Purlins and girts are the quiet backbones of steel and timber frames, yet most owners never notice them until something leaks or sags. Knowing which is which, and why each matters, saves money before the first beam is bolted.
A purlin bridges the gap between main rafters, giving roof sheets a place to land. A girt does the same job on walls, turning open posts into a stiff surface for siding. Both are light, long, and easy to overlook, but they solve different loads in different planes.
Core Structural Roles
Roof Load Path
Purlins collect every kilo of roof dead load, live load, and wind uplift, then hand that bundle to the rafters. Without them, sheets would belly between trusses and fasteners would pop within a season.
They also act as lateral braces, stopping rafters from rolling sideways when wind hits the ridge. This dual duty is why purlin spacing follows both bending limits and roof-skin flutter tests.
Wall Stability Path
Girts pick up the negative pressure that wind applies to wall cladding and ferry it back to columns. They double as attachment rails for siding, so their depth must match both structural need and the siding profile’s screw gauge.
Because girts sit vertically, they fight column twist while letting the wall breathe for condensation control. That twist control is why girts are often lapped at corners while purlins are butted at rafter peaks.
Orientation and Placement
Roof Plane Layout
Purlins run parallel to the ridge, seated on top of rafters or hung between them with cleats. Their top flange stays flush so roof sheets lie flat, preventing oil-canning that shows up as shiny ripples on sunny days.
Wall Plane Layout
Girts bolt to column faces at set heights, creating horizontal rails that look like ladder rungs from inside. They stay back from the outer column edge just enough to let siding overlap for weather tightness.
Load Direction Differences
Gravity vs Wind Uplift
Purlins feel gravity between rafters and wind uplift between fasteners. They need checks for both downward bending and upward suction, which is why bridging or sag rods appear at mid-span.
In-Plane vs Out-of-Plane
Girts fight wind pushing or pulling the wall skin perpendicular to the column. They rarely feel gravity except their own weight, so their weak axis can be smaller if siding is light.
Section Choices
C-Shapes and Zeds
Cee sections nest for cheap freight and bolt easy to rafter webs, making them the default for short-span purlins. Zeds lap over each other for continuity, turning two short pieces into one long beam without extra steel.
Timber and Hollow Sections
Timber purlins suit cathedrals where warmth matters; they notch onto rafters and accept screw-down roofing with washered screws. Rectangular hollow sections hide inside parapets where looks count, but they cost more and need welded cleats.
Connection Hardware
Cleats, Bolts, and Screws
Angle cleats give purlins four-sided support, stopping rotation when the roof acts like a wing. Two M12 bolts each end are standard; fewer bolts let the purlin twist under gusts.
Web Stiffeners
When purlin depth passes 200 mm, a square plate welded between top and bottom flanges stops the web crumpling like a soda can. The same plate doubles as a point for safety harness anchors during maintenance.
Spacing Guidelines
Roof Module Thinking
Sheet lengths come in 0.9 m increments, so purlins land at 0.9 m, 1.2 m, or 1.8 m centres without cutting panels. Keeping the same spacing on every bay lets crews swap sheets without re-measuring.
Wall Cladding Rhythm
Girt spacing matches the siding profile’s strength, not the column height. Corrugated sheets handle 1.5 m spans; composite panels need 0.6 m rails to stay flat.
Bracing Interaction
Sag Rods and Fly Braces
A rod beneath every third purlin stops them from rolling sideways under foot traffic. Fly braces from rafter to purlin web do the same job in windy bays without cluttering the ceiling.
Wall Wind Posts
Wide warehouse doors delete columns, so a girt becomes a beam carrying wind to the next post. A wind post planted beneath that girt restores the load path without thickening the wall.
Thermal and Moisture Effects
Roof Condense Control
Steel purlins create cold stripes that drip moisture onto ceilings below. A thermal spacer taped under the top flange breaks the bridge and keeps the roof sheet temperature even.
Wall Breathe Space
Girts set on packers leave a 10 mm vent gap behind vinyl siding, letting damp air escape without buckling boards. The gap also hides the girt shadow line for a cleaner facade.
Fire Rating Influence
Intumescent Clips
Where purlins support a firewall, steel clips with intumescent coating swell in a fire to hold the sheet in place. Standard clips lose strength fast, dropping the sheet and breaching the barrier.
Girt Cavity Barriers
Mineral wool stuffed between girts stops flame racing up the wall cavity. The wool sits tight to the girt flange so installers can still screw siding anywhere along the rail.
Installation Sequencing
Roof Build Order
Erect rafters first, drop purlins loosely, then square the frame by pulling diagonals before final bolting. This order lets crews walk the purlins like planks while staying inside the fall-risk zone.
Wall Build Order
Set bottom girt at finished-floor plus siding height, use it as a datum, then work upward. The bottom rail becomes a scaffold ledger, saving extra staging.
Cost Levers
Steel Grade Swap
Switching from G450 to G550 steel lets a thinner purlin carry the same load, shaving kilos and freight. The trade-off is tighter handling to avoid dents during lifting.
Lap vs Sleeve
A lapped Z-purlin uses 20 % more steel but needs half the bolts, cutting labour on tall sheds. On short spans, a sleeve joint saves steel and still passes uplift checks.
Common Site Mistakes
Upside-Down Zeds
A Z-purlin installed lips-down acts like a gutter, trapping water that rusts the web from inside. The ridge overlap must face uphill so laps drain.
Missing Girt Packers
Skipping packers on cold-formed girts pulls column flanges inward, making bolt holes oval and siding wavy. A 6 mm washer plate costs pennies compared to re-drilling.
Maintenance Access
Walkability Limits
Roof profiles rated for foot traffic still need purlin spacing under 1.2 m to stop the sheet dimpling. Mark safe paths with tape on the sheet underside so cleaners stay on the strong spots.
Wall Girt Mounts
Horizontal lifeline anchors bolt through girts, not siding, so the load ties back to the column. Anchors placed mid-span bend the girt and tear screws loose within months.
Retrofit Upgrades
Solar Panel Add-On
Old purlins sized for roof sheets alone may sag under solar rails. A sister Zed bolted alongside doubles stiffness without removing the roof.
Extra Wall Height
Raising a wall means splicing longer girts to existing ones with sleeve plates. The splice sits next to a column so the weak joint lands at near-zero moment.
Quick Field Checklist
Before Sheeting
Verify purlin laps face uphill, bridging rods are tight, and every bolt has a washer. Check girt line with a string line; a 5 mm bow now shows as a 20 mm wave in siding.
After Sheeting
Walk the roof edge and look for shiny screw heads that missed the purlin flange. On walls, tug random siding screws; a spinning screw missed the girt and will leak next storm.