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). Every load on the building, snow, wind, and the dead weight of the steel itself, travels down through these two layers in a fixed order before it reaches the foundation. Knowing which part does what is the difference between reading an engineered drawing correctly and guessing.
This guide names every framing member in a pre-engineered metal building, gives the gauges, depths, and spans the industry actually stocks, and separates the structural skeleton from the panels and trim that bolt to it. 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. A metal building is engineered so that force flows through the members in one direction, and every component is sized for its place in that chain. Getting the order right is how you read any framing plan.
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. Secondary members are light cold-formed steel. That weight difference is deliberate: the frame does the structural work, and the thin secondary steel just 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 the clear-span frames set at regular intervals (commonly 20 to 25 feet apart) down the length of the building. These are the parts an engineer sizes first, because everything else hangs off them.
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.
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 are often one continuous tapered member, deepest at the knee (the corner) where bending force peaks. This is the same job a wood or steel truss does in conventional construction, 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, 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.
- Girts run horizontally along the walls, spanning between columns, and support the wall panels. They do for the walls what purlins do for the roof. The panels that bolt to them are covered in the guide to steel building wall panels.
- Eave struts sit at the exact line where roof meets wall. Shaped like a C tilted to match the roof slope, an eave strut anchors both the top of the wall panel and the bottom edge of the roof panel, so it acts as a purlin and a girt at once.
All three are almost always cold-formed steel, most often shipped in a red oxide primer or as unpainted galvanized sections. On low-slope metal building roofs, the way these purlins set the drainage plane matters, 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 three numbers: steel gauge (thickness), section depth, and clear span. Deeper sections and thicker steel span farther and carry more load. The industry stocks a predictable range, so most buildings use off-the-shelf sizes rather than custom rolling.
Cold-formed secondary steel is typically ASTM A653 Grade 55 galvanized, in 12, 14, or 16 gauge (lower number means thicker steel: 14 gauge is about 0.075 inch, 16 gauge about 0.060 inch). Typical single-span capacities for 14-gauge C-sections:
| Section (14 ga) | Depth | Typical max single span |
|---|---|---|
| C4 | 4 in | up to 12 ft |
| C6 | 6 in | 18 to 20 ft |
| C8 | 8 in | up to 25 ft |
| C10 | 10 in | up to 30 ft |
Purlin spacing in U.S. metal buildings runs 4 to 6 feet on center, with 5 feet the most common. Spans and spacing both depend on local snow and wind load, so treat these figures as a starting point and let the engineered drawings govern.
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. C-purlins are the versatile, most-stocked profile; Z-purlins are the long-span workhorse because they nest and overlap at the supports.
| Factor | C-purlin | Z-purlin |
|---|---|---|
| Flange direction | Both flanges face the same way | Flanges face opposite ways |
| Best use | Single-span bays, light commercial, residential | Continuous multi-bay spans |
| Overlap at supports | No, butts at the rafter | Yes, nests and overlaps (often 2 to 4 ft) |
| 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 whole point of the Z shape: two Z-purlins lapped over a rafter behave like one continuous beam, which spreads bending force and lets the run reach farther than a series of separate C-purlins.
Bracing and connection hardware
Bracing and hardware are the framing components that keep the frame square and stop the light secondary steel from twisting under load. They are easy to overlook on a parts list, but skipping them is how a frame racks or a purlin rolls over. Every engineered building includes them.
- Cable or rod X-bracing runs diagonally in roof and wall bays to resist wind and seismic force along the length of the building.
- Flange braces are small angles that tie the bottom flange of a rafter or column back to a purlin or girt, stopping the primary member from buckling 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.
- Bolted clip angles, usually welded to the top of the rafter, are the standard way purlins attach to primary framing.
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, the metal roof framing is usually conventional: metal panels 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. The primary and secondary framing logic still applies, but the primary frame is wood.
The practical split is by building type. A standing-seam or exposed-fastener panel over a stick-framed or trussed roof uses conventional lumber 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. Match the framing to the panel span and the local code, and confirm the load ratings before you order steel. 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 (purlins on the roof, girts on the walls, and eave struts at the roof-to-wall line). 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.
What gauge steel is used for metal building framing?
Secondary framing (purlins, girts, eave struts) is cold-formed steel, typically ASTM A653 Grade 55 in 12, 14, or 16 gauge, with 14 gauge common. Lower gauge numbers mean thicker steel: 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?
Purlin spacing in U.S. metal buildings runs 4 to 6 feet on center, and 5 feet is the most common. The exact spacing depends on the roof panel, the snow and wind load, and the purlin depth and gauge. Deeper, thicker purlins can span farther, so spacing and section size are set together 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 is shaped like a C tilted to the roof slope so it can anchor the top edge of the wall panel and the bottom edge of the roof panel at once. That dual role means it works as a purlin and a girt combined.
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. Match the framing to the panel span and confirm the load ratings before ordering.
Reviewed by The Roofing Brief Team. Last reviewed July 2026.