C, Z or Sigma purlins: how to choose the right cold-formed section
How C, C Plus, Z, Sigma and U cold roll-formed sections differ, where each is typically used, the size ranges available, and what to send for a quote.
Updated October 16, 2026
Purlins and side rails carry roof and wall cladding loads back to the main steel frame. In most modern steel buildings they are cold roll-formed from steel strip: light, made to the exact length, punched for bolts at the mill and quick to install. The section shape decides how the member behaves on site, so it is worth choosing it deliberately rather than by habit.
The section shapes at a glance
- C section — a web (h), two equal flanges (b) and an edge lip on each flange (l). Lips stiffen the flanges against local buckling.
- C Plus (C+) — a C with a double lip: the edge lip is folded back again, which stiffens wider flanges further.
- Z section — flanges point in opposite directions. Because of this point-symmetric shape, one Z can be turned and nested over another of the same size at a support.
- Sigma (Σ) — a deeper section whose web is folded inwards in the middle. The folds stiffen the web and move the shear centre closer to the web than in a plain C.
- Sigma Plus (Σ+) — a Sigma with double lips on wide flanges.
- U channel — web and flanges without lips: used for tracks, trims and light framing rather than as a main purlin.
| Section | Web h | Flange b | Thickness t | Weight kg/m | Sizes |
|---|---|---|---|---|---|
| C | 100–400 | 50–100 | 1.5–4 | 2.55–19.61 | 84 |
| C Plus | 100–400 | 85–100 | 1.5–4 | 4.46–19.67 | 77 |
| Z | 100–400 | 50–100 | 1.2–4 | 2.25–19.61 | 80 |
| Sigma | 160–400 | 50–100 | 1.5–4 | 3.66–20.4 | 85 |
| Sigma Plus | 195–400 | 85–95 | 1.5–4 | 5.33–19.64 | 57 |
| U | 100–400 | 53–97 | 1.5–4 | 2.35–17.25 | 84 |
Every size has its own page with the developed (coil) width, weight per metre and section properties (area, Ix, Wx, Iy, Wy, radii of gyration). See the size and weight tables for the full list.
C sections: simple, versatile, single spans
A C section is symmetric about one axis only, and its shear centre lies outside the web. Under gravity load it therefore tends to twist unless the cladding, sag rods or bracing restrain it. C sections are usually detailed as single-span or sleeved members.
- Side rails (girts), eaves beams and gable posts
- Roof purlins on shorter bays
- Door and window trimmers, light framing and mezzanine joists
- Members that must sit flush back-to-back or be bolted web-to-web
Z sections: continuity over several bays
The main reason to choose Z is lapping. Over each internal rafter, two Z purlins of the same size can be nested and bolted together, so the line of purlins acts as a continuous beam over several spans instead of a chain of simple spans. Continuity generally reduces mid-span bending and deflection, which on longer buildings often allows a lighter section or a longer bay. Laps add some material, so the economy depends on the number of bays and the lap length your engineer specifies.
- Roof purlins on multi-bay industrial, logistics and agricultural buildings
- Side rails where continuity is wanted on long walls
- Projects where transport length matters: lapped Z lengths are typically one bay plus the lap
Sigma sections: deeper webs for longer spans
A Sigma section has the same lipped flanges as a C but a web with inward folds. The stiffened web resists web buckling better, and with the shear centre close to the web the section twists less under load. Sigma ranges start deeper than C and Z (from 160 mm in the mill table), so they are typically chosen for longer spans and heavier roof loads — for example snow-loaded roofs, roofs carrying solar panels or buildings with wide bays.
C Plus and Sigma Plus: double lips for wide flanges
Wide flanges give a larger bearing surface for cladding and fixings, but a wide, thin flange buckles locally more easily. The double lip on C Plus and Sigma Plus (a 25 mm lip with a 15 mm return in the mill table) keeps wide flanges stiff. They are a good fit when the cladding system needs a wider seat or when a single section has to carry more load without moving to a heavier gauge.
A practical way to choose
- Spans and loads come first. Bay spacing, roof pitch, snow and wind, cladding and any services or solar panels decide the required section. These must come from your structural engineer.
- Single span or continuous? Simple spans or sleeved joints point to C or Sigma; lapped continuity over several bays points to Z.
- Depth. When the span calls for a deep section, the stiffened web of Sigma and Sigma Plus usually uses the steel better than a plain C of the same depth.
- Flange width. If the cladding or fixings need a wider seat, look at C Plus or Sigma Plus.
- Environment. Choose the material and coating for the exposure: black steel to be painted, pre-galvanized Z275, Magnelis (ZM) or Aluzinc. See the coatings guide.
- Length and transport. Members can be cut to length up to 25 m; check what the site and the truck can handle.
What to send for a quote
- Section type and size: h × b × lip × t (mm)
- Material and coating: e.g. S350GD+Z275 or S420GD+ZM
- Cut lengths and quantity per length, or total tonnage
- Hole positions (a drawing or a punching schedule)
- Delivery country and the date you need the material on site
The minimum order is 5 t per size and material. Send your list through the quote form or the section pages and we confirm feasibility, lead time and price.
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