Sag rods and bridging

Definition and function

Sag rods are small-diameter threaded rods, and bridging is short cold-formed or angle members, installed between adjacent purlins or girts within a span. They have three purposes:

• to hold purlins and girts straight and at the correct spacing during erection and sheeting;
• to support girts and sloping purlins against their weak-axis load – the self-weight of wall cladding and the downslope component of roof load;
• to restrain the free flange under wind uplift or suction, reducing the unbraced length and increasing capacity.

Role in the load path

Cold-formed Z and C sections are efficient about their strong axis but weak and prone to twisting about their minor axis. In walls, sag rods transfer the weight of the cladding from each girt up to the eave strut and into the frame. On roofs, rods or bridging collect the downslope force from each purlin and anchor it at the ridge (with tie rods across the ridge) or at the eave strut. Under uplift, the restraint provided at rod or bridging lines reduces lateral-torsional buckling of the bottom flange.

Typical sizes and layouts

On walls, sag rods normally run from the lowest girt up to the eave strut in each bay, with diagonal rods at the top of the wall where the eave strut cannot carry the cumulative load alone. On roofs with a single slope, the full downslope force is anchored at the high or low eave rather than balanced across a ridge.

Some systems use proprietary bridging clips that bolt between purlins and provide both tension and compression restraint; others combine rod sag lines with angle anti-roll braces at eaves and ridge.

Design considerations

• Number and position of restraint lines are part of the purlin design: AISI S100, AS/NZS 4600:2018 and EN 1993-1-3 capacity tables assume particular restraint layouts.
• Downslope forces accumulate along a roof slope; the anchorage at ridge or eave must be sized for the total.
• Steeper roofs and Z sections rolling downslope need stronger restraint than shallow roofs.
• Girts in tall walls carry significant cumulative cladding weight to the top rod.
• Standing seam roofs give less lateral restraint to purlins than through-fastened sheeting, so restraint lines are more critical (see standing seam roof systems).

Fabrication and installation

World Steel punches sag rod holes in purlins and girts during roll forming, cuts and threads rods to length, and bundles them by bay. On site:

1. Install purlins or girts on their cleats.
2. Insert sag rods or bridging and tighten nuts to bring members to line and spacing – not so tight that they bow.
3. Check purlin spacing with a gauge before fastening sheets.
4. Complete ridge ties and eave anchors.

Sag rods help create a safe, stable roof structure during erection, but they do not make purlins a safe walking surface; follow site safety rules.

Common mistakes and quality checks

• Sag rods left loose, so girts sag and wall sheets show waves.
• Over-tightened rods bowing purlins.
• Missing ridge ties, allowing purlins to roll downslope.
• Rods installed only in some bays.