PEB Purlins and Girts: Functions, Layout and Detailing Considerations

Purlins and girts are the PEB’s repeating steel. Roof purlins carry sheeting and walk on the rafters. Wall girts carry wall sheeting and walk on columns or wind posts. They look like a cladding convenience until you remember that primary rafters and columns often count on them for flange restraint. Layout is therefore a structural decision, not a drafting hatch.

Most PEB secondaries are cold-formed Z or C sections, sometimes nested or lapped over frames, sometimes simple span. Eave struts sit at the roof-wall intersection and tend to be forgotten until sheeting cannot land. Sag rods or bridging keep slender Zs from rolling. Openings, units and walkways punch holes in what was a regular module. Those interruptions need trimmers the same day they appear on the architectural plan, not after approval drawings are stamped.

This guide covers function, layout logic and detailing checks. It does not prescribe span tables or section capacities; those belong to the manufacturer data and the project design basis. Read it with portal frame components and bracing systems, because purlins often double as struts in a bracing bay.

Functions: cladding support is only the obvious half

Sheeting manufacturers care about purlin spacing because panels have allowable spans. Frame designers care about the same spacing because it is a brace-point grid. Those two constraints can fight. A wide purlin spacing that still works for sheeting may be too wide for the rafter’s compression flange. A tight spacing that makes the rafter happy may over-specify sheeting supports or clash with skylights. Resolve the fight in engineering, not in a later “cladding by others” note.

Girts have the same dual life on walls, plus a third problem: doors. A forklift door is an anti-girt. Wind still arrives at that wall. Headers, jambs and wind posts must replace the missing lines. If the girt elevation still shows continuous Zs drawn through the door, the drawing is lying.

Roof purlin layout that can be built

Start from the ridge and eave, not from a random hatch. Place the eave strut, the first purlin, then a module that respects sheeting cover, insulation, walkways and known penetrations. End bays often need a different spacing because of overhangs or wind. If purlins lap over the rafter, show the lap direction and length; lapped Zs are not the same as butted simple spans.

Orientation and nesting

Z purlins have a direction. Nested or continuously lapped systems need a stated orientation so the shop punches the correct flanges. A drawing that shows a Z symbol flipped at random will be fabricated that way.

Units, walkways and solar

Rooftop equipment should land on designed steel. If a unit sits between purlins, add supports and check hanging loads. Do not let the mechanical drawing become the purlin layout.

Wall girt layout and the openings that wreck it

Girts usually run horizontally between columns, with a defined base girt or base angle and a defined eave condition. Masonry wainscot changes the lowest girt. Architectural panel systems may demand a different spacing than exposed-fastener sheeting. None of that is visible if the wall elevation is only a copy of last year’s warehouse.

Framed openings

Every opening larger than a documented typical needs jambs, a header, and a decision about whether a wind post is required. Show the remaining girt studs. A dashed architectural door is not a framed opening.

Corners and endwalls

Endwall girts may span wind columns rather than main frames. Corner details must close sheeting without assuming two girts occupy the same space. Draw the plan snippet at the corner; do not leave it to flashing.

Supports, sag rods, bridging and eave struts

Slender cold-formed purlins need help to keep their shape. Sag rods, bridging, anti-roll clips and properly detailed seats are part of the secondary system. Eave struts are the member everyone can name and then under-specify. Give them a section, a mark and a hole pattern that actually receives both roof and wall sheets.

Detailing checks before anyone punches a line of Zs

Cold-formed lines are cheap to draw and expensive to repunch. Check clashes with frame haunches (purlins that hit a deep knee), with crane brackets, and with wall bracing rods. Check that the first purlin off the eave leaves room for gutter hanging. Check that girt outer flange and column outer flange match the sheeting plane the architect thinks they bought.

• Purlin spacing versus sheeting allowable span from the specified panel, not a memory.
• Purlin spacing versus rafter flange-brace spacing used in analysis.
• Laps, nests or simple spans stated on the plan, consistent with design.
• Hole patterns for flange braces, bridging and hanging loads coordinated with the shop punch list.
• Openings: trimmers present, remaining lengths still manufacturable.
• Insulation thickness and liner panels that change seat heights.
• Expansion joints that actually break purlin continuity.
• Specials: polycarbonate, smoke vents, ridge vents, solar rails.

How secondaries should appear on the approval set

Approval drawings should show layout, spacing, section marks and openings, not every punch. Shop drawings add lengths, punching and bundles. If the approval roof plan is blank “because secondaries are typical,” the consultant cannot see whether a unit lands on steel. Put the layout on the plan and keep fabrication-level holes for later, as discussed in the approval process.

A secondary schedule — section, coating, design span condition, typical punch — saves questions. StruTools MBS Approval Package can help keep that schedule on a repeating sheet; the content still has to match this building’s sheeting and restraint assumptions.

Laying out purlins and girts on a new PEB

Work from constraints inward so the module is not a leftover dimension.

1. Lock sheeting type, cover width, insulation and liner assumptions in the input register.
2. Lock rafter and column brace-point requirements from the frame design.
3. Place eave struts and ridge conditions.
4. Lay a purlin module that satisfies both sheeting and brace points; flag conflicts instead of hiding them.
5. Repeat for girts, starting from base condition and eave, then cutting openings.
6. Add sag rods, bridging and flange-brace holes as specified, not as optional furniture.
7. Run a clash pass against haunches, cranes, bracing and rooftop units.
8. Issue the layout on approval plans with a schedule; hold punching until comments close.

Engineering tips

• Dimension purlin spacing on the slope, and say so, if that is how the sheeting is measured.
• Keep a special-load plan: hanging sprinklers, conveyors, lights, solar. Empty roofs do not stay empty.
• If Z orientation reverses at the ridge, draw the reversal; do not assume the shop knows your habit.
• Check the inside clearance from girt inner flange to column inner flange for services.
• Do not use leftover purlin stock as an eave strut unless the section is actually adequate.
• Photograph a sample bay after erection; rolled purlins are a layout lesson for the next job.

Secondary-layout mistakes that show up on site

Equal spacing that ignores the eave strut

Dividing rafter slope length by N equally can land a purlin on top of the eave member or leave a strip of sheeting overspanning.

Drawing girts through a door because the elevation was typical

The shop will punch them, the erector will cut them, and the wind post will be a site fabrication.

Flange braces detailed to a purlin that was later moved

Restraint spacing in the rafter design is now fictional. Moving purlins is a frame comment.

Simple-span design with lapped drawing

The member on site is not the member in the calculation. Pick one continuity condition and draw it.

Ignoring insulation thickness at seats

Seats that were set for sheeting-only can crush insulation or tilt the panel plane.

Purlin and girt quality checklist

Review roof plan and wall elevations together; they share the eave strut.

• Sheeting type and span direction are noted.
• Purlin spacing matches both sheeting and frame restraint assumptions.
• Eave struts are marked with their own section.
• Ridge condition and any reversal of Zs are shown.
• Laps versus simple spans are stated.
• Sag rods or bridging are shown where required.
• Flange-brace holes align with braces on the frame elevation.
• Openings have trimmers and remaining girt lengths.
• Rooftop units sit on designed steel.
• Expansion joints break secondaries on purpose.
• Coating and grade appear on the secondary schedule.
• End bays are not a blind copy of interior bays.

Frequently asked questions

Can purlin spacing be changed to suit a skylight without re-checking the rafter?

Not if the rafter’s flange brace points depend on that spacing. Treat a spacing change as a frame comment until someone confirms the unbraced length is still valid.

Are C and Z purlins interchangeable on a PEB roof?

No. They lap, nest, seat and brace differently. Substitution is a design change, including clips, sag members and punch patterns.

Why does the eave strut often differ from the purlin section?

It usually sees a different combination of roof load, wall load and sometimes axial force from bracing, and it must receive two sheeting planes. Using leftover purlin is a guess.

Do girts need to show on approval drawings if cladding is by others?

If the PEB supplier is designing the girts, yes — at least as layout and section. If girts are truly by others, the PEB package must state that and the frame design must not count on them for restraint unless the interface is specified.

Secondaries are how the frame stays the frame you analysed

Purlins and girts close the PEB envelope and they often hold the primary flanges where the calculations assumed they would be held. Layout, continuity, sag members, eave struts and opening trimmers are therefore part of the structural story. A regular hatch on a roof plan is not a secondary design.

Keep this guide beside primary and secondary members and bracing. Capacities, clip design and sheeting spans remain project-specific. StruTools does not replace manufacturer data or the responsible engineer’s review.