UK Cold-Formed Purlin Design — BS EN 1993-1-3 Light-Gauge Steel

Cold-formed steel purlins are the standard secondary structural elements in UK industrial and commercial buildings, spanning between portal frame rafters to support roof cladding. Unlike hot-rolled sections governed by BS EN 1993-1-1, cold-formed purlins are designed to BS EN 1993-1-3 "Supplementary Rules for Cold-Formed Members and Sheeting", which accounts for thin-gauge behaviour including local buckling, distortional buckling, and the stabilising influence of the attached roof sheeting. This reference covers Z- and C-section selection, buckling modes, UK manufacturer span tables, anti-sag bar requirements, and a worked example for a portal frame roof purlin.

UK Purlin Types and Geometry

Z-Sections (Zed Purlins)

Z-purlins are the predominant UK roof purlin. Their point-symmetric cross-section means that when installed with the top flange pointing up the roof slope, the principal axes are approximately aligned with the vertical and horizontal loading directions, minimising weak-axis bending. Key geometry:

• Depths: 140 mm to 350 mm (in 20-30 mm increments per manufacturer range)
• Flange widths: 55 mm to 80 mm
• Lip stiffeners: 15 mm to 25 mm (classifying the flange as a stiffened element under EN 1993-1-3 Clause 5.5.3)
• Thickness: 1.2 mm to 3.0 mm (continuously hot-dip galvanised to EN 10346, coating Z275 or Z350)
• Grade: S450GD (minimum yield 450 N/mm^2, ultimate 510-600 N/mm^2)

UK suppliers (Tata Steel ComFlor, Capital Steel RoofDek, Voestalpine Metsec) provide standardised ranges. A Z 202 x 65 x 1.8 purlin (202 mm deep, 65 mm flanges, 1.8 mm thick) weighs approximately 5.2 kg/m and is a typical choice for 7.5 m bay portal frames.

C-Sections (Channel Purlins)

C-sections are primarily used for side rails (wall cladding support) where loading is predominantly about the weak axis, and for eaves beams where the C-shape provides a natural seat for roof cladding at the eaves-to-gutter transition. C-sections are also used in mezzanine floor edge beams and partition wall head tracks where torsional restraint is provided by attached sheeting or blockwork.

Sleeved vs Butt-Connected Systems

UK purlins are manufactured as either:

• Sleeved systems: Purlins are supplied with an internal sleeve that slides inside the purlin at the support, providing continuity over the rafter. The sleeve transfers bending moment across the support, creating a two-span or three-span continuous system. Sleeved systems typically achieve 15-25% greater spanning capacity compared to simply supported purlins of the same section. Cost premium: approximately £0.50-£0.80 per metre for the sleeve and additional fabrication, offset by the reduction in purlin size.
• Butt-connected systems: Purlins butt against each other at the rafter and are connected with cleats. Simpler and lower-cost for short spans (<6 m), but significantly less efficient for standard UK portal frame bays (7.5 m+).

Buckling Modes in Cold-Formed Purlins

Cold-formed sections are susceptible to three distinct buckling modes not present in hot-rolled sections:

Local Buckling

Plate elements of the cross-section (flanges, web, lips) buckle individually between longitudinal folds. Governed by EN 1993-1-3 Clause 5.5 and EN 1993-1-5 Annex A. The effective width concept reduces the gross cross-section to an effective section that excludes portions of the plate known to buckle before the yield stress is reached.

For a Z 202 x 65 x 1.8 purlin flange (unstiffened element under uniform compression), the width-to-thickness ratio b/t = 65/1.8 = 36.1, compared to the Class 3 limit of 14 x epsilon = 14 x sqrt(235/450) = 10.1. The element is slender (Class 4), requiring effective width reduction per EN 1993-1-5.

Distortional Buckling

The entire flange-lip assembly rotates about the flange-web junction, involving both translation and rotation of the stiffened edge. This mode is specific to cold-formed sections with edge stiffeners and is not present in hot-rolled sections. EN 1993-1-3 Clause 5.5.3 provides the reduced thickness method for distortional buckling, where the stiffener thickness is reduced to account for the reduced buckling stress of the stiffener relative to the flange.

For a Z-section with a 20 mm lip stiffener, the spring stiffness K provided by the flange to the lip is calculated as:

K = E x t^3 / (4 x (1 - nu^2) x b1^2 x h_T)

Where b1 is the flange flat width, h_T is the web depth. The critical elastic buckling stress sigma_cr,s of the stiffener determines the reduction factor chi_d for distortional buckling.

Lateral-Torsional Buckling (Global Buckling)

The entire purlin section buckles laterally over its span length. For a roof purlin with attached sheeting, the lateral displacement of the top flange is restrained by the sheeting, but the bottom flange (in compression under uplift) is unrestrained. EN 1993-1-3 Clause 10.1.4 permits a simplified approach where the bottom flange is treated as a strut on an elastic foundation (the web acting as a spring).

The spring stiffness provided by the web is:

K_web = C x E x t^3 / h^3

Where C is a factor accounting for web bending, and h is the web depth. For a 202 mm deep purlin with 1.8 mm thickness, K_web is approximately 0.05-0.10 N/mm/mm, which is sufficient to restrain the bottom flange against LTB for spans up to 10 m under typical UK roof loading.

Manufacturer Safe Load Tables

In UK practice, purlin selection uses manufacturer-published safe load tables almost exclusively. These tables provide the maximum permissible span for a given purlin section under specified loading (uplift and downward) and numbers of anti-sag bar rows. The methodology is per EN 1993-1-3 Annex A "Testing", which requires:

1. Full-scale bending tests on purlin assemblies with attached sheeting
2. Statistical evaluation of test results per EN 1990 Annex D
3. Derivation of design resistance from the 5th percentile of test results divided by gamma_M = 1.0

The safe load tables are conservative by a factor of 1.25-1.50 relative to first-principles calculation because they incorporate the beneficial effect of sheeting restraint, which is difficult to quantify analytically without testing.

Typical UK Span Capacities (Z 202 x 65 x 1.8, S450GD)

Values are indicative — always consult the specific manufacturer's load table for the purlin system being specified.

Anti-Sag Bars and Bridging

Anti-sag bars (also called sag rods or bridging) are transverse ties connecting adjacent purlins at mid-span or third-points. They serve two functions:

1. During construction: Prevent the purlin from rolling over under the self-weight of the cladding before the sheeting is fully fixed.
2. Under uplift: Provide lateral restraint to the bottom flange of the purlin, which is in compression when wind uplift exceeds the dead load.

UK practice for anti-sag bar requirements:

Anti-sag bars are typically 12-16 mm diameter solid round bars with threaded ends, connected through pre-punched holes in the purlin web. Cost: approximately £3.50-£5.00 per bar including end nuts and washers, or £0.30-£0.50 per square metre of roof for a typical 7.5 m bay.

Purlin-Sheeting Interaction

The attached roof sheeting provides significant restraint to the purlin top flange through friction and mechanical interlock of the fixings. This restraint is the reason Z-purlins can span 7.5 m+ at thin gauges (1.8 mm) despite a nominal LTB slenderness that would fail by inspection for an unrestrained section.

Key design considerations for the purlin-sheeting interaction:

• Fixings: Self-drilling screws at 333 mm centres (every trough for a 1 m wide trapezoidal sheet with 3 troughs). Each screw provides approximately 1.5-2.0 kN of lateral restraint to the top flange in the plane of the sheeting.
• Diaphragm action: The roof sheeting acts as a stressed-skin diaphragm, transferring longitudinal wind loads from the gable ends to the side wall bracing. The shear flow capacity of a typical 0.7 mm trapezoidal sheet is 3-5 kN/m.
• Stress reversal (uplift): Under wind uplift, the top flange is in tension and the bottom flange is in compression. The sheeting provides no restraint to the bottom flange, so the anti-sag bars become essential. Uplift capacity without anti-sag bars may be 40-60% lower than the published table value.

Cost Data for UK Cold-Formed Purlins

Approximate supply-only costs for UK cold-formed Z-purlins in S450GD with Z275 galvanising (2026 prices):

Add approximately £0.75-£1.50/m for anti-sag bars, cleats, and fixings. Sleeve systems add £0.50-£0.80/m. Allow 10-15% for site waste (offcuts at gable ends and openings).

Worked Example — Portal Frame Roof Purlin

A 30 m span portal frame warehouse in Leicester has 7.5 m bay spacing and a roof pitch of 6 degrees. The roof cladding is a 0.7 mm trapezoidal built-up system at 0.15 kN/m^2. Select and verify a Z-purlin for the roof.

Step 1 -- Define loading:

• Purlin spacing: 1.8 m (standard for trapezoidal roof sheeting)
• Dead load: sheeting 0.15 + purlin self-weight 0.05 + services 0.05 = 0.25 kN/m^2 on slope (x 1.8 m = 0.45 kN/m)
• Snow load (Leicester): ground snow s_k = 0.50 kN/m^2
  • Shape coefficient mu_1 = 0.8 for roof pitch < 30 deg
  • Snow on plan: s = 0.8 x 0.50 = 0.40 kN/m^2 x 1.8 m spacing = 0.72 kN/m
  • Resolved onto slope: 0.72 x cos(6 deg) = 0.716 kN/m
• Wind uplift (100 m altitude, open country):
  • Peak velocity pressure qp = 0.85 kN/m^2
  • External pressure coefficient Cpe,10 = -1.3 (suction on windward roof slope, 6 deg)
  • Internal pressure Cpi = +0.2 (dominant opening on windward face)
  • Net uplift: -0.85 x (-1.3 - 0.2) = -0.85 x (-1.5) = +1.275 kN/m^2 (upward)
  • Uplift per purlin: 1.275 x 1.8 = 2.30 kN/m upward

Step 2 -- Load combinations (EN 1990 + UK NA, STR):

• Downward (snow primary): 1.35 x 0.45 + 1.50 x 0.72 = 0.608 + 1.080 = 1.688 kN/m
• Uplift (wind primary): 1.00 x 0.45 - 1.50 x 2.30 = 0.45 - 3.45 = -3.00 kN/m (net uplift)

Step 3 -- Select purlin from manufacturer table:

For 7.5 m span with 2 rows of anti-sag bars (at third-points), referring to the Tata Steel ComFlor safe load table for Z 202 x 65 x 1.8 in S450GD:

• Permissible downward UDL at 7.5 m span, 2 anti-sag rows: 3.5 kN/m > 1.688 kN/m -- OK
• Permissible uplift UDL at 7.5 m span, 2 anti-sag rows: 2.1 kN/m < 3.00 kN/m -- NOT OK

The standard Z 202 x 65 x 1.8 is inadequate for uplift. Upgrading to Z 242 x 72 x 2.0 in S450GD:

• Permissible uplift UDL at 7.5 m span, 2 anti-sag rows: 2.9 kN/m < 3.00 kN/m

Still marginal. Further upgrade to Z 302 x 78 x 2.4:

• Permissible uplift UDL at 7.5 m span, 2 anti-sag rows: 3.8 kN/m > 3.00 kN/m -- OK

Step 4 -- Cost comparison:

• Z 202 x 65 x 1.8: £7.00/m x 30 m x 34 purlin lines = £7,140
• Z 242 x 72 x 2.0: £9.50/m x 30 m x 34 = £9,690
• Z 302 x 78 x 2.4: £14.80/m x 30 m x 34 = £15,096

Alternative: retain Z 202 x 65 x 1.8 but add a third anti-sag bar row (at quarter-points). This upgrades the permissible uplift capacity from 2.1 to 2.7 kN/m -- still insufficient for 3.00 kN/m.

Alternative 2: Reduce purlin spacing from 1.8 m to 1.5 m (add 7 additional purlin lines across the 30 m span):

• Uplift per purlin at 1.5 m spacing: 1.275 x 1.5 = 1.913 kN/m
• Z 202 x 65 x 1.8 with 2 anti-sag rows: permissible uplift 2.1 kN/m > 1.91 kN/m -- OK
• Additional purlin cost: 7 lines x 30 m x £7.00/m = £1,470
• Total purlin cost: £7,140 + £1,470 = £8,610 (cheaper than upgrading all purlins to Z 302)

Step 5 -- Verify fixings:

Self-drilling screws at 333 mm centres (per trough of trapezoidal sheet):

• Lateral restraint force per screw required: approximately 1.5 kN (from purlin bottom flange LTB check)
• Screw capacity in 1.8 mm S450GD: approximately 2.8 kN in shear (manufacturer data)
• Utilisation: 1.5 / 2.8 = 0.54 -- OK

Design Resources

• UK Portal Frame Design — Complete portal frame to EN 1993-1-1
• UK Steel Warehouse Design — Portal frame warehouse with purlin integration
• UK Wind Load Guide — Wind loading to BS EN 1991-1-4 + UK NA
• UK Snow Load Guide — Snow loading to BS EN 1991-1-3 + UK NA
• UK Steel Framing Cost Guide — Cost data for UK steelwork including secondary steel
• UK Deflection Guide — Serviceability limits for UK steel structures

Frequently Asked Questions

Z-purlin or C-purlin — which should I use for a UK portal frame?

Z-purlins are standard for UK portal frame roofs. Their point symmetry means that when installed with the top flange pointing up the roof slope, the principal axes are approximately aligned with the vertical and horizontal loading directions, minimising weak-axis bending. Z-purlins also lap more efficiently at supports in sleeved systems. C-sections are used for side rails (wall cladding support) where loading is primarily about the weak axis (wind pressure/suction normal to the wall), and for eaves beams where the C-shape provides a natural seating for roof cladding at the eaves-to-gutter junction.

How many anti-sag bars do I need for UK purlins?

UK practice uses the following schedule: 0 rows for spans under 5 m (sheeting alone provides sufficient restraint); 1 row at mid-span for 5.0-7.5 m; 2 rows at third-points for 7.5-10.0 m; 3 rows at quarter-points for 10.0-12.5 m; 4 rows for spans above 12.5 m. Manufacturer load tables specify the exact number required for each purlin section and span. These are not advisory — the published safe load table values assume the stated number of anti-sag bar rows. Omitting anti-sag bars reduces uplift capacity by 40-60% because the bottom flange of the purlin (in compression under uplift) is unrestrained.

What steel grade is standard for UK cold-formed purlins?

S450GD to EN 10346 (minimum yield 450 N/mm^2, continuously hot-dip galvanised to coating Z275) is the standard UK purlin grade. The 450 MPa yield is higher than S355 hot-rolled sections (fy = 355 MPa) because cold roll-forming introduces strain hardening that elevates the yield strength, particularly at the corner radii where plastic deformation is concentrated. S350GD (350 MPa) is used for lighter-duty applications where the higher strength is unnecessary. S550GD (550 MPa) is available for heavily loaded purlins or where weight minimisation is critical. All UK purlin manufacturers stock S450GD as their standard grade.

Do I need to calculate purlin capacity from first principles?

In UK practice, purlin capacity is determined from manufacturer-published safe load tables almost exclusively — not from first-principles calculation. These tables are derived from full-scale structural testing per EN 1993-1-3 Annex A, which captures the complex interaction between the purlin, the attached roof sheeting, and the anti-sag bars. The partial restraint provided by the sheeting (through friction at the fixings and diaphragm stiffness of the profiled sheet) cannot be reliably quantified by calculation. Full-scale testing with the actual sheeting system is the only recognised method for establishing purlin resistance. First-principles calculation to EN 1993-1-3 is used only for non-standard configurations (e.g., purlins supporting solar panels, walkways, or underslung services) that fall outside the scope of the tested configurations. Even then, designers should apply a conservatism factor of at least 1.25 to analytical results to account for the unquantified restraint benefit in the tested configurations.

Educational reference only. All design values are per BS EN 1993-1-3:2006 + UK National Annex, BS EN 1991-1-3, BS EN 1991-1-4, and EN 10346. Purlin safe load tables are manufacturer-specific and must be obtained from the specific supplier (Tata Steel, Voestalpine Metsec, Capital Steel, etc.) for the actual purlin system being specified. Designs must be independently verified by a Chartered Structural Engineer registered with the Institution of Structural Engineers (IStructE) or the Institution of Civil Engineers (ICE). Results are PRELIMINARY -- NOT FOR CONSTRUCTION without independent professional verification.