A hot-rolled C-channel and a C purlin share a shape but not much else. The C-channel is rolled at a steel mill with thick flanges and web and is designed as structural steel under CSA S16. Purlins, girts and Z-channels are cold-formed from thin sheet or strip and designed under CSA S136, the standard for cold-formed steel. Cold-formed sections are light and efficient for roofs and walls over a primary frame, while hot-rolled channel is used where a member carries heavier loads, frames openings or has to take connections and welding.
Two ways to make a channel
- Hot-rolled. A steel billet or bloom is reheated and passed through rolls until it becomes the final shape. The result has thick, solid flanges and a web several millimetres thick, rounded fillets at the corners and sloped inside flange faces. Canadian drawings name these C200x17, C250x23 and so on; see the C-channel size chart.
- Cold-formed. Flat coil is fed through roll-forming stands (or bent on a press brake) at room temperature. The section has the thickness of the coil throughout, with tight bends instead of fillets, and often stiffening lips on the flange edges. Purlins, girts, eave struts, steel studs and track are all made this way, frequently from galvanized coil.
The 2024 Ontario Building Code reflects the split. Article 4.3.4.1 sends structural steel to CSA S16, and Article 4.3.4.2 sends cold-formed steel members to CSA S136. The two standards treat the members differently because thin sections fail differently, often by local buckling of the flat parts long before the steel yields.
C channel vs C purlin
| Hot-rolled C-channel | Cold-formed C purlin | |
|---|---|---|
| How it's made | Hot-rolled at a mill | Roll-formed from coil |
| Thickness | Web and flanges several mm, flanges thicker than web | Same thin gauge throughout |
| Flange edges | Plain, sloped inside face | Usually stiffening lips |
| Design standard | CSA S16 | CSA S136 |
| Designation | Depth and mass, such as C200x17 | Manufacturer's depth, flange, lip and thickness |
| Weight for its depth | Heavy | Light |
| Typical role | Framing, headers, stringers, ledgers, equipment supports | Roof purlins, wall girts, light secondary framing |
The practical point is that outside dimensions tell you very little. A 203 mm cold-formed C purlin and a C200x17 can look similar from across a yard, but their thicknesses, properties and capacities are not in the same league. Never substitute one for the other without the engineer's approval, and never assume a purlin can carry a point load such as a hoist, heater or solar racking unless it was designed for it.
C purlins vs Z purlins
Cold-formed secondary framing comes in two common profiles:
- C (lipped channel). Both flanges point the same way. Cs are easy to fasten back to back, to frame around doors, windows and roof openings, and to use at building ends and edges where a flat back is handy.
- Z. The flanges point in opposite directions, with a lip on each. The big advantage is nesting: one Z can slip partly inside the next, so purlins are lapped over each rafter or frame line and bolted together. A lapped run behaves like a continuous member over several supports, which reduces bending at mid-span and lets lighter sections span further. Zs also stack tightly for shipping.
Many manufacturers roll Z sections with one flange slightly wider than the other so that nested laps fit snugly. Sizes are given by depth, flange widths, lip length and thickness, and every roll-former publishes its own catalogue and load tables. That is why a "steel Z channel sizes" search turns up different lists: there is no single national size series for cold-formed Zs the way there is for hot-rolled channels.
Purlins, girts and where hot-rolled channel fits in
In a typical steel building:
- Purlins run across the roof between the main frames and carry the roof sheeting or deck.
- Girts run horizontally along the walls between columns and carry the wall cladding, transferring wind load to the frame.
- Eave struts run along the top of the walls where roof and wall meet.
- Bracing (rods, cables or angles) keeps the whole system square and stable.
Cold-formed C and Z sections do most of this secondary work. Hot-rolled channel still turns up in the same buildings where loads or details demand it:
- Framed openings. Jambs and headers around overhead doors and large openings often need more strength and stiffer connections than a girt can provide.
- Equipment and mechanical supports. Units, piping and hoists hung from the roof need members designed for point loads.
- Mezzanines, platforms and stairs. Channel stringers, edge members and framing are hot-rolled work; see our steel mezzanine guide.
- Repairs and reinforcement. A damaged or overloaded purlin is sometimes reinforced or replaced with a heavier section, but only to an engineer's design.
The main frame itself, the columns and rafters, is usually built from hot-rolled W-shapes or welded plate sections rather than channel. For how the primary members drive a building's cost, see steel-framed building cost.
Pre-engineered buildings vs conventional steel
Pre-engineered metal buildings are supplied as a system: the manufacturer designs and supplies the frames, purlins, girts, bracing and cladding together. In Ontario, Article 4.3.4.3 of the building code requires steel building systems to be manufactured by companies certified under CSA A660. If you are adding to or modifying a pre-engineered building, the original manufacturer's design or a structural engineer has to confirm what the existing purlins and frames can take.
Conventional steel buildings are designed by an engineer and fabricated from hot-rolled members to project drawings, with secondary framing chosen to suit. Either way, the drawings tell you which members are hot-rolled and which are cold-formed.
How to read what's on your drawings
- A C or MC designation with depth and mass (C200x17, MC12x10.6) means hot-rolled structural channel.
- A manufacturer's code or a description with a gauge or thickness (for example a depth, flange width and "14 ga") means a cold-formed section from a roll-former's catalogue.
- Purlin or girt schedules usually list cold-formed sections, lap lengths and bolt patterns.
- Connection details at openings and equipment usually show the hot-rolled pieces.
A purlin or girt that looks undersized is not necessarily wrong. Cold-formed members are designed with the cladding, sag rods and bracing that hold their flanges in place, so they can't be judged on their own or by comparison with hot-rolled tables. If anything is unclear, ask the designer before ordering. For the wider family of channel names, including track and U sections, see C channel vs U channel.
Hot-rolled channel for your building
Our C-channels are hot-rolled structural channel, cut and fabricated to your drawings with holes, clips, copes and welded plates in our CWB certified shop. We can prime or galvanize to suit and deliver across the GTA and Ontario. Cold-formed purlins and girts come from roll-formers and building system suppliers; we handle the structural steel that frames openings, supports equipment and builds platforms around them.
Send your drawings for a quote, or browse our steel products, including channels, beams, plates and posts.
Frequently asked questions
Is C channel and C purlin the same?
No. A C-channel on a structural drawing is a hot-rolled section with thick flanges and web, designed under CSA S16. A C purlin is cold-formed from thin sheet, usually with stiffening lips on its flanges, and is designed under CSA S136. They share a shape, not a strength.
What is the difference between C and Z purlins?
C purlins have both flanges pointing the same way, which makes them easy to frame around openings and at the ends of a building. Z purlins have flanges pointing in opposite directions, so one can nest inside the next and be lapped over a support, which lets a run of purlins act continuously over several spans.
What is a girt?
A girt is a horizontal member on a wall that supports the cladding and transfers wind load to the columns. Purlins do the same job on a roof. Both are commonly cold-formed C or Z sections in steel buildings.
Can a hot-rolled C-channel be used as a purlin or girt?
Yes, where the designer specifies it. Hot-rolled channels are heavier per metre than cold-formed sections of the same depth, so they are typically used where loads are higher, spans are short, or the member also frames an opening or supports equipment.
