Purlins

What purlins do

Purlins are the horizontal secondary members of the roof. They span between the rafters of adjacent frames, support the roof sheeting or standing seam panels, and transfer roof loads to the primary frame. They also do several jobs that are less obvious:

• restrain the rafter top flange and, through flange braces, the inner flange;
• act as struts in the roof bracing system;
• carry collateral loads such as sprinklers, lighting and ducts.

In pre-engineered buildings purlins are almost always cold-formed Z or C sections roll-formed from galvanised or bare high-strength coil.

Purlins in the load path

Roof sheeting carries dead, live, snow and wind load to the purlins at each fastener line. Purlins span 6–9 m (up to about 12 m with deep sections) between frames and deliver their reactions through cleats bolted to the rafter. Under wind uplift the load reverses, and the unrestrained bottom flange of the purlin is in compression – usually the governing case in tropical and cyclonic regions.

Z purlins can be nested and lapped over the frames, making them continuous across several spans. Continuity lowers midspan moments and deflection, and the double thickness at the lap is placed where the support moment is largest.

Typical sizes and materials

Purlin spacing is usually 1.2–1.8 m, set by the sheeting span capacity and wind zone; it is reduced near eaves, ridges and corners where wind suction is highest. More detail on materials is in cold-formed steel for purlins and girts.

Z versus C sections, laps and special purlins

• Laps: lap length is set by design – longer laps carry more support moment – and the lap is bolted at the support and at both lap ends. Typical total laps are in the order of 0.6–1.8 m depending on span and load.
• End bays carry higher moments because they lack continuity on one side; heavier thickness or longer laps are used there.
• Ridge purlins are placed a short distance either side of the ridge to support the ridge cap and any ridge ventilator.
• Purlin extensions cantilever beyond the endwall to support a gable overhang; see roof extensions.
• Strut purlins are purlins (often doubled or heavier) at braced bays that also carry axial force from the roof bracing.

Design considerations

• Restraint from sheeting: through-fastened sheeting restrains the top flange; AISI S100 gives an R-factor method for uplift, and standing seam roofs are evaluated by the base test method in AISI S908. EN 1993-1-3 Section 10 and AS/NZS 4600:2018 cover the equivalent cases.
• Sag rods or bridging reduce the unbraced length of the free flange under uplift; see sag rods and bridging.
• Lap length and bolt pattern must match the design assumption of continuity.
• Anti-roll restraint: on steeper slopes Z purlins tend to roll downslope and need restraint at the ridge or anti-sag systems.
• Deflection limits under wind and gravity, typically span/150 to span/240.
• Point and hanging loads must be applied at the shear centre or via approved clips.

Fabrication, coating and installation

World Steel roll-forms Z and C purlins on CNC roll-forming lines that punch holes and cut to length in one pass from coil, so every purlin matches the 3D model (see cold roll forming). Pre-galvanised coil is the normal finish; hot-dip galvanising after forming or painted finishes are available for aggressive environments.

On site, purlins are bolted to rafter cleats with M12 or M16 bolts, laps are bolted as detailed, and sag rods or bridging are installed before sheeting so the roof stays straight. Purlins should be laid with the correct flange direction – Z purlins usually face upslope as shown on the erection drawings.

Common mistakes and quality checks

• Lap bolts omitted, turning a continuous design into simple spans.
• Purlins installed in the wrong flange direction, reversing the designed roll behaviour.
• Walking on unbraced purlins – fall and stability hazard.
• Wrong thickness used in end bays, where the design often uses heavier sections.
• Unauthorised hanging of services from the bottom flange.