What Is a Purlin? C Purlin vs Z Purlin for Industrial Roofing

A purlin is a horizontal secondary structural member that transfers roof and wall loads from the cladding (roofing sheets or wall panels) to the primary structural frame (rafters, trusses, or columns). In metal building construction, purlins are typically cold-formed from high-tensile steel (550 MPa yield strength) into C or Z profiles.

Why Purlins Matter

Purlins serve three critical functions:

Load transfer: They carry the weight of roofing material, insulation, and imposed loads (wind, snow, maintenance traffic) to the main frame
Roof sheet support: They provide the continuous bearing surface for roof sheets at specified intervals
Stability: They brace the compression flange of the main rafters, preventing buckling under load

A purlin failure can lead to roof collapse — proper specification is essential for structural safety.

C Purlin vs Z Purlin: Which to Choose

Factor	C Purlin	Z Purlin
Shape	Channel (C-shaped) with lips	Zee (Z-shaped) with lips
Max span (simple)	Up to 6m	Up to 6m
Max span (continuous)	Not used for continuous spans	Up to 10m
Connection	Bolts directly to rafter flange	Overlaps and nests at supports
Material efficiency	Less efficient for long spans	15-20% lighter for same span
Best for	Small sheds, canopies, wall girts	Large industrial roofs, warehouses

Standard Purlin Sizes in India

Designation	Depth mm	Flange mm	Lip mm	Thickness mm	Max Span
C100x50x15x2.0	100	50	15	2.0	6m
C125x50x15x2.5	125	50	15	2.5	8m
C150x65x20x2.5	150	65	20	2.5	10m
Z150x65x20x2.0	150	65	20	2.0	8m
Z150x65x20x2.5	150	65	20	2.5	10m
Z200x75x20x3.0	200	75	20	3.0	12m

Purlin Spacing Guidelines

The spacing between purlins is determined by the roof sheet’s load capacity:

0.35mm sheets: 1.2m maximum spacing
0.47mm sheets: 1.5m maximum spacing
0.55mm sheets: 1.8m maximum spacing
0.60mm sheets: 2.0m maximum spacing

According to JSW Steel’s structural design manual, improper purlin spacing accounts for approximately 30% of premature roof sheet failures in industrial buildings, typically caused by sheet deflection exceeding allowable limits under rain loading.

Technical Frequently Asked Questions

What is a purlin in construction?

A purlin is a horizontal structural member that runs across the main building frames (rafters or trusses) to support the roof sheeting or wall cladding. Purlins transfer the weight of the roof and any imposed loads (wind, snow, maintenance) to the main structural frame. They are typically made from cold-formed steel sections, most commonly C-shaped or Z-shaped profiles.

What is the difference between C purlin and Z purlin?

C purlins (channel-shaped) are used for simple-span applications up to 6 meters and are typically bolted directly to the rafter flange. Z purlins (zee-shaped) are designed for continuous-span applications up to 10 meters — they overlap and nest at the supports, creating structural continuity that allows longer spans. For most industrial sheds in India, Z purlins are preferred for roof spans above 6 meters.

What size purlin do I need for my shed?

For a standard industrial shed with 1.5m purlin spacing and 0.47mm roofing sheets, use C100x50x15x2.0mm for up to 6m bay spacing, or Z150x65x20x2.5mm for up to 10m bay spacing. The required size depends on roof span, sheet profile, wind uplift loads, and snow loads. Always consult the manufacturer's load span table for your specific configuration.

How far apart should purlins be spaced?

For 0.47mm BMT roofing sheets, purlins should be spaced at 1.5m maximum on center. For 0.55mm sheets, spacing can extend to 1.8m. Wider spacing risks sheet deflection, oil-canning, and failure under wind uplift or rain loading. The purlin spacing also affects the required purlin section size — wider spacing needs deeper purlins.