Choosing Coil Thickness for Purlins (C & Z Section Guide)

Choosing Coil Thickness for Purlins

Complete Structural & Roll Forming Guide

1️⃣ Why Purlin Thickness Is Critical

Unlike roofing sheets, purlins are structural members.

They support:

• Roof loads

• Wind uplift

• Dead loads

• Live loads

• Equipment loads

Incorrect thickness can cause:

• Excessive deflection

• Buckling

• Connection failure

• Structural non-compliance

Purlin thickness is not chosen by “feel” — it must match load conditions.

2️⃣ Common Purlin Thickness Ranges

Typical base metal thickness (mm):

Thickness (mm)	Typical Use
1.2 mm	Light duty
1.5 mm	Small span
1.8 mm	Medium span
2.0 mm	Commercial buildings
2.5 mm	Heavy duty
3.0 mm+	Large span / industrial

Always specify in millimeters (not gauge).

3️⃣ Factors That Determine Thickness

Thickness selection depends on:

• ✔ Span length
• ✔ Load (kN/m²)
• ✔ Steel grade
• ✔ Section depth
• ✔ Flange width
• ✔ Bracing condition
• ✔ Building code requirements

Thickness alone does not determine strength — section geometry matters.

4️⃣ Thickness & Section Modulus

Structural strength increases with:

• Depth of section

• Thickness of steel

Section modulus increases significantly with thickness.

Even small thickness increase dramatically increases load capacity.

5️⃣ Span vs Thickness Relationship

Longer span requires:

• Greater section depth

• Greater thickness

• Or higher steel grade

Example (generalized):

• 4m span → 1.5 mm may suffice
• 6m span → 2.0 mm or more
• 8m span → 2.5 mm+

Exact values require structural calculation.

6️⃣ Steel Grade Interaction

Common grades:

• G250

• G350

• G450

• G550

Higher grade allows:

• Same thickness but higher load capacity

• Or thinner section for same load

However:

Higher grade increases forming difficulty.

Thickness and grade must be evaluated together.

7️⃣ Local Buckling & Thickness

Thin purlins are prone to:

• Flange buckling

• Web buckling

• Distortional buckling

Increasing thickness improves:

• Buckling resistance

• Load capacity

• Structural stability

Buckling is often the governing design limit.

8️⃣ Deflection Control

Building codes often limit:

• Span / 180
• Span / 240
• Span / 360

Thicker purlins reduce deflection.

Even if strength is adequate, deflection may govern thickness choice.

9️⃣ Connection & Bolt Bearing

Thicker steel improves:

• Bolt bearing capacity

• Hole deformation resistance

• End reaction strength

Thin purlins can deform around bolts under load.

🔟 Wind Uplift Considerations

In cyclone / hurricane zones:

Wind uplift loads may govern thickness.

Roof systems with large tributary areas require:

Heavier purlins.

Underestimating uplift loads is a common mistake.

1️⃣1️⃣ Cost vs Structural Optimization

Thicker steel:

• ✔ Higher material cost
• ✔ Higher transport cost
• ✔ Higher forming load

But:

• ✔ Greater structural reliability
• ✔ Reduced deflection
• ✔ Increased safety margin

Optimization is key.

1️⃣2️⃣ Roll Forming Machine Impact

Thicker purlins require:

• ✔ Larger shaft diameter
• ✔ Higher motor torque
• ✔ Stronger frame
• ✔ Heavier bearings
• ✔ Stronger punching system

A machine designed for 1.5 mm may not safely run 2.5 mm.

Always verify machine thickness capacity.

1️⃣3️⃣ Punching Force Increases with Thickness

Purlins often include:

• Bolt holes

• Slots

• Service holes

Punching force increases with thickness and grade.

Hydraulic capacity must be sized correctly.

1️⃣4️⃣ Thickness & Developed Width

Thicker material increases:

Bend allowance slightly.

In fold-heavy C or Z sections, this can add several millimeters to developed width.

Blank width must be calculated per thickness range.

1️⃣5️⃣ Practical Thickness Selection Guide

Light Agricultural Buildings

1.2 – 1.5 mm

Small Commercial Units

1.8 – 2.0 mm

Medium Industrial

2.0 – 2.5 mm

Large Industrial / Long Span

2.5 – 3.0 mm+

Always confirm via structural calculation.

1️⃣6️⃣ Common Mistakes

• ❌ Selecting thickness based only on span
• ❌ Ignoring wind uplift
• ❌ Ignoring steel grade
• ❌ Not checking machine capacity
• ❌ Assuming thicker always solves deflection
• ❌ Using gauge instead of mm

Structural engineering must guide thickness choice.

1️⃣7️⃣ Engineering Summary

Coil thickness for purlins affects:

• ✔ Bending strength
• ✔ Buckling resistance
• ✔ Deflection
• ✔ Bolt performance
• ✔ Machine load
• ✔ Punching force
• ✔ Developed width

Thickness must match:

Load + span + grade + building code + machine capability.

Purlins are structural members — thickness selection is engineering-driven.

FAQ Section

Is 1.5 mm enough for purlins?

Only for short spans and light loads.

Is thicker always better?

Structurally yes, but cost and machine load increase.

Does steel grade matter?

Yes. Higher grade can reduce required thickness.

Does thickness affect blank width?

Slightly, through bend allowance.

Can my roofing machine run purlin thickness?

Usually no. Purlin machines are heavier duty.

Should I calculate thickness myself?

No. Use structural engineering design.