By Iris Xu, Sales Engineer at ZTRFM | Last Updated: September 15, 2026
Last year I reviewed a set of drawings for a 24-meter span warehouse in the Philippines. The designer had specified C-section purlins at 1.2m spacing for the entire roof. It would have worked -- but barely. The deflection was right at the limit. I changed the mid-span purlins to Z-sections lapped at the supports, and the deflection dropped 35%. Same steel weight, better performance.
I'm Hu Xianzhe, Cold Roll Forming Technical Engineer at ZTRFM. I design cold formed steel buildings and review other people's designs. The difference between C, Z, Sigma, and Hat sections isn't just about shape -- it affects how you connect them, how they carry load, and which machine makes them. Let me walk through each one.
C-Section (Cee)
The most common cold formed steel section. Looks like the letter C -- a web with two flanges and two lips (small returns at the flange edges to prevent local buckling).
Typical sizes: Web height 80-300mm, flange width 40-80mm, lip length 10-25mm. Material thickness 1.5-3.0mm. The most common sizes for purlins are C120, C140, C160, C180, C200, C220, C240, C300 (the number is the web height in mm).
Section properties (C200 x 70 x 20 x 2.0 as example):
- Cross-section area: 725 mm²
- Moment of inertia (Ix): 5,120,000 mm⁴
- Section modulus (Wx): 51,200 mm³
- Weight: 5.7 kg/m
Best for: Wall girts, roof purlins on smaller spans (under 6m), framing members. Easy to connect because the open side is accessible for bolts.
Limitation: Not efficient for lapped connections at interior supports. Two C-sections can't nest, so you lose the continuous beam advantage.
Z-Section (Zee)
Same elements as C-section (web, flanges, lips) but the flanges point in opposite directions -- one left, one right. This is the key difference, and it's the reason Z-sections are preferred for longer spans.
Why Z-sections win on multi-span roofs: Because the flanges are symmetric but opposite, two Z-sections can nest (overlap) at interior supports. You lap them by 10-15% of the span length, bolt them together, and you've created a continuous beam. This reduces the maximum bending moment by 25-30% compared to simply-supported C-sections.
Typical sizes: Same depth range as C-sections (80-300mm). Common: Z120, Z140, Z160, Z180, Z200, Z220, Z250. Flange width is typically narrower than C-sections of the same depth (because the section derives more of its stiffness from the depth).
Best for: Multi-span roof purlins (6-12m spans), where lapping at supports is possible. Also the standard choice for solar panel mounting structures.
Limitation: Can't be used as wall girts as easily (the asymmetry makes connecting cladding trickier). Also more difficult to connect at end walls.
Sigma Section (Σ)
A newer profile that looks like the Greek letter Sigma. It has a web with two re-entrant folds (inward bends) in the middle of the web, creating a more efficient section.
The re-entrant folds increase the moment of inertia without adding material. A Sigma section at the same weight as a C-section can have 15-20% higher load capacity. The trade-off is that the profile is more complex to form and requires more roller stands.
Best for: Long-span applications where weight matters (shipping to remote sites), and where the customer wants to reduce steel tonnage for cost or environmental reasons.
Availability: Less common than C and Z. Not every manufacturer makes Sigma tooling. Check availability before specifying it in a design.
Hat Section (U-Channel / Furring Channel)
A simple U-shape -- a web with two flanges and no lips. The simplest cold formed section to make.
Typical sizes: Web height 25-75mm, flange width 20-50mm. Very thin material (0.5-1.2mm).
Best for: Furring channels for drywall ceilings, hat channels for wall furring, small structural supports, and as backing for cladding attachment.
Limitation: Without lips, the flanges are prone to local buckling under compression. Not suitable for primary structural members or long spans.
Comparison Table
| Property | C-Section | Z-Section | Sigma | Hat |
|---|---|---|---|---|
| Lapped connection | No | Yes (nesting) | Limited | No |
| Efficiency (strength/weight) | Medium | Medium-High | High | Low |
| Max practical span (purlin) | 6m | 12m (lapped) | 10m | 2m |
| Forming stands needed | 14-18 | 16-20 | 20-24 | 8-12 |
| Machine price tier | $$ | $$ | $$$ | $ |
Design Standards
If you're designing with cold formed steel, these are the standards that govern section properties and design methods:
- AISI S100 (North America): The reference standard for cold formed steel design. Used in the US, Canada, and Mexico.
- EN 1993-1-3 (Europe): Eurocode 3, Part 1-3. Covers supplementary rules for cold formed members.
- AS/NZS 4600 (Australia/NZ): Based on AISI but with local modifications.
- GB 50018 (China): Technical code for cold formed thin-wall steel structures.
All four standards use the Effective Width Method or Direct Strength Method to account for local buckling in thin sections. If you're buying purlins, make sure the supplier can provide section properties calculated to the standard your project requires.
Quick Q&A
Can a C-section machine make Z-sections? No. The roller designs are fundamentally different. You need either a dedicated Z-section machine or a "C/Z quick-change" machine that has both roller sets on a shared frame.
What's a C/Z quick-change machine? A machine with two sets of rollers. You unbolt one set and bolt on the other. Changeover takes 2-4 hours. Costs about 60% more than a single-profile machine but gives you both profiles.
What steel grade should I specify? For structural purlins: S350GD or S550GD (galvanized, minimum yield 350 or 550 MPa). For non-structural: S220GD is sufficient. The "GD" suffix means galvanized.
What's the tolerance on purlin dimensions? Per EN 10162: web height ±1.5mm, flange width ±1.5mm, length ±3mm, straightness 1/500 of length. These are standard and your supplier should meet them.
Frequently Asked Questions (FAQ)
Can a C-section machine make Z-sections?
No. The roller designs differ fundamentally. A dedicated Z-section machine or a C/Z quick-change machine with both roller sets is required.
What is a C/Z quick-change machine?
It is a machine with two roller sets on a shared frame. Changing from one set to the other takes about 2-4 hours and the machine costs about 60% more than a single-profile machine.
What steel grade is recommended for structural purlins?
The article recommends S350GD or S550GD galvanized steel with minimum yield strength of 350 or 550 MPa for structural purlins, while S220GD is sufficient for non-structural use.
What dimensional tolerances are stated for purlins?
The article cites EN 10162 tolerances of plus or minus 1.5 mm for web height and flange width, plus or minus 3 mm for length, and straightness of 1/500 of length.
