Strip the sheeting off a pre-engineered building and most of what remains is purlins. They are the cold-formed steel members that span from rafter to rafter, parallel to the eave, and carry the roof. Girts do the same job on the walls. Together they outnumber every other member type in the building.
Calling them secondary framing undersells the role. Purlins collect every load the roof sheeting sees and deliver it to the rafters. They restrain the rafter flanges that the frame design assumes are held. A good purlin scheme saves real tonnage; a poor one shows up as a wavy roofline and sheeting fixings that work loose.
This guide covers what purlins and girts do, the C and Z sections they are rolled from, why lapped Z-purlins save steel, and the checks that size the section. The governing check is usually wind uplift, not gravity.
What purlins and girts actually do
A purlin is a cold-formed steel section, rolled from galvanised coil typically 1.5 to 2.5 mm thick, spanning between the primary frames. Roof purlins sit on the rafters at a spacing set by the sheeting's spanning capacity, commonly 1.2 to 1.8 m for metal roof cladding. Girts are the same sections fixed across the columns to carry wall sheeting.
They do three jobs at once:
- Carry the cladding loads. Dead load, collateral load from services and insulation, live load and wind all arrive through the sheeting and travel through the purlins into the rafters.
- Restrain the primary frames. Flange braces from purlin to rafter hold the rafter's inner flange, and the frame design counts on that restraint being there.
- Distribute longitudinal loads. Wind on the gable ends travels along the purlin lines and roof bracing back to the braced bays and down to the foundations.
C-sections and Z-sections
Nearly every PEB purlin is one of two profiles. The C-section is the familiar channel, both flanges on the same side of the web. The Z-section runs its flanges in opposite directions, and the two flanges are rolled to slightly different widths on purpose: rotate one Z through 180 degrees and it nests inside the next.
That one geometric property decides where each section goes.
| Aspect | C-section | Z-section |
|---|---|---|
| Lapping | Cannot nest; each span designed on its own | Rotates 180° and nests, lapping into continuous lines |
| Typical use | End bays, framed openings, girts at doors | Main roof and wall runs, bay after bay |
| Capacity for weight | Set span by span | Continuity cuts design moments; laps double the section at supports |
| Transport | Sturdy shape, packs cleanly | Bundles nest tightly |
Lap continuity: where Z-purlins save steel
A single-span purlin sees its full design moment at mid-span, wL²/8, and one section size carries it the whole way. Make the line continuous over the frames and the picture changes: peak moments migrate to the supports and drop, heading towards wL²/10 in the end spans and wL²/12 internally.
Lapped Z-purlins exploit both effects at once. At every frame, two purlins overlap and bolt together, so the section is doubled exactly where the hogging moment peaks. The lap runs past the points of contraflexure, and between laps a single, lighter section carries the reduced mid-span moment. The same roof, designed as lapped continuous Z lines instead of single spans, comes out lighter — which is why PEB roofs run Z-purlins bay after bay and keep C-sections for the ends and openings.
Sag rods and lateral restraint
A purlin is stiff about its major axis and weak about its minor one. On a sloped roof, part of the load acts down-slope in the plane of the sheeting, bending the purlin about that weak axis. Sag rods deal with it: round bars threaded through the purlin webs at mid-span or third points, tying every purlin in the bay to the next and cutting the weak-axis span to a fraction of the bay.
The rods run up the slope to the ridge, where the pull from the two slopes balances. They earn their keep twice. During erection they hold the purlin lines straight before the sheeting arrives; in service they act as the restraint points the uplift check depends on.
The purlin-to-rafter connection
Purlins bolt to short cleats welded to the rafter top flange in the shop. Everything at height is bolted; no site welding. The bolt holes are punched during rolling, which is why detailing accuracy matters so much: one wrong hole pitch repeats across hundreds of identical members before anyone reaches site. At a lap, both purlins bolt through the same cleat, with additional bolts near the lap ends so the doubled section acts as one.
Wind uplift usually governs
Under gravity, the purlin's top flange is in compression and the screwed-down sheeting restrains it continuously. That is the strong case. Reverse the load and the situation degrades: under wind uplift the bottom flange goes into compression, and nothing restrains it except the sag rods or discrete braces. The capacity falls accordingly.
PEB roofs make it worse. The dead load of a metal-clad roof is small, so there is little weight to offset suction, and the net uplift on light roofs in windy regions is substantial. The uplift case, checked at the unrestrained flange's effective length, usually decides the purlin section rather than the gravity case the roof spends its life under.
Beyond uplift, thin cold-formed sections bring their own checks: local buckling of slender elements, distortional buckling of the flange-and-lip assembly, web crippling at cleats and bolt bearing in thin material — all covered by the cold-formed steel design codes rather than the hot-rolled rules. Deflection limits commonly sit around span/150 to span/180 for metal-clad roofs, with each code and cladding specification setting its own values.
None of this is hard analysis in isolation. The difficulty is scale: a mid-sized PEB carries hundreds of purlins and girts, each with a lap, a hole pattern, sag rod positions and a place in the bill of materials. Purlin layout, section checks against gravity and uplift, and the fabrication drawings should come out of the same model as the primary frames, so a change to the bay spacing reworks every purlin line automatically instead of by hand.
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FAQs
What is the difference between a purlin and a girt?
They are the same cold-formed sections doing the same job in different planes. Purlins span between rafters and carry the roof sheeting; girts span between columns and carry the wall sheeting. The design checks and connections are near-identical, with girts seeing wind pressure and suction rather than roof loads.
Why are Z-purlins used instead of C-purlins on PEB roofs?
A Z-section's flanges are rolled to slightly different widths, so one purlin rotated 180 degrees nests into the next. That allows lapped, continuous lines over the frames: design moments drop below the single-span wL²/8, and the section is doubled at the supports where the hogging moment peaks. C-sections cannot nest, so they are kept for end bays, framed openings and girts.
What do sag rods do in a purlin system?
They tie the purlins in a bay together at mid-span or third points, cutting the weak-axis span for the down-slope component of roof load. They also hold the purlin lines straight during erection, and in service they provide the restraint points the wind-uplift check relies on.
Why does wind uplift govern purlin design?
The sheeting restrains only the outer flange. Under uplift the inner flange goes into compression with no continuous restraint, so capacity drops to what the sag-rod or brace spacing can deliver. Combined with the low dead weight of a metal-clad roof, the uplift combination usually sizes the section.
