Secondary Framing
Sag Rods and Bridging for Purlins and Girts
Definition: Sag rods and bridging are light bracing elements fixed between purlins or girts to keep them straight, correctly spaced and restrained against rotation. They control sag during erection and help secondary members resist the forces caused by gravity and wind loads.
Key facts
- Sag rods are commonly threaded steel rods; bridging may be angles, channels or straps.
- Lines of bridging are placed at intervals along the purlin span.
- Restraint forces must be anchored at the eave, ridge or frames.
- They help maintain purlin spacing during roof installation.
- Girt sag rods support wall girt weight before cladding.
Role in the structure
Cold-formed Z and C sections twist and move laterally under load. Sag rods, angles or straps connect adjacent members so the restraint provided by the roof or wall panels and the anchors at the eave or ridge is shared along the line. Accumulated forces are carried to the frames, eave strut or ridge, where they are anchored.
Why do purlins need sag rods or bridging?
Z-purlins have inclined principal axes, so gravity load makes them move downslope and twist. Wind uplift causes the free flange to buckle sideways. Bridging lines reduce these effects and help the purlins reach their design capacity.
During erection, before the roof panels are fixed, bridging also prevents purlins from rolling and keeps them at the right spacing.
How are restraint forces anchored?
Forces along a bridging line accumulate from purlin to purlin. They are anchored at the eave strut, at the ridge where opposite sides balance, or with diagonal anchors to the frames.
Anchorage details and their capacities are part of the secondary framing design, especially for standing seam roofs, where the panel provides less restraint.
Are sag rods used in walls?
Yes. In walls, sag rods or straps support the self-weight of girts and wall panels, reducing deflection about the weak axis.
They are anchored at the eave strut and often run down to the base girt or to the lowest girt, depending on design.
What problems do missing sag rods cause?
Without bridging, purlins can roll during roof installation, spacing drifts, and panel fasteners miss the flanges. Under service loads, unrestrained purlins may deflect or twist more than designed, which shows up as wavy roof lines or leaking laps.
For this reason, bridging lines are treated as part of the structural design and checked during erection like any other member.
Materials & coatings
- Threaded steel rods with nuts and washers, galvanized or zinc plated.
- Cold-formed angles or channels for rigid bridging.
- Galvanized steel straps for light bracing.
Design considerations
- Number of bridging lines per purlin span.
- Anchorage of accumulated forces.
- Roof system restraint: through-fastened or standing seam.
- Wind uplift and gravity cases.
- Erection stability before panels are installed.
Fabrication
- Cut rods to length and thread ends.
- Cut and punch bridging angles or channels.
- Punch matching holes in purlins and girts.
Erection
- Install bridging at the same time as purlins.
- Tighten rods to keep purlins straight, without bowing them.
- Fix anchors at eave or ridge before roofing.
Inspection & quality
- Verify bridging line locations and count.
- Check rod tightness and nut engagement.
- Confirm purlin spacing and alignment.
What to include in your RFQ
- Roof system type.
- Wind and snow data.
- Any ceiling or service attachments to purlins.
- Coating finish for bridging components.
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. Illustrative geometry for coordination and learning. Final member sizes come from the project design.
Frequently Asked Questions
Can I omit sag rods if the roof panels are screwed down?
Only if the design allows it. Through-fastened panels provide restraint, but bridging may still be needed for uplift or during erection.
Do standing seam roofs need more bridging?
Often yes, because clip-fixed panels provide less lateral restraint. The engineer determines the requirement.
How tight should sag rods be?
Snug enough to keep members straight and spaced. Over-tightening can bow the purlins.
Engineering content is general guidance. Every World Steel building is designed by qualified engineers to the code and loads of the project.
