Key takeaways
- Standard strut channel is 41x41 mm in both metric and imperial — the slot pitch (21 or 25 mm) must match the clamps and spring nuts being used.
- Hot-dip galvanising to ISO 1461 is standard for outdoor solar mounting; coastal and aggressive environments require heavier coating or stainless.
- Wind uplift, not panel dead weight, is usually the governing load case for solar mounting structure design.
- On-site roll forming is cost-effective above approximately 10 MW where transport and waste costs exceed the equipment cost.
- Carport structures span 5–12 m and require C-purlin or hot-rolled primary sections — strut channel alone is not adequate for the primary span.
Why solar mounting structure design matters
A photovoltaic system is only as reliable as the structure that holds it. The mounting structure must keep panels at the design tilt angle for 25–30 years through wind, snow, thermal cycling and UV exposure without fatigue failure, corrosion or significant movement. It must also allow installation quickly, since labour is a major component of system cost in all markets.
Cold-formed steel and aluminium strut channels (C-channel, slotted strut, or unistrut) are the standard material for ground-mount and rooftop solar mounting systems globally. The sections are produced by roll forming and are available in standard lengths from distributors, or made to order from a roll forming line on site or in a factory.
The main structure types
Ground-mount fixed-tilt: the most common large-scale installation. Driven piles, ground screws or concrete ballasts carry horizontal rails on which panels are tilted at a fixed angle optimised for the site latitude. Strut channel runs north-south on the piles; east-west purlins carry the panel mounting clips.
Rooftop ballasted: panels are mounted on a ballast frame sitting on a flat or low-slope roof without roof penetrations. The frame must be heavy enough to resist wind uplift without anchoring through the roof membrane. Strut channel is used for both the frame and the tilt legs.
Rooftop penetrating: panels are fixed through the roof to rafters or purlins below. Faster than ballasted and better for high-wind zones, but requires watertight flashing at each penetration.
Carport: structure spans between columns to provide vehicle cover while supporting panels above. C-purlins or hot-rolled sections carry the long span; strut channel is used for secondary framing and panel mounting.
| Structure type | Typical span | Primary material | Key design factor |
|---|---|---|---|
| Ground-mount fixed tilt | 2–4 m N-S rail | Strut channel, C-purlin | Wind uplift, pile spacing |
| Rooftop ballasted | 1–2 m frame | Strut channel | Wind uplift, roof load limit |
| Rooftop penetrating | Rafter spacing | L-bracket, rail | Watertight flashing at fix points |
| Solar carport | 5–12 m span | C-purlin, hot-rolled | Vehicle impact, long-span deflection |
Strut channel (unistrut) dimensions and load capacity
Standard strut channel is 41 x 41 mm (1-5/8 in x 1-5/8 in) in the imperial series and 41 x 41 mm or 40 x 40 mm in metric, in plain or slotted (punched) configurations. Heavier-duty sections are 41 x 82 mm (double channel or back-to-back) for longer spans. Wall thickness is typically 1.5–3.0 mm depending on load requirements.
The slotted pattern at 21 mm or 25 mm pitch allows fixings at any position without drilling, which is critical for rapid solar installation. The clamp or spring nut slides into the channel and locks when tightened.
A standard 41x41 strut at 2.5 mm wall thickness in S350 steel has an allowable uniform distributed load of approximately 1.5–2.0 kN/m at 2 m span. Panel loads are typically 0.3–0.5 kN/m2; combined with wind uplift, the governing load case is usually uplift rather than dead weight.
Material and corrosion protection
For ground-mount and outdoor rooftop applications, hot-dip galvanised (HDG) strut channel is standard. The zinc coating provides 20–30 years of corrosion protection in most environments. In coastal or aggressive industrial environments, a heavier coating (Class C per ISO 9223 or equivalent) or stainless steel is required.
Aluminium strut channel is used in markets where weight is critical (rooftops with low structural capacity) or where aluminium finishes are required. Aluminium is lighter than steel but has lower yield strength, so sections must be heavier to achieve the same load capacity.
Pre-galvanised (Sendzimir-galvanised) material is sometimes used for indoor or sheltered applications where the galvanised strip is roll-formed without post-galvanising. The zinc coating is lighter (typically 275 g/m2 = approximately 20 µm per side) and does not provide the same protection as hot-dip for exposed outdoor use.
On-site roll forming for solar mounting
On large solar farms (above approximately 10 MW), the volume of strut channel and rail required makes on-site roll forming economically attractive. A portable or container-based roll forming line is set up at the project site, forming strut channel and rails from coil to the required lengths without transport damage and with near-zero off-cut waste.
On-site forming reduces logistics cost significantly on projects far from processing centres, and avoids the damage to punched holes and cut ends that can occur during transport of pre-cut sections. The line typically produces one standard profile and cut length, operating continuously during the installation phase.