Secondary Framing
Z-Section Purlins for Metal Building Roofs
Definition: A Z-purlin is a cold-formed steel roof member with a Z-shaped profile that spans between rigid frames and supports the roof panels. Because Z-sections can nest and lap over frames, they form continuous beams that are light, efficient and easy to ship.
Key facts
- Z-sections can be lapped over supports to create continuous multi-span beams.
- Typical spacing is commonly between about 1.2 and 1.8 m, set by the roof panel and loads.
- Depth and thickness are selected by design to suit bay spacing and loads.
- Usually supplied pre-galvanized or with a primer finish.
- Sag rods or bridging control rotation and buckling of the purlin.
- Roof panels fastened to purlins can contribute to their stability.
Role in the structure
Roof panels deliver gravity, snow and wind uplift loads to the purlins, which span from frame to frame and pass the loads to the rafters. Purlins also restrain the rafter top flange, provide anchor points for flange braces, and act as struts in the roof bracing system, carrying longitudinal forces between braced bays.
Why are Z-purlins preferred over C-purlins on roofs?
A Z-section has flanges pointing in opposite directions, so two purlins of the same depth can nest inside each other. Lapping them over a frame doubles the section where the negative moment is greatest and makes the purlin line continuous.
Continuity reduces both moments and deflections compared with simple spans, which lets lighter purlins carry the same load. C-sections cannot nest in the same way and are more often used for simple spans, endwalls or framed openings.
How are Z-purlins braced?
Under load a Z-purlin tends to rotate and move sideways because its principal axes are inclined. Through-fastened roof panels resist much of this movement, while sag rods, bridging or anti-roll anchors hold the purlin line in position and transfer accumulated forces to the frames or eave.
Standing seam roofs with floating clips provide less restraint, so the purlin design and bracing differ. The roof system must therefore be decided before purlins are designed.
What loads govern purlin design?
Gravity loads include roofing, insulation, collateral services, roof live load and snow, including drifts next to parapets or roof steps. Wind uplift is often critical, especially at edges and corners, where the free flange of the purlin is in compression.
Point loads from solar panels, hung services or equipment need to be declared. The design method for cold-formed members, including web crippling at supports and distortional buckling, follows the governing cold-formed steel standard.
How are purlins connected to the frames?
Purlins are bolted to purlin clips welded on the rafter top flange, or directly through the flange. At laps, bolts through both purlins create the continuous connection.
Over interior frames, stiffeners or support angles may be used to prevent web crippling under heavy loads. All bolts and laps are shown on the roof framing plan.
Materials & coatings
- Cold-formed structural steel strip, pre-galvanized or zinc-aluminum coated, grade selected by design.
- Uncoated steel with shop primer for dry interior environments.
- Coated steel to sheet standards such as ASTM A653, AS 1397 or EN 10346 families.
- Zinc-plated or galvanized lap and clip bolts.
Design considerations
- Bay spacing, purlin spacing and lap length.
- Gravity, snow drift and wind uplift loads, including edge zones.
- Roof system restraint: through-fastened or standing seam.
- Bracing layout and anchorage of sag rods or bridging.
- Deflection limits for roofing, ceilings and drainage.
- Strut forces from the roof bracing system.
- Point loads from solar, services and equipment.
Fabrication
- Coil is roll-formed to profile and cut to length.
- Holes for laps, clips and sag rods are punched in line.
- Members are bundled by roof zone and labeled to the framing plan.
- Coating condition and profile dimensions are checked during production.
Erection
- Install purlins starting at the braced bay.
- Bolt all laps and clips before placing roof panels.
- Fix sag rods or bridging to keep lines straight and spaced correctly.
- Avoid stacking heavy panel bundles on unbraced purlins.
Inspection & quality
- Check purlin size, thickness and lap lengths against drawings.
- Verify that all lap bolts and clips are installed.
- Confirm straightness and spacing before roofing.
- Inspect coating damage at cuts and repair as required.
What to include in your RFQ
- Roof system type and panel profile.
- Collateral loads and any solar or equipment loads.
- Site snow and wind data.
- Coating type for the environment.
- Ceiling or liner requirements affecting deflection.
3D model & IFC of this component
Rotate the model, isolate the component and download its IFC4 file for your BIM coordination model.
IFC entity: IfcMember (PURLIN). Illustrative geometry for coordination and learning. Final member sizes come from the project design.
Frequently Asked Questions
What is purlin lapping?
Lapping is overlapping two Z-purlins over a frame and bolting them together. It makes the purlin continuous across the support and doubles the section where moment is highest.
Can I hang services from purlins?
Light services can be hung if included in the collateral load. Concentrated loads should be checked by the engineer and may require spreader members.
Why are some purlins thicker than others?
End bays, edge zones and drift areas see higher loads, so the designer uses thicker or deeper purlins there and lighter ones elsewhere.
Engineering content is general guidance. Every World Steel building is designed by qualified engineers to the code and loads of the project.
