If you've ever walked onto a construction site for a steel warehouse or a large shed, you've likely noticed the purlin girt network making up the "skeleton" between the heavy main frames. While the massive I-beams get most of the glory for keeping the building upright, these smaller secondary members are actually doing the heavy lifting when it comes to supporting the roof and walls. Without them, you'd just have a few giant steel hoops standing in a field with nowhere to attach your siding or roofing panels.
In the world of metal buildings, these components are the glue that holds everything together. They aren't just there for decoration; they serve a vital structural purpose by transferring loads from the exterior "skin" of the building—like the metal sheets—back to the primary structural frame. Let's break down how this whole system works and why you should care about the difference between a purlin and a girt.
The Basic Difference Between a Purlin and a Girt
It's actually pretty simple, though people tend to use the terms interchangeably if they aren't in the trade. A purlin is a horizontal beam used in the roof structure. It runs parallel to the building's ridge and supports the roof decking or sheeting. If you look up while standing inside a finished metal building, those rows of steel bars running across the ceiling are your purlins.
On the flip side, a girt is basically the same thing but for the walls. These are the horizontal members attached to the side wall and end wall columns. They give you a place to screw in your wall panels. So, in short: roof equals purlin, wall equals girt. Even though they perform almost identical functions in different planes, their engineering requirements can vary because a roof has to deal with snow and rain, while a wall mostly deals with wind.
The Shape of the Steel: C-Sections and Z-Sections
When you look at a purlin girt system, you'll notice the steel isn't just a flat bar or a solid beam. It's almost always "cold-formed" steel, meaning it's shaped from thin sheets of steel at room temperature. The two most common shapes are C-sections and Z-sections.
Why the "Z" Shape is So Popular
Z-purlins are probably the most common sight on modern job sites. The reason is pretty clever: they can be "nested." Because of the way the flanges are angled, one Z-section can fit perfectly inside another. This allows for lapping at the supports. When you overlap two Z-purlins over a mainframe column, you're essentially doubling the thickness of the steel at the point of highest stress. It makes the whole roof much stiffer and allows you to use thinner, lighter steel over longer spans. It's an efficient way to save money without sacrificing strength.
The Reliable C-Section
C-sections (often called C-channels, though that's technically a different thing in hot-rolled steel) look like the letter C. These are usually used for the "Eave Purlins"—the ones right at the edge of the roof—or around door and window openings. Since they have a flat back, they're much easier to bolt onto a flush surface. They don't nest like Z-sections do, so they're usually used in simpler "simple span" configurations where the steel just goes from one column to the next without overlapping.
How They Handle the Elements
The engineering behind a purlin girt layout is all about load paths. Think about a massive windstorm hitting the side of a warehouse. That wind hits the metal wall panels first. The panels bend slightly and push against the girts. The girts then take that pressure and transfer it to the heavy main columns, which are bolted into the concrete foundation.
In a roof scenario, it's often about "gravity loads." If you live in a place that gets heavy snow, those purlins are carrying thousands of pounds of white stuff. They have to be spaced correctly—usually every 4 to 5 feet—to make sure the roof panels don't crinkle under the weight.
One thing people often forget is "uplift." In a high-wind event, a roof acts like an airplane wing, and the wind actually tries to suck the roof off the building. The purlins have to be bolted down securely enough to resist being pulled upward. It's not just about holding the roof up; it's about holding the roof down.
Installation Realities and Bridging
If you've ever tried to install these, you know they can be a bit floppy until they're fully secured. Because they are made of relatively thin, cold-formed steel, they have a tendency to twist or "roll" when you put weight on them. This is where bridging or "sag rods" come into play.
Bridging consists of small rods or angles that connect the purlins or girts to one another mid-span. This keeps them from twisting and ensures they stay perfectly vertical (or perpendicular to the roof slope). If your girts start to sag, your wall panels will look wavy, and your screw lines will be a mess. More importantly, a twisted purlin loses a huge chunk of its structural integrity. It's like trying to stand on the edge of a piece of cardboard versus standing on it when it's folded into a box—geometry is everything.
Bypass vs. Flush Girts
When it comes to the walls, you have a couple of choices on how to mount your purlin girt system.
- Bypass Girts: These are mounted on the outside of the columns. This is the most common method because it creates a continuous run of steel and allows for that "lapping" we talked about with Z-sections. It also creates a small gap between the column and the wall panel, which is great for running electrical conduit or tucking in extra insulation.
- Flush Girts: These are installed between the columns. This is a bit more labor-intensive because you have to clip each girt to the side of the column. However, it saves space. If you have a tight property line or you just want a clean, flat interior wall without columns sticking out, flush girts are the way to go.
Corrosion and Protection
Since these members are the bones of your building, you don't want them rusting away. Most purlin girt components come in one of two finishes: red oxide primer or galvanized steel.
Red oxide is the classic "red iron" look. It's basically a shop-applied primer that protects the steel during shipping and construction. It's cost-effective, but if the building is going to be open to the elements or in a humid environment, it might eventually need a top coat.
Galvanized steel is dipped in zinc. This is the gold standard (well, the silver standard) for longevity. It won't rust even if it gets scratched, thanks to the way zinc sacrificially protects the steel underneath. If you're building near the ocean or in a chemical-heavy industrial zone, going galvanized is a no-brainer.
Why Quality Spacing Matters
You might be tempted to save a few bucks by spacing your purlins further apart. "Hey, if I space them 6 feet apart instead of 5, I save 20% on steel!" Don't do it. The spacing is calculated based on the "gauge" (thickness) of your roof panels and the local weather codes.
If your purlins are too far apart, the roof panels will flex every time someone walks on them. Over time, that flexing pulls at the fasteners, widens the holes, and—you guessed it—leads to leaks. A solid purlin girt foundation is what makes a metal building last 50 years instead of 15.
Wrapping It All Up
At the end of the day, the purlin girt system is the silent workhorse of the metal construction world. They aren't as flashy as the big rafters or as visible as the painted wall panels, but they provide the essential bridge between the skin and the frame. Whether you're using Z-sections for their nesting capabilities or C-sections for those tricky corners, getting the secondary framing right is the difference between a building that stands the test of time and one that rattles every time the wind blows.
Next time you're inside a big steel shed, take a second to look up. Now you'll know exactly what you're looking at: a finely tuned system of purlins and girts keeping everything straight, true, and—most importantly—above your head.