Purlin Types for UK Construction

Loading — BS 6399-3 and BS EN 1991-1-4

UK roof purlins resist vertical loads (cladding self-weight 0.10-0.25 kN/m^2, insulation, services, snow load to BS EN 1991-1-3, access load 0.6 kN/m^2 over 1 m^2 per BS 6399-3 Clause 6.3) and wind uplift. Per BS 6399-2, external pressure coefficient C_pe for a duo-pitch roof ranges from -0.6 (windward) to -1.3 (leeward, steep pitch). For a Manchester-area site at 25 m altitude: q_s = 0.613 times 23^2 = 324 Pa, design suction approx 0.8-1.2 kN/m^2.

Cold-Formed Purlin Design — BS 5950-5

Major Axis Bending

For a simply supported purlin of span L with factored load w_Ed: M_y,Ed = w_Ed times L^2 / 8. The bending resistance M_cy,Rd depends on whether the compression flange is restrained by the roof cladding. With trapezoidal steel cladding fixed at every trough (300-333 mm spacing), the flange is fully restrained.

Minor Axis Bending

Roof slope theta introduces: M_z,Ed = w_Ed times L^2 / 8 times sin(theta).

Combined Bending — BS 5950-5 Clause 6.5

M_y,Ed / M_cy,Rd + M_z,Ed / M_cz,Rd <= 1.0

Anti-Sag Rod Design

Anti-sag rods restrain the purlin compression flange against lateral-torsional buckling under wind uplift reversal. Per SCI P341: rod sizing typically 12-16 mm diameter S275 round bar at mid-span or third-points. Rod force: P_rod = 0.02 times w_Ed times L (2% rule per BCSA guidance). For w_Ed = 1.5 kN/m and L = 6.0 m: P_rod = 0.18 kN. A 12 mm S275 rod (T_r = 23.3 kN) is more than adequate. Rods must be anchored to the ridge purlin or a purpose-made cleat at the eave.

Deflection Limits for UK Purlins

Per BS 5950-1 Table 8 and SCI P341:

Worked Example 1 — Cold-Formed Z Purlin

Problem: Design cold-formed purlins spanning 6.0 m at 1.8 m centres. Roof pitch 6 degrees. Cladding: 0.7 mm trapezoidal steel (0.10 kN/m^2). Insulation: 0.05 kN/m^2. Snow: 0.5 kN/m^2. Wind suction: -0.8 kN/m^2. Steel grade S280GD+Z.

Step 1 — Loads: Dead G = (0.10 + 0.05) times 1.8 = 0.27 kN/m. Snow S = 0.5 times 1.8 = 0.90 kN/m. Gravity ULS: w_Ed = 1.35(0.27) + 1.5(0.90) = 1.715 kN/m.

Step 2 — Moments: M_y,Ed = 1.715 times 6.0^2 / 8 = 7.72 kN.m. M_z,Ed = 7.72 times sin(6 deg) = 0.81 kN.m.

Step 3 — Try Tata Steel Multibeam 175 Z 1.6 mm (Z17516): W_el,y = 45.3 cm^3, W_el,z = 10.3 cm^3, f_yb = 280 MPa.

M_cy,Rd = 45.3 times 10^3 times 280 / 1.0 = 12.68 kN.m. M_cz,Rd = 10.3 times 10^3 times 280 / 1.0 = 2.88 kN.m.

Interaction: 7.72/12.68 + 0.81/2.88 = 0.609 + 0.281 = 0.890. OK.

Step 4 — Deflection: w_SLS = 1.17 kN/m. delta = 23.6 mm < L/200 = 30 mm. OK.

Step 5 — Wind Uplift: M_y,Ed (uplift) = 8.63 kN.m. The unrestrained bottom flange must be checked for lateral-torsional buckling. For Z17516 with L_e = 6000 mm, M_b,Rd approx 8.9 kN.m > 8.63 kN.m. OK.

Selected: Multibeam 175 Z 1.6 mm (S280GD+Z) at 1.8 m centres. Anti-sag rods 12 mm dia S275 at mid-span.

Worked Example 2 — Hot-Rolled Angle Purlin

Problem: UK industrial building, 5.0 m span, equal angle purlins at 1.5 m centres. Roof pitch 10 degrees. Fibre cement cladding (0.20 kN/m^2). Snow 0.6 kN/m^2. Steel grade S275.

Step 1 — Loads: G = 0.20 times 1.5 = 0.30 kN/m. S = 0.6 times 1.5 = 0.90 kN/m. w_Ed = 1.35(0.30) + 1.5(0.90) = 1.755 kN/m.

Step 2 — Try 100x100x8 EA: M_y,Ed = 1.755 times 5.0^2 / 8 = 5.48 kN.m. Z_el = 20.1 cm^3. M_cy,Rd = 5.53 kN.m. Utilisation = 0.991. Marginal.

Step 3 — Upgrade to 100x100x10 EA: Z_el = 24.6 cm^3. M_cy,Rd = 6.77 kN.m. Utilisation = 0.809. OK.

Step 4 — Deflection: I_y = 174 cm^4 (100x100x10 EA). delta = 26.7 mm > L/240 = 20.8 mm (fibre cement limit). FAIL.

Step 5 — Upgrade to 120x120x10 EA: I_y = 307 cm^4. delta = 15.1 mm < 20.8 mm. OK.

Selected: 120x120x10 EA (S275) at 1.5 m centres. Sag rods 16 mm dia S275 at mid-span.

Angle purlins are less efficient than cold-formed Z or C sections for the same mass. Specify only where hot-rolled sections are required: high-temperature environments, traditional construction, or robustness requirements.

Purlin-to-Rafter Connection Design

UK purlin connections typically use:

1. Cold-formed cleats: 90x90x8 EA angle cleats bolted to rafter web with 2-M16 Grade 8.8 bolts.
2. Hot-rolled angle purlins: Seat angle cleats (90x90x8 EA) with 2-M16 bolts to rafter and purlin.
3. Sleeved systems: Proprietary sleeve connectors for multi-span continuity. Sleeve length = 10-15% of span per manufacturer specification.

FAQ

What is the difference between Z and C purlins? Z-sections allow lapping at supports for moment continuity (15-25% span capacity increase). C-sections are for single-span applications (eaves, ridges) or sleeved systems.

When are anti-sag rods required? When wind uplift produces compression in the unrestrained flange. For Z-sections spanning > 4.0 m, sag rods at mid-span are standard UK practice. For heavily loaded purlins, use third-point sag rods.

What steel grade for UK cold-formed purlins? S280GD+Z (280 MPa yield, zinc-coated) and S350GD+Z (350 MPa) per BS EN 10346. S280GD+Z is standard stock; S350GD+Z provides 25% strength increase but may require minimum order quantities.

How to handle eave and ridge purlins? Eave purlins carry higher wind suction (C_pe up to -2.0 at the eave per BS 6399-2) plus gutter and fascia loads. Typically one section size up or at reduced spacing. Ridge purlins anchor the anti-sag rod system.

Can I use Tata Steel load tables instead of hand calculations? Yes — for standard spans, spacings, and roof slopes. The tables incorporate BS 5950-5 design checks. For unusual loading (heavy cladding, high-altitude snow, walk-on access), project-specific design is required.

What is the typical purlin spacing for a UK portal frame? Standard spacings are 1.5-2.0 m for 0.7 mm profiled steel cladding, 1.2-1.5 m for fibre cement, and 1.5 m for built-up insulated panel systems. The cladding manufacturer's span tables govern the maximum purlin spacing.

Purlin Lapping and Continuity — UK Best Practice

Z-section purlins achieve greater spanning efficiency through lapping at the supports. The lap length directly affects the moment capacity over the intermediate support:

Lap length = 10% of span is the traditional UK rule of thumb per SCI P341. The lap must be bolted with at least 2-M12 Grade 8.8 bolts through both purlin webs within the lap zone. For multi-span purlin runs, alternate the lap direction at each support to avoid cumulative fit-up errors.

Double-Span vs Single-Span Economics

For a typical UK portal frame at 6.0 m bay centres:

• Single-span C purlins: 6 x 200 Z 2.0 mm at 1.8 m centres, no laps. Simpler fabrication, 10-15% heavier.
• Double-span Z purlins: 12 x 175 Z 1.6 mm at 1.8 m centres, 500 mm laps. 25-30% lighter steel weight, higher erection labour.

The total installed cost (material + labour) typically favours double-span Z purlins for spans > 5 m and runs of 6+ purlins. Single-span C sections are preferred for short runs, irregular bays, and repair/retrofit work where lap complexity is not justified.

Corrosion Protection for UK Purlins

UK building practice requires specific corrosion protection based on the internal environment classification per BS EN ISO 12944:

Standard UK cold-formed purlins are supplied with Z275 (275 g/m^2 zinc coating) which is adequate for internal C1-C2 environments. For external canopies, unheated warehouses, and agricultural buildings, specify Z350 or Z450 with an organic topcoat. The cut ends of purlins are unprotected — specify touch-up zinc-rich paint applied on site per manufacturer instructions.

Related Pages

• UK Cold-Formed Steel Design — BS 5950-5
• UK Wind Load Calculation — BS 6399-2
• UK Snow Load — BS EN 1991-1-3 UK NA
• UK Beam Design — BS 5950 Flexural Members
• UK Steel Properties — fy for Cold-Formed Grades
• UK Deflection Limits — Serviceability Criteria
• UK Portal Frame — Industrial Building Design
• UK Stub Column — EN 1993-1-1 Axial Capacity
• UK Web Bearing & Buckling — EN 1993-1-5
• UK Anchor Bolts — Holding-Down Design
• Beam Calculator — SFD, BMD & Reactions

Educational reference only. Purlin design per BS 5950-5:1998 and SCI P341. Verify against current Tata Steel purlin load tables. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent Chartered Engineer verification.