How to Specify a C Purlin (Complete Structural Guide)

Learn how to specify a C purlin including section size, thickness, material grade, punch pattern, coil width and machine requirements.

Complete Engineering & Procurement Guide

C Purlins are used for:

Roof support systems
Wall girts
Mezzanine framing
Industrial buildings
Steel structures

They are structural members — not just sheet profiles.

Specification errors can cause structural failure.

1️⃣ What Defines a C Purlin?

A C purlin is defined by:

✔ Web depth
✔ Flange width
✔ Lip size
✔ Thickness
✔ Steel grade
✔ Length
✔ Punch configuration

Common naming format:

C200 x 70 x 20 x 2.0

Meaning:

200 mm web
70 mm flange
20 mm lip
2.0 mm thickness

All dimensions must be defined.

2️⃣ Standard Section Sizes

Common web depths:

100 mm
120 mm
150 mm
200 mm
250 mm
300 mm

Common flange widths:

50 mm
60 mm
70 mm
75 mm

Common lip sizes:

15 mm
20 mm
25 mm

Each variation changes load capacity.

Never specify only web depth.

3️⃣ Thickness Range

Common thicknesses:

1.2 mm
1.5 mm
1.6 mm
1.8 mm
2.0 mm
2.5 mm
3.0 mm

Heavy structural may exceed 3.0 mm.

Machine must support maximum thickness + grade.

4️⃣ Material Grade

Common structural grades:

G350
G450
G550

Higher strength allows:

Higher load capacity
Thinner section

But increases:

Forming load
Punch tonnage requirement

Grade must be specified before tooling and machine selection.

5️⃣ Coating Type

Common coatings:

Z275 galvanized
Z450 galvanized
AZ coating (less common for purlins)

Indoor applications may use:

Black steel (uncoated)

Coating affects:

Roll wear
Punch wear
Corrosion resistance

Always specify coating mass.

6️⃣ Typical Coil Width

Coil width depends on:

Web depth + 2 flanges + 2 lips + bend allowance.

Example:

C200 x 70 x 20

200 + (70 × 2) + (20 × 2) = 380 mm
Add bend allowance → approx. 400–420 mm coil

Exact developed width must be calculated.

Never assume coil width.

7️⃣ Punch Pattern Specification

C purlins often require punching:

✔ Bolt holes
✔ Slotted holes
✔ Service holes
✔ Cleat holes

Specify:

Hole diameter
Hole spacing
Edge distance
Hole type (round or slot)
Hole tolerance

Punching affects machine tonnage requirement significantly.

8️⃣ Length Specification

Common lengths:

6 m
9 m
12 m
Custom cut-to-length

Length tolerance must be defined:

±2 mm typical.

Longer lengths require:

Stronger run-out tables.

9️⃣ Structural Load Requirement

You must define:

✔ Roof load
✔ Wind load
✔ Snow load
✔ Span length
✔ Deflection limit

Section size must be engineered, not guessed.

Never select section without load calculation.

🔟 Machine Engineering Requirements

Typical C purlin line:

14–20 forming stands
80–95 mm shafts
22–45 kW motor
Servo punching unit
Hydraulic cut-off

Heavy thickness (3 mm+) requires:

Stronger frame
Larger shafts
Higher motor power

Machine capacity must match maximum thickness + grade.

1️⃣1️⃣ Tolerance Requirements

Typical tolerances:

Web depth ±1–2 mm
Flange width ±1 mm
Straightness tolerance defined
Length ±2 mm

Structural alignment depends on tight tolerance.

Poor tolerance causes installation issues.

1️⃣2️⃣ Lip Size Importance

Lip increases:

Section stiffness
Buckling resistance

Changing lip size changes:

Structural capacity
Developed width
Punch alignment

Lip must be defined clearly.

1️⃣3️⃣ Z vs C Clarification

Some projects require:

C purlin
Z purlin

They are not interchangeable.

C purlins are typically used for:

Short spans
Wall girts

Always confirm whether C or Z required.

1️⃣4️⃣ Export Market Variations

Australia uses:

High-strength G550 commonly.

Africa uses:

G550 widely.

Middle East may use:

G350–G450.

Europe may follow:

EN structural standards.

Always confirm local structural code.

1️⃣5️⃣ Common Specification Mistakes

❌ Not specifying thickness
❌ Not specifying grade
❌ Ignoring punch layout
❌ Not defining lip size
❌ Selecting section without structural calculation
❌ Assuming coil width

These mistakes cause project delays and structural issues.

1️⃣6️⃣ Developed Width Reminder

Developed width includes:

✔ Web
✔ Flanges
✔ Lips
✔ Bend allowance
✔ Thickness compensation
✔ Springback correction

Incorrect developed width causes:

Wrong coil ordering
Machine overload
Section size inaccuracy

1️⃣7️⃣ Final C Purlin Specification Checklist

Before tooling or machine approval:

✔ Confirm web depth
✔ Confirm flange width
✔ Confirm lip size
✔ Confirm thickness range
✔ Confirm steel grade
✔ Confirm coating type
✔ Calculate developed width
✔ Confirm coil availability
✔ Define punch pattern
✔ Define length tolerance
✔ Confirm structural load requirement
✔ Confirm production speed target

Only then proceed.

FAQ Section

What is the most common C purlin size?

C200 x 70 x 20 in many industrial buildings.

Is G550 required?

Common in many markets for higher strength.

How thick can purlins be?

Often up to 3 mm on light gauge lines.

Does punch layout matter?

Yes — affects structural performance and machine design.

Can one machine run multiple sizes?

Yes — if adjustable tooling system is used.

Is coil width easy to calculate?

Only if full geometry and bend allowance are included.