A purlin is a horizontal member that runs between the main roof frames and carries the roof sheeting. It is the roof’s equivalent of a floor joist: closely spaced, repetitive, and sized to span the gap between the big members rather than to hold up the building on its own.
In a steel building they are almost always cold-formed sections rolled from sheet, and they come in two shapes that look similar and behave very differently: the C and the Z.
What a Purlin Actually Carries
Roof load arrives as pressure on the sheeting. The sheeting spans the short distance between purlins; the purlins span between rafters or the rigid frames of a metal building; the frames carry it to the columns and into the foundation. Each step gathers load from many members above and hands it to fewer, larger ones below, which is the same chain described in our guide to girders, beams and joists.
Two things make purlins different from floor joists. Roof load includes uplift as well as gravity, so a purlin can be pushed up as hard as it is pushed down. And the roof slopes, so load does not arrive square to the section: part of it acts sideways, which is why a purlin wants to twist rather than simply bend.
C Purlin vs Z Purlin
That nesting is the whole reason Z purlins dominate metal buildings. Because two Z sections overlap at a support, a purlin line can be lapped and bolted into something that behaves as a continuous beam across several bays instead of a row of separate simple spans. A continuous member carries more for the same section, and the lap puts the doubled steel exactly where the bending is highest, over the support.
| C purlin | Z purlin | |
|---|---|---|
| Flange direction | Both the same way | Opposite ways |
| Symmetry | About one axis | Point symmetric |
| Can nest at a lap | No | Yes |
| Usual span arrangement | Simple spans | Lapped and continuous |
| Typical use | End bays, framed openings, wall girts, short runs | Main roof purlin runs in metal buildings |
| Handling | Stacks flat | Nests, so it ships compactly |
C sections still have their place. They are used where a run ends and there is nothing to lap into, around framed openings, and very commonly as wall girts. A manufacturer’s secondary framing catalog shows both: Nucor Building Systems publishes Z and C section property tables along with standard purlin and girt spacing (checked September 2026). Section properties vary between manufacturers, so the tables that matter are the ones from whoever is supplying the building.
The Roof Is Part of the Structure
A cold-formed purlin on its own is a thin, open section with very little torsional stiffness. Load it off-axis and it will roll over long before it reaches the bending capacity you would calculate for a braced member.
It does not fail that way in a real roof because it is not on its own. Through-fastened sheeting screwed to the top flange restrains that flange and stops it moving sideways. Bridging, sag rods or angle bracing run perpendicular to the purlins and hold them upright between frames. The design calculation assumes all of that is present.
This is why the bracing is not optional trim. Leaving the bridging out of a run, or swapping through-fastened sheeting for a standing seam roof that lets the panel slide, changes what the purlin can carry. The same purlin, the same span, a different restraint assumption, and a different answer.
Cold-Formed Is Not the Same as Hot-Rolled
A C purlin and a C channel from the AISC catalog are different animals, and the names cause real confusion on site.
| Cold-formed C purlin | Hot-rolled C channel | |
|---|---|---|
| Made by | Roll-forming steel sheet at room temperature | Rolling hot steel through a mill |
| Wall | Thin and uniform | Thicker, with a tapered flange |
| Has stiffening lips | Usually yes | No |
| Designed to | AISI cold-formed steel specification | AISC specification |
| Governing concern | Local buckling of thin elements | Yielding and overall buckling |
The distinction matters because the design rules are genuinely different. Thin cold-formed elements can buckle locally well before the steel yields, so the capacity depends on an effective section rather than the full one. Hot-rolled shapes rarely behave that way. If you want the hot-rolled families and what each is used for, see types of steel beams, and how to read steel beam sizes for what the designations mean.
Purlins and Rust
Purlins are thin, and thin steel has less material to lose before it matters. They also sit in the part of the building where condensation forms, on the underside of a cold roof.
Most secondary framing is therefore supplied pre-galvanized. The American Galvanizers Association describes a zinc coating as giving barrier and cathodic protection plus a zinc patina, typically two to five times longer between maintenance than paint alone (checked September 2026). On a member you can only reach with a lift, that difference is the whole argument. There is more on how the coating behaves in our article on whether galvanized steel rusts.
Frequently Asked Questions
What is the difference between a purlin and a rafter?
The rafter runs up the slope from eave to ridge and is part of the main frame. The purlin runs horizontally across the rafters and carries the sheeting. Load goes sheeting to purlin to rafter, so the rafter is the larger member.
What is the difference between a purlin and a girt?
Nothing, except which way the wall faces. The same C and Z sections are called purlins in the roof and girts in the wall. Girts carry wall sheeting and resist wind pressure and suction rather than gravity load.
How far apart should purlins be?
Spacing is set by what the sheeting can span and by the loads, so it is an output of the design rather than a rule of thumb. Manufacturers publish standard spacings for their own systems, and those assume their panel, their purlin and their bracing.
Can I use a wood purlin instead?
Timber purlins are normal in pole barns and traditional timber roofs. The principle is identical; only the material and the design standard change. Steel dominates where spans get long or where the building is a pre-engineered metal system.
Why are Z purlins lapped instead of butted?
Because the lap makes the run continuous and doubles the steel over the support, where bending is greatest. Butting two purlins end to end would leave every span acting alone, which needs a heavier section for the same load.
The Short Version
Purlins carry the roof sheeting to the frames. Z sections nest, so they get lapped into continuous runs and do most of the work in metal buildings; C sections handle end bays, openings and wall girts. They are thin, they rely on the sheeting and bridging to stay upright, and they are designed to a different standard from anything in the AISC catalog. Sizing one is engineering work, and a licensed structural engineer has to do it.
Reference information only, not engineering advice. Disclaimer.