A pre-engineered metal building is a steel building system whose frames, secondary members and panels are designed and fabricated by one manufacturer for a specific set of loads, then shipped to site and bolted together. Instead of an architect and engineer detailing every beam from standard rolled sections, the manufacturer designs the whole package, usually with tapered welded frames, cold-formed purlins and girts, and metal roof and wall panels.
This guide explains how the system is put together, how it differs from conventional structural steel, what the common frame types are, and what to pin down before ordering one.
What “Pre-Engineered” Actually Means
The name is slightly misleading. The building is not engineered in advance and pulled off a shelf. It is engineered by the manufacturer for your site’s loads, dimensions and code, using the manufacturer’s own standard components and design software. What is “pre” about it is the system: the member types, connection details and panel profiles are standardized, so only the sizes and layout change from job to job.
The Metal Building Manufacturers Association (MBMA) describes these systems as primary and secondary steel framing that is fabricated off-site and assembled on site by bolting rather than field welding (checked September 2026). You will also see the terms “metal building system,” “PEMB” and “steel building kit.” A kit usually means a smaller, simpler package sold for DIY assembly, but the structural idea is the same.
The Parts of a Metal Building System
MBMA’s design resources page (checked September 2026) lists the basic components as primary frames, secondary purlin and girt members, and metal cladding. Here is what each does.
Primary frames
The main frames are usually built-up I-sections welded from plate, not hot-rolled beams. Because the manufacturer cuts its own web and flange plates, it can taper the web: deep at the knee, where the column meets the rafter and bending is highest, and shallower elsewhere. That puts steel where the forces are and saves weight compared with a constant-depth rolled section. If you want to compare with standard rolled shapes, the wide flange beam sizes chart lists what a conventional frame would be built from, and types of steel beams covers the other shape families.
Secondary framing: purlins and girts
Purlins run along the roof between frames, and girts run along the walls. Both are usually cold-formed C or Z sections, and Z purlins are often lapped over the frames to act continuously. Our guide to C purlins vs Z purlins goes into how they work and why the lap matters.
Bracing
Rigid frames resist sideways load across the building. Along the length of the building, wind and seismic load are usually carried by rod or cable X-bracing in the roof and walls, or by portal frames where a bay has to stay open for doors. Flange braces, short angles from the purlins or girts to the inside flange of the frame, keep the frame from buckling sideways.
Cladding
Roofs are commonly standing seam or through-fastened panels, and walls are usually through-fastened panels, though masonry, tilt-up or other wall materials can be used with the frame. Coating choice matters for long-term appearance and corrosion; our article on whether galvanized steel rusts covers how zinc-based coatings protect steel.
Common Frame Types
| Frame type | What it looks like | Typical use |
|---|---|---|
| Clear span | A single rigid frame with no interior columns | Arenas, hangars, warehouses, anywhere open floor space matters |
| Multi-span (modular) | Rigid frame with one or more interior columns | Very wide buildings where interior columns are acceptable, cutting frame weight |
| Single slope | Roof slopes one way only | Strip retail, additions, buildings that drain to one side |
| Lean-to | A smaller frame that leans on a main building’s columns | Additions, covered storage along one side |
| Beam and post end wall | Lighter framing at the end of the building instead of a full rigid frame | End walls that will never need to be removed for expansion |
If you might extend the building later, ask for a full rigid frame at the expansion end wall. A beam and post end wall is lighter and cheaper, but it is not designed to become an interior frame.
Pre-Engineered vs Conventional Steel
| Pre-engineered metal building | Conventional structural steel | |
|---|---|---|
| Who designs the steel | The manufacturer’s engineers, for the loads given to them | The project’s structural engineer of record |
| Main members | Tapered built-up plate sections | Hot-rolled W-shapes, HSS, channels, angles |
| Secondary members | Cold-formed C and Z purlins and girts | Rolled beams, joists, or cold-formed members |
| Connections | Standard bolted end-plate connections | Designed per job, bolted or welded |
| Best fit | Low-rise, simple, repetitive buildings | Multi-story, irregular or heavily loaded buildings |
| Flexibility later | Changes must be checked against the manufacturer’s design | Easier for another engineer to analyze and modify |
MBMA’s own material claims shell completion in 30 to 50 percent less time than competing framing systems (MBMA homepage, checked September 2026). Treat that as the industry association’s figure; your schedule will depend on the manufacturer’s lead time, the foundation and the erector.
What to Pin Down Before You Order
Most problems with metal buildings come from what the manufacturer was or was not told. The design is only as good as the load information it was based on.
- Building code and loads. The local building department sets the code edition, wind speed, snow load and seismic category. Confirm the building is designed to those numbers, not a generic default.
- Collateral and future loads. Sprinklers, lighting, ductwork, a future ceiling, solar panels or a hoist all add load. If they are not in the design, they are not in the frame.
- Openings. Large overhead doors change where the bracing can go and how the walls are framed. Decide sizes and locations early.
- Cranes and mezzanines. A bridge crane or mezzanine has to be part of the original design. Adding either later usually means reinforcing frames.
- Foundation design. The manufacturer supplies anchor bolt layouts and column reactions, but the foundation is typically designed separately by a local engineer. Make sure someone owns it.
- Sealed drawings. Ask for drawings and calculations sealed by an engineer licensed in your state, which most building departments require for permit.
Erection and Connections
Frames arrive in pieces, typically columns and rafter segments with end plates already welded on. On site, the erector stands columns on the anchor bolts, bolts rafter segments together at their end plates, then adds purlins, girts and bracing before the panels go on. The end-plate connections are high-strength structural bolts tightened to the manufacturer’s requirements; our bolt grades guide explains how structural bolts differ from hardware-store grades.
The erection sequence matters. Until the first braced bay is complete, frames are unstable and rely on temporary bracing. Follow the manufacturer’s erection drawings rather than improvising.
Modifying an Existing Metal Building
Because the frames and purlins are designed close to their loads, pre-engineered buildings have less hidden reserve than many conventional buildings. Cutting a new door through a braced bay, hanging new equipment from purlins, adding a roof-mounted unit or removing a rod brace can all overload something. MBMA publishes guidance on retrofitting and reinforcing metal building systems for exactly this reason. Any change to the structure should be checked by a licensed structural engineer, ideally with the original manufacturer’s drawings and design loads in hand.
Frequently Asked Questions
What is the difference between a pre-engineered building and a metal building kit?
Both use the same idea of manufacturer-designed frames, purlins and panels. “Kit” usually refers to smaller, simpler buildings sold for owner or DIY assembly, while “pre-engineered metal building” is the broader industry term that also covers large commercial and industrial buildings.
Are pre-engineered buildings built with I-beams?
The main frames are I-shaped, but they are usually welded from steel plate and tapered, not standard hot-rolled I-beams. Purlins and girts are cold-formed C or Z sections.
Can a metal building be expanded later?
Yes, if it was planned for. Ordering a full rigid frame at the end wall that will be removed makes expansion much simpler. Otherwise the end wall framing may need to be replaced.
Who designs the foundation for a pre-engineered building?
The manufacturer normally provides column reactions and anchor bolt layouts. The foundation itself is typically designed by a local engineer who knows the soil conditions. Confirm this in the contract.
Can I hang a hoist or add a mezzanine in an existing metal building?
Only after a licensed structural engineer has checked the frames, purlins and foundation for the new load. These buildings are designed for specific loads, and additions that were not included usually need reinforcement.
Reference information only, not engineering advice. Disclaimer.