By The Roofing Brief Team. Last reviewed: October 2026.
Metal building framing components fall into two structural layers: primary framing (the columns and rafters that form the rigid skeleton) and secondary framing (the purlins, girts, and eave struts that span between the primary members and carry the roof and wall panels). In a standard pre-engineered building the secondary members are cold-formed Z and C sections 8, 10 or 12 inches deep, per Nucor Building Systems and Ceco Building Systems literature, and every load (snow, wind, and the dead weight of the steel) travels through these two layers in a fixed order before it reaches the foundation.
The member-by-member breakdown below uses published manufacturer data: section depths, nominal thicknesses, weights per foot and spacing limits from Nucor’s secondary framing manual, gauges and finishes from Metal Sales, and eave strut shapes from Ceco. For the visible cladding and clips that attach on top of this frame, see the companion breakdown of metal roof system components.
Primary vs secondary framing: how the load path works
Primary framing carries the building. Secondary framing carries the panels and hands their load to the primary frame. In the Nucor Building Systems standard, Z purlins and girts are shop punched for field bolting and attach to the rafters and columns with shop-welded bypass clips as standard, so the connection points are fixed before the steel ships.
The load path in a standard rigid-frame building runs in five steps:
- Roof and wall panels catch snow, wind, and rain load at the surface.
- Purlins and girts (secondary framing) collect that load from the panels.
- Rafters and columns (primary framing) receive the load from the secondary members.
- Base plates and anchor bolts transfer it into the concrete.
- The foundation carries it to the ground.
Primary members are hot-rolled or built-up I-sections, often tapered. Secondary members are light cold-formed steel: Nucor’s lightest 8-inch Z purlin (08Z054) weighs 2.65 lb per foot and its heaviest 12-inch Z (12Z120) weighs 8.33 lb per foot. The frame does the structural work, and the thin secondary steel spans the gaps and holds the skin.
Primary framing components: columns and rafters
Primary framing is the rigid steel skeleton, and its two core members are columns and rafters. Together they form clear-span or modular frames set at regular bays down the length of the building. Nucor’s general frame information says its modular rigid frames work with Z purlins for maximum economy on bay spacing of up to 40 feet, or with bar joist purlins for 60 foot spans with 50 foot bays.
Columns are the vertical members that carry load down to the foundation. Three types appear in a typical building:
- Rigid-frame (sidewall) columns connect to the rafter to form the main clear-span bent along each sidewall.
- Endwall columns stand at the two gable ends and support the endwall framing and panels.
- Interior columns appear only in wider buildings that cannot economically clear-span in a single reach; Nucor’s standard interior columns are pipes.
Rafters are the sloped or horizontal members that span overhead, tying the column on one sidewall to the column on the opposite wall. In a rigid frame the column and rafter often meet in a deep tapered knee where bending force peaks. Nucor lists tapered beam (TCG) frames for buildings under 70 feet wide, clear-span rigid frames (RCG) for the widest column-free spans, and open-web TrussFrame rafters for clear spans of 250 feet or more. The wood equivalent is covered in the guide to roof trusses and rafters.
Secondary framing components: purlins, girts, and eave struts
Secondary framing is the light cold-formed steel that spans between the primary frames and gives the panels something to fasten to. The three members are purlins, girts, and eave struts, typically 8 to 12 inches deep, and each sits in a different plane of the building.
- Purlins run horizontally across the roof, perpendicular to the rafters, and support the roof panels. They carry roof load (snow, wind uplift, dead weight) back to the rafters at a standard maximum spacing of 5 ft 0 in in Nucor’s system.
- Girts run horizontally along the walls, spanning between columns, and support the wall panels. Nucor Building Systems uses Z girts as standard and C girts for framed openings. The panels that bolt to them are covered in the guide to steel building wall panels.
- Eave struts sit at the line where roof meets wall and anchor both the top of the wall panel and the bottom edge of the roof panel. Ceco’s roll-formed eave strut is a C section 8 inches deep with 3 1/4 inch flanges; its brake-formed versions are 10 or 12 inches deep with a 3 1/4 inch top and 4 1/2 inch bottom flange.
All three are cold-formed steel, and the Metal Sales secondary framing catalog lists them in 12, 14 and 16 gauge with red oxide primer or galvanized finishes. On low-slope metal building roofs, the purlin layout sets the drainage plane, which is why pitch and snow load drive the design of single-slope metal building roofs.
Purlin and girt sizing: gauge, depth, and span
Purlins and girts are specified by section depth, flange width and steel thickness, written as a code such as 10Z075 (10 inch Z, 0.075 inch thick). Metal Sales stocks Z and C members with web heights of 4, 6, 8, 9, 10 and 12 inches and flange widths of 2.5 to 4 inches; Nucor’s standard pre-engineered members come in 8, 10 and 12 inch depths.
Lower gauge numbers mean thicker steel. Nucor’s tables run from 0.054 to 0.120 inch nominal thickness, which brackets the 16 gauge (about 0.060 inch), 14 gauge (about 0.075 inch) and 12 gauge (about 0.105 inch) members Metal Sales lists. The table below gives selected sections from Nucor’s secondary framing manual, whose properties are calculated under AISI S100 and shown as fully braced capacities.
| Section | Depth x flange | Nominal thickness | Weight | Allowable moment Ma (ASD) | Source |
|---|---|---|---|---|---|
| 08Z054 | 8 in x 2.5 in | 0.054 in | 2.65 lb/ft | 52.9 in-kip | Nucor 4.3 |
| 08Z075 | 8 in x 2.5 in | 0.075 in | 3.68 lb/ft | 81.2 in-kip | Nucor 4.3 |
| 10Z060 | 10 in x 2.5 in | 0.060 in | 3.35 lb/ft | 77.5 in-kip | Nucor 4.3 |
| 10Z075 | 10 in x 2.5 in | 0.075 in | 4.19 lb/ft | 110.5 in-kip | Nucor 4.3 |
| 10Z105 | 10 in x 2.5 in | 0.105 in | 5.86 lb/ft | 161.5 in-kip | Nucor 4.3 |
| 12Z075 | 12 in x 3.5 in | 0.075 in | 5.21 lb/ft | 133.3 in-kip | Nucor 4.3 |
| 12Z120 | 12 in x 3.5 in | 0.120 in | 8.33 lb/ft | 265.1 in-kip | Nucor 4.3 |
| 08C075 | 8 in x 2.75 in | 0.075 in | 3.68 lb/ft | 76.2 in-kip | Nucor 4.3 |
| 10C075 | 10 in x 2.75 in | 0.075 in | 4.19 lb/ft | 103.4 in-kip | Nucor 4.3 |
Two patterns stand out in Nucor’s data: going from 8 to 10 inches deep at the same 0.075 inch thickness raises allowable moment by about 36 percent (81.2 to 110.5 in-kip) for 0.51 lb more steel per foot, while going from 0.075 to 0.105 inch at 10 inches deep raises it about 46 percent for 1.67 lb more per foot. Spacing is set by the roof system: Nucor’s standard maximum purlin spacing is 5 ft 0 in with purlins under its CFR roof (UL Class 90, UL 580 Construction No. 590), or 5 ft 6 in with joists, and odd end spaces run from 1 ft 10 in to 5 ft 6 in when the slope is 1/2:12 or steeper. Your engineered drawings govern the final size and spacing.
C-purlin vs Z-purlin: which shape and why
The two purlin profiles are the C (Cee) and the Z (Zee), and the choice comes down to whether the run is a single span or a continuous multi-bay span. Nucor makes Z purlins and girts its standard secondary framing and uses C girts for framed openings and special applications; Metal Sales sells both shapes in the same 12, 14 and 16 gauges.
| Factor | C-purlin | Z-purlin |
|---|---|---|
| Flange direction | Both flanges face the same way | Flanges face opposite ways |
| Best use | Single-span bays, framed openings, light commercial | Continuous multi-bay spans (Nucor standard) |
| Overlap at supports | No, butts at the rafter | Yes, nests and overlaps at the rafter |
| Span efficiency | Good for one bay | Longer spans or more load over the same span |
| Connection | Simple bolted clip | Bolted lap plus anti-roll clips |
The overlap is the reason for the Z shape: two Z-purlins lapped over a rafter act as one continuous beam, which spreads bending force and lets the run reach farther than a series of separate C-purlins. In Nucor’s tables an 8 inch C and an 8 inch Z of the same 0.075 inch thickness weigh the same 3.68 lb per foot.
Bracing and connection hardware
Bracing and hardware keep the frame square and stop the light secondary steel from twisting under load. Nucor’s bracing system manual calls full height X-bracing the lowest cost longitudinal bracing option and puts it in the first interior bay on buildings three bays or longer.
- Cable or rod X-bracing runs diagonally in roof and wall bays to resist wind and seismic force along the length of the building.
- Portal frames replace X-bracing in a sidewall bay where Nucor’s guide calls for more than 10 ft of clear height under the bracing; they cost more than X-bracing.
- Flange braces are small angles tying a rafter or column bottom flange to a purlin or girt so it cannot buckle sideways.
- Sag rods (sag angles) run at mid-span between purlins or girts to hold them in line before the panels go on.
- Anti-roll clips keep Z-purlins from rotating under load.
- Base plates and anchor bolts connect each column to the foundation and transfer load into the concrete.
- Clips: Nucor uses shop-welded bypass clips on purlins and girts as standard, with bolted clips available; its cold-formed box beams, which can replace the eave strut, connect with 1/2 inch A325 bolts.
Metal building framing decision table
| If your building has | Use | Source |
|---|---|---|
| Width under 70 ft | Tapered beam (TCG) frame with straight columns | Nucor 4.1.3 |
| Wide floor, no interior columns allowed | Clear-span rigid frame (RCG); TrussFrame for 250 ft or more | Nucor 4.1.4 |
| Wide floor, interior columns acceptable | Modular rigid frame (RMG) with pipe interior columns | Nucor 4.1.5 |
| Bays up to 40 ft | Z purlins | Nucor 4.1.5 |
| Bays over 35 ft, or a mezzanine | Open-web joists (ClearBay up to 60 ft) | Nucor 4.3 |
| Bays up to 65 ft from Ceco | Long Bay System, 12 to 44 in deep | Ceco |
| Framed openings in the wall | C girts | Nucor 4.3 |
| Sidewall bay that needs over 10 ft of clear height under bracing | Portal frame instead of X-bracing | Nucor 4.4.4 |
| Width 100 ft or less, low eave 18 ft or less, slope 1:12 or less | Fixed base corner columns as a bracing option | Nucor 4.4.5 |
The Ceco figures come from Ceco’s secondary framing explainer (October 2019), which also describes Z girts and purlins with stiffening lips formed at 50 degrees to the flange.
Framing a metal roof on a house: residential vs pre-engineered
Not every metal roof rides on a pre-engineered steel frame. On a house, metal panels usually fasten to wood rafters or trusses, either directly to solid decking or across horizontal wood or steel purlins (sometimes called battens) spaced to match the panel, and the 2021 IRC sets the slope floor in Section R905.10.2: 3:12 for lapped, non-soldered panels without lap sealant, or 1/2:12 with applied lap sealant. For member sizes and spacing, see our guide to roof purlins. For steel options, see our guide to metal roof trusses for post-frame buildings.
The practical split is by building type. A standing-seam or exposed-fastener panel over a stick-framed or trussed roof uses lumber sized from IRC Tables R802.4.1(1) through (8) for the structure and light purlins or a solid deck as the fastening base. A pre-engineered metal building uses the full steel system above: rigid-frame columns and rafters with cold-formed purlins, girts, and eave struts 8 to 12 inches deep. To go deeper on any term here, start at the Learn About Roofing hub.
Frequently asked questions
What are the framing components of a metal building?
A metal building has primary framing (columns and rafters that form the rigid skeleton) and secondary framing (cold-formed purlins on the roof, girts on the walls, and eave struts at the roof-to-wall line, typically 8, 10 or 12 inches deep). Bracing, flange braces, sag rods, base plates, and anchor bolts complete the frame. Panels and trim are cladding, not framing.
What is the difference between purlins and girts?
Purlins and girts are the same kind of cold-formed steel member in different planes. Purlins run horizontally across the roof and support the roof panels, carrying load to the rafters. Girts run horizontally along the walls and support the wall panels, carrying load to the columns. An eave strut does both jobs where the roof meets the wall, and Metal Sales sells all three in 12, 14 and 16 gauge.
What gauge steel is used for metal building framing?
Secondary framing (purlins, girts, eave struts) is cold-formed steel that Metal Sales lists in 12, 14 and 16 gauge. Nucor’s standard sections run from 0.054 to 0.120 inch nominal thickness; 14 gauge is roughly 0.075 inch and 16 gauge about 0.060 inch. Primary columns and rafters are heavier hot-rolled or built-up sections sized by the engineer.
How far apart are purlins spaced?
Nucor’s standard maximum purlin spacing is 5 ft 0 in with purlins under its CFR roof, or 5 ft 6 in with joists, and odd spaces run from 1 ft 10 in up to 5 ft 6 in on slopes of 1/2:12 or steeper. The exact spacing depends on the roof panel, the snow and wind load, and the purlin depth and gauge, so it is set on the engineered drawings.
What is an eave strut?
An eave strut is the secondary framing member at the line where the roof meets the wall. It anchors the top edge of the wall panel and the bottom edge of the roof panel at once, so it works as a purlin and a girt combined. Ceco’s roll-formed eave strut is a C section 8 inches deep with 3 1/4 inch flanges.
Can you frame a metal roof over wood rafters?
Yes. On houses, metal panels commonly fasten to wood rafters or trusses, either over solid decking or across horizontal purlins or battens spaced to the panel. This is different from a pre-engineered metal building, which uses steel columns, rafters, purlins, and girts throughout. Under IRC R905.10.2, lapped panels without sealant need at least a 3:12 slope.