. Each is a legitimate choice — in the right application. Using the wrong one will not fail in year 1. It will fail in year 10 or year 15, quietly and expensively.
This guide compares all three across the dimensions that matter for EPC engineers and procurement teams.
1. Structural Strength: Yield Strength, Stiffness, Weight
Steel is stronger than aluminum — full stop. Structural carbon steel (S235/S355) carries a yield strength of 235–355 MPa. Aluminum alloy (6005-T5, the standard for solar extrusions) yields at around 240–260 MPa. On paper, the gap is smaller than most people expect. But stiffness is where the difference is decisive. Steel's elastic modulus is approximately 200 GPa; aluminum's is around 70 GPa. Steel is three times stiffer for the same cross-section.
In practice, this means aluminum sections must be significantly larger in cross-section to match the deflection limits of an equivalent steel member. For long-span carport beams (8m or more), that stiffness deficit becomes a real design constraint. For columns and purlins at standard 5–7.5m bay spacings, aluminum handles the loads comfortably.
ZAM steel uses the same S235/S355 structural grade as HDG steel. The ZAM coating is a surface treatment, not a material change. Strength is identical to HDG for the same section size.
Weight: Aluminum is roughly one-third the density of steel (2.7 vs 7.85 g/cm³). A 6m aluminum purlin weighs approximately 4–6 kg. The steel equivalent weighs 12–18 kg. This matters enormously for shipping, rooftop handling, and labor cost.
2. Corrosion Resistance: Coastal, Industrial, High-Humidity Sites
Corrosion determines lifespan. This is where the three materials diverge most sharply.
Aluminum forms a self-healing alumina oxide layer on exposure to air. In moderate environments (ISO C1–C3), anodized aluminum requires almost no maintenance and can exceed 30 years without coating degradation. In coastal and marine environments (C4–C5), uncoated aluminum performs well against atmospheric corrosion but is vulnerable to galvanic coupling — particularly where aluminum contacts carbon steel fasteners or brackets without isolation. Proper isolation at all contact points (EPDM pads, neoprene gaskets, A4 stainless fasteners) is essential.
HDG steel relies on a zinc coating deposited by hot-dip galvanizing per ISO 1461. Minimum coating thickness is 45–85 μm depending on section size. Zinc provides sacrificial cathodic protection: even if the coating is scratched, surrounding zinc corrodes first, protecting the steel base. In C3 environments, a standard ISO 1461 coating (85 μm average) lasts 25–30 years before maintenance is needed. In C4/C5 coastal or industrial environments, zinc loss rates increase significantly. ISO 9223:2012 gives first-year zinc corrosion rates of 2.1–4.2 μm/year in C4, 4.2–8.4 μm/year in C5 and 0.1–0.7 μm/year in C2; salt spray testing (ISO 9227) ranks coatings against each other but does not give a field loss rate. Structures in high-humidity tropical environments or near industrial emissions will need inspection every 3–5 years and localized re-coating at any damage points.
ZAM steel (Zn-Al-Mg ternary alloy coating) outperforms standard HDG in corrosion resistance by a significant margin. The Mg content drives formation of simonkolleite (Zn₅(OH)₈Cl₂·H₂O), a dense, adherent corrosion product that self-seals surface defects. In comparative salt spray testing, ZAM coatings (120–185 g/m² coating weight) reach 3,000–5,000+ hours before red rust — compared to 1,000–1,500 hours for equivalent HDG sections. In coastal C4 environments, ZAM structural steel can sustain 25–30 years without re-coating, with minimal annual maintenance. ZAM also performs well at cut edges and fastener holes, where HDG coatings are at their most vulnerable.
ZAM's corrosion profile makes it the preferred choice for cable trays, walkways, and secondary framing in harsh environments — exactly where cut surfaces and drilled holes are most common. Solaracks ZAM cable trays and maintenance walkways leverage this property: the fine micro-crystalline matte grey finish holds up at punched slots and oblong drainage perforations without requiring edge sealing.
3. Weight and Shipping Cost
Aluminum's one-third weight advantage is most valuable for two scenarios: air freight (rare for large projects but occasionally relevant for emergency supply) and projects where site access or rooftop load limits are constraining. For standard sea-freight solar carport projects, the difference in FOB cost matters more than shipping weight.
Steel (HDG or ZAM) is denser, but steel's structural efficiency per unit cost means less material is needed for equivalent structural performance on long spans. On a cost-per-strength basis, steel wins on large-span primary members. On a cost-per-kilogram shipped basis, steel is heavier but structurally delivers more per kilogram.
For international procurement, HDG and ZAM steel components are typically palletized and shipped flat — straightforward for 20ft or 40ft containers. Aluminum extrusions can be bundled in longer lengths with less bracing, occasionally fitting more linear meters per container.
4. Fabrication and On-Site Installation
Aluminum extrusions are manufactured to near-net shape — purlins, rails, and clamp profiles exit the die already dimensioned for solar panel mounting. Site cutting is clean (standard carbide blades), produces no sharp burrs, and requires no coating touch-up after cutting. Bolted assembly only — no welding in the field. Thermal expansion is significant (23×10⁻⁶/°C vs 12×10⁻⁶/°C for steel), requiring slotted holes and expansion gaps, particularly for long canopy runs in high-temperature climates.
HDG steel requires welding, drilling, and cutting before galvanizing — the sequence matters. All fabrication must be completed before the zinc bath. Any on-site modification (cutting to fit, drilling additional holes) breaks the coating and requires zinc-rich paint touch-up. This is the primary field disadvantage of HDG: damaged areas are common during installation and must be treated immediately.
ZAM steel is typically roll-formed after coating. Light-gauge ZAM sections (purlins, cable trays, walkway planks) can be cut on-site with standard shears or cold saws with minimal coating damage at the cut edge. The alloy's self-sealing behavior at cut edges reduces the urgency of touch-up, but best practice still recommends zinc-rich paint on exposed edges for C4/C5 sites.
5. Cost Comparison
HDG structural steel is the most cost-effective option on a per-kilogram basis for structural members. It is widely manufactured, competitively priced globally, and the galvanizing process adds relatively modest cost to standard carbon steel sections. For large carport projects in benign environments (C2–C3), HDG steel delivers the lowest CAPEX.
ZAM steel carries a premium over HDG — typically 20–35% higher per tonne — but often eliminates the need for periodic re-coating in harsh environments, reducing 25-year OPEX. For projects in coastal zones or tropical high-humidity markets, the lifecycle cost of ZAM steel is competitive with or better than HDG.
Aluminum is the most expensive option per kilogram and per unit of structural strength. However, for secondary components (purlins, mounting rails, accessories), the weight savings in shipping and handling can partially offset material cost on large remote projects. For columns and primary beams on commercial carports with spans above 6m, aluminum is rarely cost-competitive with steel.
6. Lifespan
| Material | Typical Lifespan | Maintenance Requirement |
|---|---|---|
| HDG Steel (ISO 1461) | 25 years (C3) / 15–20 years (C4) | Inspect every 3–5 years; touch-up at damage |
| ZAM Steel | 25–30 years (C3–C4) | Minimal; inspect every 5–7 years |
| Aluminum Alloy (6005-T5) | 30+ years (C1–C4 with isolation) | Minimal; check galvanic joints annually |
7. Best-Use Scenarios
Choose HDG steel for: inland sites, C2–C3 environments, large-span primary structure (columns and main beams), budget-driven projects where lifecycle cost in the first 20 years is the primary criterion. HDG remains the structural workhorse for the majority of commercial carport projects globally.
Choose ZAM steel for: coastal installations, tropical high-humidity environments (C4), secondary framing elements such as purlins, cable trays, walkways, and gutter profiles where cut edges are frequent. ZAM is particularly suited to O&M-light projects where re-coating is not feasible (remote sites, long service contracts, island deployments).
Choose aluminum for: rooftop carport projects where dead load is constrained, premium architectural installations where aesthetics matter, markets where steel anti-dumping duties apply, and components where field modification is expected. Aluminum's advantage is most pronounced in accessories and sub-structure rather than primary columns and beams.
Material Selection at Solaracks
Solaracks supplies all three materials across its solar carport range and does not prescribe a single solution. Material selection is driven by site corrosivity category (ISO 9223), structural span, local code requirements, and budget across the project lifecycle.
For every carport inquiry, Solaracks provides a 24-hour structural design with static analysis to the applicable standard (e.g. ASCE 7-22 or JIS C 8955:2017), using SAP2000 where simulation is needed. Material specification is part of that design — not an afterthought.
Submit your site parameters and we will recommend the right material combination for your project.