Purlins Selection Guide

Choosing the wrong purlin can lead to issues such as the inability to install roof panels, failure to support the required loads, or premature failure due to corrosion within just two years. Below, we clearly outline the differences, suitable applications, and selection criteria for four common types of cold-formed thin-walled steel purlins.

Quick Identification of Four Purlin Types

Detailed Characteristics of Each Type

1. C-section Steel (Most Common)

Characteristics: Symmetrical cross-section; simple to process and install. Application: Simply supported purlins. According to National Standard Atlas 11G521-1, the applicable span for C-section purlins ranges from 4 to 9 meters, with an economical span of 4 to 6 meters; specific dimensions are determined by load calculations. Selection Points: Height (100–300mm), flange width (40–80mm), lip/stiffener (10–20mm), and thickness (1.5–3.0mm). Load-bearing capacity depends primarily on the cross-section height.

2. Z-section Steel (Suitable for Long-span Continuous Purlins)

Characteristics: Top and bottom flanges face opposite directions; lapping allows for the formation of continuous purlins, resulting in superior structural performance. Application: Continuous roof purlins; spans of 6–9 meters; commonly used in industrial factory buildings. Selection Points: Lap length should be no less than 10% of the span (per GB 51022-2015) and no less than 600mm. Bolted connections at the Z-section overlap must be secure.

3. U-shaped Steel (Auxiliary Component)

Characteristics: Channel-shaped cross-section, no edge lips, relatively low strength. Applications: Door/window frames, edge trim, struts, connectors, equipment suspension. Selection notes: Generally not used as primary load-bearing purlins; ensure proper anti-corrosion treatment.

4. "几"-shaped Steel (Specialized for Secondary Purlins)

Characteristics: Cross-section resembles the Chinese character "几" (inverted U-shape with flanges); high rigidity; usually installed perpendicular to primary purlins. Applications: Roof secondary purlins, PV panel supports, roof panel supports. Cannot replace primary purlins for supporting heavy roof panel loads. Selection notes: Common specifications include 41×41 and 52×52; thickness 1.5–2.5 mm; pay attention to the connection method with primary purlins.

Selection Summary

For roof primary purlins, prioritize Z-shaped steel (for long continuous spans) or C-shaped steel (for small-to-medium simply supported spans); use C-shaped steel for wall purlins; use U-shaped steel for door/window openings and edge trim; use "Ji"-shaped steel for PV panels and secondary supports. Keep three principles in mind: span determines section depth, load determines material thickness, and environment determines the anti-corrosion grade. When in doubt, consult a professional structural engineer.

For more information needed or any inquiry,please feel free to contact Yumisteel team.