How to Specify a C Purlin (Complete Structural Guide)
Learn about how to specify a c purlin (complete structural guide) in roll forming machines. Profile Guide guide covering technical details
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:
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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.