Key takeaways
- Sigma carries more load than a C or Z section of the same depth because its stiffening folds put more steel away from the neutral axis.
- Z sections win on long roofs because they lap into a continuous beam; sigma does not lap in the same way.
- If depth is free, a deeper C or Z purlin is usually cheaper than a sigma section for the same capacity.
- If depth is constrained by the cladding system or headroom, sigma is the right answer.
How the sections differ
A C purlin is a simple channel. A Z purlin is the same channel rotated into a point-symmetric form so it can lap. A sigma purlin adds longitudinal stiffening folds in the web and sometimes in the flanges, which stiffens the section without increasing its overall depth.
That single difference — stiffening folds — is the whole comparison. Everything else follows from it.
| Criterion | CZ purlin | Sigma purlin |
|---|---|---|
| Section shape | Plain channel (C or Z) | Channel with stiffening folds |
| Capacity at equal depth | Baseline | Higher — more steel away from neutral axis |
| Lapping for continuity | Z laps into a continuous beam | Not designed for the same lap detail |
| Cost per metre at equal load | Lower if depth is available | Higher, but uses less depth |
| Best for | Long roofs, standard spans | Restricted depth, cladding rails, heavier loads |
Capacity: what the folds actually buy you
Bending capacity of a cold-formed section depends mainly on how far the material sits from the neutral axis. A plain channel accumulates most of its steel in the web, close to the neutral axis, where it contributes little to bending strength.
The sigma folds move material outward, so the section resists bending more effectively for the same amount of steel. The gain is real but it is not unlimited — typically the equivalent of going one or two thickness steps up in a C section.
The practical consequence: if your span table says you need a 250 mm C purlin at 3.0 mm and you can only fit 200 mm of depth, a 200 mm sigma section will often do the job. If you have 300 mm of depth available, a deeper C or Z purlin will usually be cheaper.
Continuity: where Z still wins
The Z section's point symmetry lets adjacent purlins lap over the rafter, creating a continuous beam. On a long roof that roughly halves the bending moment compared with single spans, which is a far larger benefit than the modest capacity gain from sigma folds.
Sigma sections are not normally lapped in the same way, so they behave as single spans unless you design a specific connection. On a long roof, a lapped Z section frequently beats a sigma section of the same depth — not because it is a better section, but because the structural system around it is better.
This is why the two products coexist rather than compete. Z purlins for long roofs with free depth; sigma for constrained depth and for cladding rails, where spans are short and continuity is less useful.