C Shaped Steel and Z Shaped Steel Purlin Selection Guide for Steel Buildings

Purlins are the least glamorous members in a steel building and among the most influential. They support the cladding, transfer wind and snow loads into the frame, and provide the lateral restraint that lets slender rafters reach their full capacity. Specifying C shaped steel and Z shaped steel correctly is a low-cost decision with a high structural payoff.

Because purlins are cold-formed from coil, they are also flexible products: depth, flange width, lip size and thickness can all be adjusted within a single mill set-up. That flexibility is useful, but it means the buyer must state the full section designation - otherwise quotations are not comparable.

C Shaped Steel Versus Z Shaped Steel

Both sections are cold-formed from galvanized strip, and both are produced in similar depth ranges, typically 100-300 mm with 1.5-3.0 mm thickness. The difference is structural behaviour at the supports.

How to Read a Purlin Designation

A purlin is defined by four numbers: depth, flange width, lip (curl) depth and material thickness. A typical order reads C200 x 70 x 20 x 2.5 or Z250 x 75 x 20 x 2.5. Always quote all four, plus the steel grade and the zinc coating mass - for example Q345 with Z275 galvanizing.

• Depth: the main driver of stiffness; increasing depth is usually more efficient than increasing thickness.

• Flange width: affects fixing surface for cladding screws and lateral stability.

• Lip size: stiffens the free edge of the flange and improves load capacity.

• Thickness: governs local bearing and durability; 2.0-2.5 mm is typical for industrial roofs.

• Coating: Z275 (275 g/m2) hot-dip galvanizing is the standard for external purlins.

Spacing Rules and Their Interaction with Cladding

Purlin spacing is not chosen in isolation - it is dictated by the cladding. Single-skin color steel tile typically needs 1.0-1.5 m spacing, while insulated sandwich panels can span 1.5-2.5 m or more. Because purlin count drives both material and labour, matching the cladding choice to the purlin layout is one of the easiest ways to optimise a building's cost.

Lapping, Bracing and Accessories

Z purlins achieve continuity by overlapping at each support, usually by a length equal to about 10% of the span or a fixed 500-900 mm, bolted through both members. The lap must be oriented so that the sections nest correctly - a surprisingly common site error. Anti-sag rods, bracing straps and spacers complete the system and should be included in the material schedule, not improvised on site.

• Anti-sag rods: reduce mid-span sag in steep or long-slope roofs.

• Bracing / stay bars: provide lateral restraint to the rafter flange and resist wind suction.

• Cleats and bolts: galvanized to match the purlin; specify grade and diameter.

• Sealing washers: prevent water ingress at every cladding fixing point.

Typical Sizing Guide

The following is an indicative starting point only - final sizes must always come from a structural calculation using the project's actual wind and snow loads.

Frequently Asked Questions