What Is a Purlin? Types, Sizes and Applications

A purlin is a horizontal structural member that supports the roof covering (or wall cladding) and transfers its load to the main building frame. In steel buildings, purlins are almost always cold-formed C or Z sections roll formed from galvanized steel coil. They are spaced along the roof slope, usually 1-1.5 m apart, and span between rafters or portal frames. Purlins are also used horizontally on walls, where they support cladding sheets and are sometimes called girts.

C Purlins vs Z Purlins

C purlins: C-shaped sections with lipped edges. They are simple to connect (bolted through the web), need no overlap for splicing, and are ideal for short spans (up to about 6-8 m) and simple roofs.
Z purlins: Z-shaped sections that nest and overlap at the joints. The overlap creates a stronger continuous member, so Z purlins suit longer spans, higher loads and larger buildings; they are the standard choice for portal frame structures in many markets.

See C vs Z Purlin: Key Differences for a full comparison of the two sections.

Common Purlin Sizes

Purlin depth is the main dimension and follows the load and span. Standard sizes range from 75 mm to 350 mm depth:

Depth	Typical use	Common thickness
75-100 mm	Small spans, canopies, mezzanines	1.5-2.0 mm
100-150 mm	Warehouse and factory roofs	1.6-2.5 mm
150-200 mm	Longer spans, heavier roofs	2.0-3.0 mm
200-350 mm	Large industrial buildings	2.5-3.5 mm

Flange width is typically 40-80 mm depending on the system, and standard purlin lengths are cut to order (commonly up to 12 m, or double-lengths for Z overlap).

How Purlins Are Made

Purlins are roll formed from galvanized coil on a purlin roll forming machine. The line forms the C or Z section progressively through rollers, punches the bolt/splice holes in line, and cuts each purlin to length automatically. Punching patterns follow the purlin manufacturer's or engineer's fixing schedule — a CNC punching line gives full flexibility for custom hole patterns. Typical production speeds are 8-25 m/min depending on thickness.

Purlin Applications

Industrial buildings: warehouse and factory roofs and walls.
Agricultural buildings: poultry and livestock sheds, storage barns.
Commercial buildings: shopping centers, showrooms, offices with steel frames.
Canopies and mezzanines: light C purlins for small spans.
Solar mounting: purlins carry solar panel mounting structures on industrial roofs (see our solar channel guide).

Installation and Fixing

Purlins are fixed to the frame with bolts or cleats; Z purlins are lapped and bolted through both webs at the overlap, and roof sheets are screwed directly to the purlin flange. Correct spacing, lapping and bolt patterns follow the engineer's design — our C & Z purlin installation guide covers spacing and fixing in detail.

FAQ

What is a purlin used for?

A purlin supports the roof or wall cladding and transfers its load to the main building frame; purlins run horizontally along the roof slope.

What is the difference between C and Z purlins?

C purlins are simple, non-overlapping sections for short spans; Z purlins overlap and splice into a continuous member for longer spans and heavier loads.

What sizes do purlins come in?

Common depths are 75-350 mm with thickness 1.5-3.5 mm, cut to any length per the building design.

How are purlins manufactured?

They are roll formed from galvanized coil with in-line hole punching and automatic cut-to-length on a purlin machine.

Can purlins be used for solar mounting?

Yes — purlins and strut channels carry solar mounting structures on industrial roofs; solar channel sections are a related product family.

What purlin machine do I need?

It depends on the section depth and thickness range, hole pattern and speed — send your section drawings for a quotation.

Get a Quote

Contact ZTRFM with your purlin section drawings for a quotation.

What Is a Purlin? Types, Sizes and Applications