A C-purlin and a Z-purlin are both roll-formed from flat steel, and both are specified by depth, flange, lip and thickness. The difference is the flanges. A C-purlin’s two flanges face the same way, so it has a flat back and one piece can’t slide inside another of the same size. A Z-purlin’s flanges face opposite ways and are usually made slightly unequal, so the pieces nest in a bundle and lap over each other at supports. That is why lapped Z-purlins are the usual choice for continuous roof and wall framing on metal buildings, while C-purlins go into simple spans, framed openings, studs, racking and solar rails.
This guide covers where each shape fits, how the trade, AISI and SSMA naming systems describe the same section, why a gauge number is not a thickness, and what to put on a drawing so a roll former quotes the section you mean.
C vs. Z at a glance
| Feature | C-purlin (Cee) | Z-purlin (Zee) |
|---|---|---|
| Flanges | Both face the same way, usually equal widths | Face opposite ways, often slightly unequal |
| Over a support | Usually a simple span; continuity needs a lap or splice detail from the engineer | Laps over the next piece and bolts through the webs, so the line acts as a continuous beam |
| In a bundle | Same-size pieces don’t nest inside each other | Turned 180°, one Z fits against the next, so bundles pack tight |
| Typical uses | Simple-span purlins, framed openings, studs and joists, racking and solar rails | Continuous roof purlins and wall girts on metal buildings |
| How it’s specified | Depth × flange × lip × thickness | Depth × each flange × lip × thickness |
Both shapes are open sections that twist or roll under load in ways a closed tube does not. Bracing, sheathing and the connections handle that, and they are part of the structural design, which stays with the engineer of record.
When a C-purlin fits
- Simple spans. Where each piece runs from one support to the next and nothing needs to lap, a C does the job with plain end connections.
- Openings, ends and edges. The flat back of a C frames door and window jambs, headers and the ends of a run cleanly, and two Cs bolted back to back make a symmetric built-up member when the engineer calls for one.
- Studs and joists. SSMA’s “S” members, the studs and joists of cold-formed framing, are lipped C sections.
- Racking and solar rails. Rails and racking members are often C sections punched along the web for brackets, with the hole pattern set by the racking drawing.
When a Z-purlin fits
- Continuous roof purlins and wall girts. On a metal building, Z-purlins lap over each frame and are bolted through the webs, so a line of purlins works as one continuous beam across several bays. That continuity is a large part of why metal building roofs use Z sections.
- Long runs of repeated bays. The same piece repeats bay after bay, which suits roll forming: one section, one punch pattern at each end, many pieces.
- Bundling and handling. Because Z sections nest, a bundle of Zs takes less space than the same count of Cs.
Two details matter on a Z order. First, say which flange is wider and which one faces up, because the wider flange is what lets one piece lap over the next. Second, the lap is bolted through the webs, so the punch pattern near each end of the piece matters as much as the section itself. Send the lap holes with the profile, not as an afterthought.
Cee purlin vs. C-channel vs. hot-rolled channel
“C-channel” means three different products in US steel buying. A Cee purlin is a roll-formed C with a stiffening lip on each flange. An unlipped C-channel, also called track or U-channel, is roll-formed the same way without lips. An AISC hot-rolled C or MC channel is a fixed structural size such as C8×11.5, ordered by depth and weight per foot. If the drawing gives a gauge, a lip or a purlin depth, it is a roll-formed section; see C-purlins, C-channels and Z-purlins. If it gives an AISC designation, see hot-rolled steel channel sizes.
How sections are named
The same C section can carry three different names, depending on who wrote the drawing.
| System | Format | Example | How to read it |
|---|---|---|---|
| Trade shorthand | Depth, shape letter, gauge | 8C14 |
8 in. C in 14 gauge; flange, lip and decimal come from that supplier’s catalog |
| AISI design manual (D100) | Depth, CS / CU / ZS / ZU, flange, “x”, thickness in thousandths of an inch | 8CS2.5x070 |
8 in. deep lipped C, 2-1/2 in. flange, 0.070 in. thick |
| SSMA framing | Depth and flange in hundredths of an inch, style letter, mils | 800S250-68 |
8.00 in. web, S member (C-stud or joist with lips), 2.50 in. flange, 68 mil minimum thickness |
Reading 8CS2.5x070
In AISI’s naming, CS is a C section with lips and CU is a C without lips; ZS and ZU are the same for Z sections. The number after the “x” is the thickness in thousandths of an inch, so x070 is 0.070 in. The name does not give the lip length or the corner radius; AISI’s tables define those for each listed section.
Reading 800S250-68
SSMA gives web depth and flange width in hundredths of an inch, so 800 is 8.00 in. and 250 is 2.50 in. The style letter is S for studs and joists, T for track, U for channel and F for furring channel. For track, the depth is measured inside to inside. The last number is mils: the minimum base steel thickness in thousandths of an inch, which SSMA sets at 95 percent of the design thickness. SSMA also fixes the lip by flange width: 5/8 in. on a 2-1/2 in. flange, shorter than the lips in the purlin catalogs we reviewed.
A quick way to check a name against a published weight is the strip-width method in our purlin weight calculator. For 800S250-68, the sides add up to 8 + 2.5 + 2.5 + 0.625 + 0.625 = 14.25 in.; four bends take off 4 × 2 × 0.0713 = 0.570 in.; and 13.680 × 0.0713 × 3.4032 gives 3.32 lb/ft. SSMA’s table lists 3.33 lb/ft for the same section.
Gauge vs. decimal thickness
A gauge number is not a measurement. The Manufacturers’ Standard Gauge for sheet steel is based on weight, at 41.82 lb per square foot per inch of thickness, and SSMA framing is ordered by mils. Galvanized sheet has a gauge of its own that includes the zinc, so 14 gauge galvanized is 0.0785 in. on the galvanized steel gauge chart, which matters for a pre-galvanized G90 purlin. The Manufacturers’ Standard Gauge and SSMA land on different decimals for the same gauge:
| Gauge | Manufacturers’ Standard Gauge (in.) | SSMA mils | SSMA design thickness (in.) | SSMA minimum thickness (in.) |
|---|---|---|---|---|
| 20 | 0.0359 | 33 | 0.0346 | 0.0329 |
| 18 | 0.0478 | 43 | 0.0451 | 0.0428 |
| 16 | 0.0598 | 54 | 0.0566 | 0.0538 |
| 14 | 0.0747 | 68 | 0.0713 | 0.0677 |
| 12 | 0.1046 | 97 | 0.1017 | 0.0966 |
| 10 | 0.1345 | 118 | 0.1242 | 0.1180 |
SSMA lists the gauge numbers for reference only, and its 20 gauge row is the 33 mil structural product (30 mil is a drywall product). Purlin catalogs add a third set of decimals: one we reviewed lists 14 gauge as 0.075 in. and 12 gauge as 0.100 in.
The practical rule: write the decimal on the drawing, say whether it is nominal or minimum, and keep the gauge as a reference in parentheses, for example “0.0747 in. nominal (14 ga)”. See the full sheet metal gauge chart for other gauges.
Typical US market sizes
These are conventions you will see in published tables and catalogs, not a list of sizes Mona Steel stocks or runs.
| Where you’ll see it | Depths | Flanges | Lips | Thickness |
|---|---|---|---|---|
| Metal building purlin and girt catalogs | 4 to 12 in., in 2 in. steps; some catalogs go deeper | Usually 2 to 3-1/2 in. | About ¾ to 1-3/16 in. | 16, 14 and 12 ga; 10 ga often special order |
| SSMA framing (S members) | 1-5/8 to 16 in. | 1-1/4 to 3-1/2 in. | 3/16 to 1 in., set by flange width | 18 to 118 mil |
AISI’s Cold-Formed Steel Design Manual (D100) also tabulates generic sections for engineers to design with, from lipped Cs such as 4CS2x059, 9CS2.5x059 and 12CS3.5x105 to lipped Zs such as 8ZS3.25x059 and 12ZS3.25x105. A custom profile does not have to match any of these; it needs a dimensioned drawing.
Coatings
G90 is an ASTM A653 coating designation for pre-galvanized sheet: 0.90 oz of zinc per square foot of sheet, total of both sides, which the American Galvanizers Association puts at about 0.77 mil per side. Hot-dip galvanizing after fabrication to ASTM A123 is much heavier; its minimums for structural shapes run from 1.8 to 3.9 mils, depending on steel thickness. Pre-galvanized steel is coated before it is formed, so cut ends and punched holes are bare edges, while hot-dip galvanizing after fabrication coats every surface.
G90 pre-galvanized coil is cost-effective for most indoor and moderate environments. For harsh or coastal exposure, hot-dip galvanizing after fabrication or zinc-magnesium coated steel may be the better choice. Give the exposure and design life with the drawing so the coating is quoted with the section.
Quote C or Z profiles with Mona Steel
Mona Steel LLC, headquartered in Anaheim, California, roll-forms C-purlins, C-channels and Z-purlins, plus U, hat and sigma sections, from 20 gauge to ¼ in. They come pre-galvanized (ASTM A653, e.g., G90) or hot-dip galvanized (ASTM A123), pre-punched, slotted and cut to length, with mill test reports and heat-number traceability on every order. Volume work is our focus: production runs and repeat releases of profiles that run into thousands of linear feet, delivered across California and shipped to Texas, Arizona, Nevada and nationwide. We also quote smaller orders when we can help.
Your engineer of record sizes the member; Mona Steel doesn’t provide stamped engineering, but we can optimize the profile geometry for strength-to-weight and cost within that design. Laser-cut clips, splice plates and connection plates can ship with the profiles on one purchase order.
See C-purlins, C-channels and Z-purlins, convert tons to feet with the purlin weight calculator, work through the RFQ checklist, or send a profile drawing.
Sources
- AISI D100, Cold-Formed Steel Design Manual (2017): section naming and section tables, as used in AISI research report RP21-01.
- AISI D111, Design Guide for Cold-Formed Steel Purlin Roof Framing Systems: lapped Z-purlins and unequal flanges for nesting.
- Steel Stud Manufacturers Association, Product Technical Guide: product naming, steel thickness table, lip lengths and section weights.
- Manufacturers’ Standard Gauge for Sheet Steel, based on 41.82 lb per square foot per inch of thickness, as tabulated in Machinery’s Handbook.
- American Galvanizers Association, Continuous sheet galvanizing: ASTM A653 designations and ASTM A123 thickness minimums.
- Published C- and Z-purlin catalogs from three US metal building component suppliers, reviewed October 2026, for the market conventions above.