How to Install C & Z Purlin: Sizes, Spacing & Fixing

C Purlin vs Z Purlin: Which to Use

C purlins have a C-shaped cross-section with flanges on one side. They are used where the purlin ends are simply supported — typically on walls, for gable-end rafters, and for short spans where the purlin is cut and joined between supports. C purlins are easy to butt-join and are the default choice for wall girts and small buildings.

Z purlins have a Z-shaped cross-section and are the standard choice for roof purlins on sloping roofs. The key advantage is that Z purlins can be lapped and nested at the joints — the end of one purlin slides inside the next — which creates a continuous beam across the supports. This gives Z purlins a strength advantage over C purlins of the same size, and they carry most roof loads in portal frame buildings.

Rule of thumb: use C for walls and simple spans, use Z for roof purlins on slopes and long spans.

Standard Purlin Sizes and Section Properties

Purlin designation	Typical section (mm)	Typical thickness (mm)
C100 / Z100	100 x 50	1.5-2.0
C150 / Z150	150 x 60	1.5-2.5
C200 / Z200	200 x 65-75	2.0-3.0
C250 / Z250	250 x 75	2.0-3.0
C300 / Z300	300 x 85-90	2.5-3.5

The section depth (100-300 mm) is chosen from the span and load tables, and the flange width and thickness follow the section. Always size purlins from the manufacturer's load tables or a structural engineer's calcs — never by guesswork.

Purlin Spacing Rules

Purlin spacing is driven by the roof covering's maximum span, not by the purlin itself:

Metal roof sheets — 1.2-1.5 m spacing is typical (light corrugated can span 1.5 m; heavier profiles less)
Sandwich panels — 1.0-1.5 m depending on panel thickness and load
Clay tile and heavy coverings — 0.9-1.2 m, check the covering manufacturer's spec

At eaves, ridge and penetrations (roof lights, vents), spacing is reduced by roughly 50% for 2-3 bays to handle the local loads. Snow and wind load regions can require tighter spacing — follow the project's structural drawings.

Fixing and Connections

To the portal frame — purlins are fixed with M12-M16 bolts (or self-drilling screws with cleats) through pre-punched holes at the supports. Use two bolts minimum per connection.
Laps (Z purlins) — lap length is typically 10% of the span (minimum 600 mm) and the lap is bolted through both webs. Laps go over the frame connection point.
Bracing — angle or rod bracing (fly braces) is fitted at supports to stop purlin rotation and twisting, especially on slopes above 15 degrees.
Sag rods — for long wall girts and valley conditions, sag rods or sag straps tie the members together to share the load.
Fixings for roof sheets — self-drilling screws or rivets per the sheet manufacturer's pattern; always fix into the flat pan area, not the ribs, unless the sheet spec says otherwise.

Pre-punched holes are a big time-saver on site. Purlin machines with a CNC punching unit (like our steel frame purlin CNC punching machine) punch the holes in the factory to your drawings, so the erectors just bolt up.

Common Installation Mistakes

Wrong orientation — Z purlins must face the right way up on each slope (the web should not twist the roof plane); mixing orientations at hips and valleys causes fitting problems
Oversized spacing — exceeding the covering's span limit makes sheets drum or sag between purlins
Laps not over supports — lapping between supports creates a weak point instead of a continuous beam
Missing bracing — skipping fly braces is the most common cause of purlin rotation failures
Cutting with a torch — never cut purlins with a flame; it destroys the galvanizing at the cut. Use a saw or shears and touch up with zinc paint.

For the machine side of production, see our guide C Profile vs Z Profile Purlin: How to Choose the Right One, and explore the C & Z purlin roll forming machine.

FAQ

What is the difference between C and Z purlin?

C purlins have a C-shaped section used for walls and simple spans; Z purlins have a Z-shaped section that laps and nests at joints, making them the standard for roof purlins on slopes and long spans.

What is the standard purlin spacing?

1.2-1.5 m for metal roof sheets, 1.0-1.5 m for sandwich panels, and 0.9-1.2 m for heavy coverings. Reduce spacing near eaves, ridges and penetrations.

How long should a Z purlin lap be?

Typically 10% of the span (minimum 600 mm), with the lap positioned over the frame connection and bolted through both webs.

How are purlins fixed to the frame?

With M12-M16 bolts or self-drilling screws through pre-punched holes at the supports, two bolts minimum per connection, plus fly braces to stop rotation.

Do I need a punching unit on the purlin machine?

For site efficiency, yes — a CNC punching unit pre-punches holes to your drawings so erectors bolt up without drilling on site.

How do I get a quotation for a purlin machine?

Send your purlin sections and punching drawings to us by WhatsApp/Tel +86 13166567923 or email steven@zt-rfm.com, and we will confirm the machine configuration, price and delivery time.