Steel Purlins in Construction: Types, Sizes, and Key Benefits

As one type of steel structure in buildings, steel purlins are widely used on roofs and walls. Although they are secondary structural members, steel purlins in construction also play an important role.

What Is a Steel Purlin?

A steel purlin is a horizontal structural beam used in steel construction to support roof and wall panels. They help distribute loads, prevent sagging, and provide resistance against wind, snow, and other external forces. Installed perpendicular to main rafters or beams, purlins are critical for the stability and safety of large-span metal roofs.

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Why Use Steel Purlins in Construction?

Steel purlins are often referred to as the “backbone of the roof.” They provide more than just support:

• Even Load Distribution—Spread roof weight evenly across the structure.

• Enhanced Stability—Act as a skeleton for the roof, improving durability.

• Cost Savings – Optimize steel usage and reduce material costs.

• Fast Installation—Prefabricated design makes them quick to install.

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Types of Steel Purlins

Steel purlins are categorized by cross-sectional shape. The two most common are:

C-Purlins

• Best for small to medium roofs

• Simple installation

• Limited for long spans

Z-Purlins

• Best for large-span roofs & high-wind areas

• Overlapping connections create stronger joints

• Superior load distribution

Comparison Table: C-Purlins vs Z-Purlins

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Common Sizes of Steel Purlins

Steel purlins are available in different sizes depending on the project:

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Spacing and Span of Steel Purlins

• Purlin Spacing – Distance between adjacent purlins; smaller spacing = stronger stability.

• Purlin Span – Length a purlin covers between supports; affects thickness and size requirements.

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Common Structural Layouts

• Single-Span – Simple, short-span projects.

• Continuous-Span – Large roofs, efficient load distribution.

• Overlapping Layout – Z-purlins for stronger joints.

• Cantilever & Double-Span – For extended or complex designs.

• Curved Layout – Used in stadiums or special designs.

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Connection Methods for Steel Purlins

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Seven Key Advantages of Steel Purlins

1. High strength & durability

2. Lightweight & easy to install

3. Versatile for all building types

4. Excellent fire resistance

5. Cost efficiency & long service life

6. Corrosion resistance in harsh environments

7. 100% recyclable & eco-friendly

Steel Beams vs. Steel Purlins

Feature	Steel Beams	Steel Purlins
Main Role	Carry major structural loads	Support roof and wall panels
Typical Position	Floors, roofs, frames, bridges	Between main rafters or columns
Load	Carries heavy primary loads and transfers them to columns/supports	Carries roof/wall loads and transfers them to main beams or rafters
Structural Level	Primary structural member	Secondary structural member
Common Shapes	I, H, T, Box, C	C or Z
Typical Span	Medium to long spans	Short to medium spans between main frames
Typical Applications	Buildings, bridges, warehouses, stadiums	Metal roofs, warehouses, factories, wall systems
Installation	Usually connected to columns, foundations, or other main members	Usually fixed to rafters or main beams
Main Benefit	High load-bearing capacity	Lightweight, economical, and fast to install

Steel beams = the main skeleton of the building.
Steel purlins = the secondary support system for the roof and walls.

Learn more about steel beams in construction: types, Uses

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