A metal building is made of four groups of components: the primary frames that carry the main loads, the secondary framing (purlins, girts and eave struts) that spans between frames, the bracing that keeps it all square, and the roof and wall panels that close it in. Anchor bolts tie the whole system to a concrete foundation.
Knowing what each part does makes it much easier to read a quote, compare suppliers, or understand why changing one dimension affects the price. This guide covers every major component from the slab up, with a labeled diagram and a summary table.
How the Parts Work Together
Load in a metal building follows a simple path. Snow, wind and the weight of the roof press on the roof panels. The panels pass that load to the purlins, the purlins carry it to the main frames, and the frames carry it down their columns into the anchor bolts and foundation. Wind on the walls takes the same route through the wall panels and girts. Bracing handles the forces that try to rack the building along its length. In a pre-engineered metal building, the manufacturer designs all of these pieces as one system, which is why swapping a single member is not a do-it-yourself decision.
Primary Framing
The primary frames are the heavy members that give the building its shape. They repeat along the length of the building at a set spacing called the bay.
Rigid frames (main frames)
Most pre-engineered buildings use rigid frames: a column on each side joined to sloped rafters by stiff bolted connections, so the frame resists both gravity and sideways wind without diagonal bracing in the open bays. The members are usually built-up I-sections welded from plate, and they are often tapered, deeper at the knee where the column meets the rafter and shallower where bending is lower. That tapering puts steel only where the frame needs it. Rigid frames depend on stiff joints, the same idea covered in our post on the moment connection.
Clear-span vs multi-span frames
A clear-span frame has no interior columns, which suits arenas, hangars and shops that need an open floor. Wider buildings often use multi-span (modular) frames with one or more interior columns, which shortens the rafter spans and usually saves steel. The choice affects both cost and how the floor can be used, so it is worth settling early.
Endwall framing
The end walls can use a full rigid frame, which makes it easy to extend the building later, or lighter post-and-beam framing with endwall columns and a rake beam, which is cheaper but has to be reworked if the building grows.
Secondary Framing
Secondary members span from frame to frame and give the roof and wall panels something to attach to. They are usually cold-formed steel, roll-formed from galvanized or primed coil into Z or C shapes.
Purlins
Purlins run along the roof, parallel to the ridge, and carry roof loads to the rafters. Z-shaped purlins can nest inside each other, so they are usually lapped over the frames to act as continuous beams. A 2025 STRUCTURE magazine article on metal building roof systems (checked November 2026) explains that lapping gives continuity with simple connections, and that the roof panels help stabilize the purlins so the roof works as a system. Our purlin guide covers shapes and spacing in more detail.
Girts
Girts are the wall version of purlins. They run horizontally between columns, carry wind pressure from the wall panels back to the frames, and frame the tops and bottoms of openings. They can be bypass girts, set outside the column flange, or flush girts, set within the column depth to save wall thickness.
Eave struts
The eave strut sits at the top of each side wall where roof meets wall. Its top flange supports the edge of the roof panels and its web supports the top of the wall panels, so it does the job of both a purlin and a girt.
Bracing
Rigid frames resist wind across the building, but something has to stop it racking along its length. That is the bracing system:
- Rod or cable X-bracing in at least one roof bay and wall bay, running diagonally between frames.
- Portal frames where an X-brace would block a door, using a stiff frame in the wall plane instead.
- Flange braces, small angles from purlins or girts to the inside flange of the frame, which stop the slender frame members from buckling sideways.
- Panel diaphragm action, where the roof and wall panels themselves stiffen the building, depending on panel type and fastening.
Bracing is often removed or relocated during renovations to add a door. That changes how the building resists wind and must be checked by an engineer first.
Roof and Wall Panels
Through-fastened panels
Ribbed panels screwed directly to purlins and girts with exposed fasteners. They are economical and common on walls and on many farm, storage and shop roofs. According to the STRUCTURE article, through-fastened panels restrain the purlins, which simplifies purlin design.
Standing seam roofs
Panels held down by concealed clips and joined by raised seams, so there are few holes through the roof. The clips let the roof expand and contract with temperature, which helps on long buildings, but they also give the purlins less restraint, so the design has to account for that.
Panels are usually coated steel such as Galvalume or galvanized sheet, often with a paint finish. Trim, closures, gutters and downspouts finish the edges and seal the panel ribs against weather and pests.
Foundation, Anchor Bolts and Base Plates
Every frame column ends in a base plate that sits on the foundation and is held down by cast-in anchor bolts. Because metal buildings are light and have large roof areas, wind uplift on the anchor bolts can matter as much as the downward load. The manufacturer usually supplies the anchor bolt layout and the column reactions; in many projects a separate engineer uses those reactions to design the foundation.
Openings, Insulation and Accessories
- Framed openings: jambs and headers around roll-up doors, walk doors and windows, tied into the girts.
- Insulation: faced blanket, liner systems or insulated metal panels. Options and trade-offs are in our guide on how to insulate a metal building.
- Ventilation and light: ridge vents, wall louvers and translucent skylight panels.
- Extensions: lean-tos, canopies and roof overhangs, which add loads the main frames must be designed for.
- Crane and mezzanine supports: must be specified at the time of order, because they change the frame design.
Metal Building Components at a Glance
| Component | Where it is | What it does | Typical material |
|---|---|---|---|
| Rigid frame (columns and rafters) | Across the building at each bay | Carries roof and wall loads to the foundation | Welded built-up or hot-rolled I-sections |
| Endwall framing | Each end wall | Supports end wall panels and roof edge | Rigid frame or posts and rake beam |
| Purlins | Roof, parallel to ridge | Support roof panels and carry load to rafters | Cold-formed Z or C sections |
| Girts | Walls, horizontal | Support wall panels and resist wind | Cold-formed Z or C sections |
| Eave struts | Top of side walls | Support roof edge and top of wall panels | Cold-formed sections |
| Bracing | Selected roof and wall bays | Stops racking along the building length | Rods, cables, angles, portal frames |
| Panels | Roof and walls | Weather skin; can stiffen the structure | Coated steel sheet |
| Base plates and anchor bolts | Column bases | Transfer loads and uplift to the concrete | Steel plate and threaded rod |
Who Designs the Components
For a pre-engineered building, the manufacturer’s engineers size the frames, secondary framing and bracing for the loads at your site, using industry references from the Metal Building Manufacturers Association. The MBMA design resources page (checked November 2026) lists its Metal Building Systems Manual, 2024 edition, which it says conforms to the 2024 International Building Code, alongside a free Roof Framing Design Guide for cold-formed purlin systems. Snow, wind and seismic loads vary by location, and the local building department sets which code edition applies. A licensed structural engineer has to design and check the building and its foundation; this guide is only an overview of the parts.
Metal Building Components FAQ
What is the difference between a purlin and a girt?
Both are secondary framing members spanning between the main frames. Purlins are on the roof and support roof panels; girts are on the walls and support wall panels.
What is primary and secondary framing in a metal building?
Primary framing is the main frames, columns and rafters that carry loads to the foundation. Secondary framing is the lighter purlins, girts and eave struts that span between frames and support the panels.
Can I remove X-bracing to add a door?
Not without an engineer. The bracing keeps the building from racking in the wind. It can often be moved to another bay or replaced with a portal frame, but that has to be designed and checked.
Are metal building frames made from I-beams?
They are I-shaped, but on most pre-engineered buildings they are built up by welding plates together and tapered to suit the loads, not cut from standard rolled wide flange beams. Smaller buildings and some endwalls do use rolled shapes.
What holds a metal building down?
Anchor bolts cast into the foundation, connected to the base plate of each column. They resist both the building’s weight and wind uplift, so the foundation and bolt design must match the manufacturer’s reactions.
Reference information only, not engineering advice. Disclaimer.