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How to Install Z & C Purlins — Step-by-Step Installation Guide
Time Required
5 minutes
Estimated Cost
INR 0
Materials
Tools
- Calibrated torque wrench or torque-limiting screwdriver
- Tape measure and chalk line
- Safety harness and edge protection
- 1
Verify purlin schedule and mark rafter centres
Before handling any material, compare the delivered purlin sizes and lengths against the engineer's purlin schedule. Mark each rafter centre line with a chalk line — the pre-punched holes must align with these marks.
Pro tip
Snap the chalk line from eave to ridge in one pass. A sagging chalk line will misposition every purlin in that bay.
- 2
Position the first purlin and check alignment
Place the first Z or C purlin on the rafter with the pre-punched holes centred on the chalk line. The purlin flange must bear flat on the rafter top flange across its full width. Use packing shims if the rafter has a variable depth profile.
- 3
Fix the purlin with self-drilling screws
Drive one 5.5 × 32mm hex-head self-drilling screw through each pre-punched hole into the rafter flange. The screw must penetrate a minimum of 16mm into the steel. Do not over-drive — the washer face should be flush with the purlin web, not recessed.
Pro tip
Two screws per purlin-to-rafter connection is the minimum. Three are required when the purlin schedule specifies a Type A or B eave connection.
- 4
Install the lap joint at internal rafters
For Z purlins at internal rafters, nest the adjacent bay's purlin over the first — the Z-profile is designed to stack. The lap length is specified in the purlin schedule (typically 10–15% of bay span). Fix through both webs with two M12 hex bolts and flat washers.
- 5
Install sag rods for spans above 6 metres
For bay spacings above 6 metres, install sag rods at mid-span between adjacent purlins. Pass the sag rod through the pre-punched web holes, fit the nut and washer on each side, and tension finger-tight plus one quarter turn. Do not over-tension — sag rods prevent lateral buckling; they are not tie-rods.
Pro tip
Sag rods are installed before sheeting begins. Attempting to install them after sheeting is in place is extremely difficult and dangerous.
- 6
Verify torque and inspect alignment
Using a calibrated torque wrench, verify that all self-drilling screws are torqued to 6 N·m. Check purlin alignment with a string line across the top flange — the maximum permitted deviation is ±3mm over a 6m span. Sign off the checklist before proceeding to sheeting.
Introduction
Installing cold-formed Z and C purlins correctly determines both the structural integrity and the long-term weatherproofness of the roof. An incorrectly fixed purlin — wrong lap length, missing sag rods, or over-driven screws — can fail under wind uplift loading without any visible warning. This guide covers the installation procedure step by step, using the standard Tata Steel and JSW purlin ranges available from our stockyard.
Before you start: Confirm you have the signed purlin schedule from the structural engineer. This document specifies the exact section size, thickness, lap lengths, screw count per connection, and sag rod spacing for your specific building. Never install purlins from memory or from another project's schedule.
Safety Requirements
Working at height on a partially clad roof is a high-risk activity. Before any purlin installation begins:
- Edge protection must be in place on all open edges and openings
- Each worker must wear an inspected harness with a lanyard attached to a certified anchor point
- No material is to be left on the roof at the end of a work session — wind can convert loose purlins into projectiles
- Wet or iced steel surfaces must not be worked on without anti-slip measures
Material Check Before Installation
Before any fixing begins, verify the delivered materials against the purlin schedule:
| Check | Expected | Action if wrong |
|---|---|---|
| Section depth (Z150, C200, etc.) | Per schedule | Do not install — return to supplier |
| Thickness (1.5mm, 2.0mm, etc.) | Per schedule | Do not install — return to supplier |
| Length | Per schedule ±3mm | Check if short end is acceptable; reject if too short |
| Coating | Z275 galv or pre-painted | Verify MTC — bare steel must not be installed |
| Pre-punched holes | Present, aligned | Verify hole pattern matches rafter centres |
After Installation — Inspection Checklist
Before the sheeting crew begins:
- All purlin sizes match the schedule
- All lap lengths match the schedule
- Lap bolts are M12 and have flat washers both sides
- All self-drilling screws are flush (not over-driven or under-driven)
- All screw connections have been torque-checked (6 N·m)
- Sag rods installed at all specified mid-span positions
- Sag rod nuts are tensioned (not loose, not over-tightened)
- String line alignment check completed (max ±3mm deviation)
- Signed inspection record completed by site supervisor
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Frequently Asked Questions
Can Z and C purlins be mixed in the same bay?▾
Yes, but only at end bays where lapping is not required. Internal bays should use consistently Z or C sections — mixing types at a lap joint causes alignment problems.
What is the maximum purlin spacing for a 1.5mm Z purlin?▾
For a Tata Steel 1.5mm S350 GD Z purlin carrying a combined dead plus live load of 0.55 kN/m², the maximum spacing is approximately 1,400mm at a 7.5m bay. Consult the purlin schedule in your structural drawings for the exact spacing for your load case.
How many self-drilling screws are needed per purlin?▾
Two screws per purlin-to-rafter connection (one each side of the web centreline) is the standard minimum. Eave and ridge purlins typically require three screws as specified in the connection schedule.
Do sag rods need to be removed before sheeting?▾
No. Sag rods remain in place permanently. They prevent lateral-torsional buckling of the purlin and are part of the permanent structure.
What tools are needed for installation?▾
A calibrated torque wrench or torque-limiting screwgun, tape measure, chalk line, M12 spanner for sag rod nuts, and appropriate safety equipment including harness and edge protection.