Key takeaways
- Z-purlins lapped over multiple spans carry 30–40% more load than C-purlins of the same nominal size due to continuity.
- Deflection, not bending stress, often governs size selection on spans above 6 m.
- Steel grade affects the load table — a 200Z15 in S450 outperforms a 200Z20 in S350.
- Anti-sag bridging is required above roughly 5 m span and directly changes which load table applies.
- Always confirm depth, gauge AND steel grade when ordering — the section designation alone does not specify grade.
What governs purlin size
A purlin carries the roof load from the cladding down to the primary frame. The size you need depends on four things: span between primary frames, load (dead load from cladding plus live load from wind, snow and maintenance access), steel grade, and whether the purlin is continuous over multiple spans or simply-supported.
Getting the size wrong in either direction creates problems. Undersizing causes excessive deflection or failure under wind uplift. Oversizing adds weight and cost without structural benefit. The selection process is not guesswork — it follows standard load tables published by steel section manufacturers, cross-checked against national structural codes.
C-purlin versus Z-purlin: choosing the section type first
Before selecting a size, confirm the section type. C-purlins (sometimes called C-channels) are symmetric and simply-supported — each span is independent. Z-purlins are asymmetric and designed to lap over the primary frame, creating continuity that significantly improves the load capacity of the same nominal size.
A Z-purlin lapped over two spans typically carries 30–40% more load than a single-span C-purlin of the same depth and gauge. If your building has frames at 6 m or 7.5 m centres and the roof load is moderate, a 200Z15 (200 mm deep, 1.5 mm gauge) will often outperform a 250C20 (250 mm, 2.0 mm) at lower material cost — purely because of the continuity effect.
The practical trade-off is connection complexity: Z-purlins require a lap connection detail, while C-purlins bolt independently. Specifiers in hurricane or heavy snow regions often prefer the simpler C-connection even at higher material cost, because the connection detail is more predictable under extreme load.
| Section | Spans | Continuity | Typical use |
|---|---|---|---|
| C-purlin | Single-span | None — simply supported | Short spans, heavy load, simple detailing |
| Z-purlin | Multi-span (lapped) | Yes — lap creates continuity | Common spans 5–9 m, standard portal frame roofs |
| Sigma purlin | Multi-span | Manufacturer-specific | Proprietary systems, longer spans, higher loads |
Standard size ranges and what they cover
Purlins are sized by depth (mm) and material thickness (mm or gauge). Common metric ranges run from 140 mm to 300 mm depth in 1.2 mm to 3.0 mm gauge. Imperial sizes follow a different series — 6 in to 12 in depth, 14 to 18 gauge.
The table below shows representative metric sizes and typical span applications. These are indicative — always verify against the load table for your specific load case, steel grade and lap configuration.
| Depth (mm) | Gauge (mm) | Typical span (m) | Notes |
|---|---|---|---|
| 140 | 1.5–2.0 | 3.5–5.0 | Light cladding, short spans |
| 160 | 1.5–2.5 | 4.0–6.0 | Residential and light industrial |
| 200 | 1.5–2.5 | 5.0–7.5 | Standard industrial portal frame |
| 250 | 2.0–3.0 | 6.0–9.0 | Heavy-load or wide-span industrial |
| 300 | 2.5–3.0 | 7.5–12.0 | Long-span or snow country applications |
Steel grade and its effect on size selection
Most structural purlins are produced from S350 or S450 (yield strength 350 or 450 MPa) in metric markets, or ASTM A653 Grade 50 or Grade 80 in North America. Higher-strength steel allows a smaller section to carry the same load — a 200Z15 in S450 can replace a 200Z20 in S350 without changing depth.
Lower-strength steel (S235 or Q235) is sometimes used for non-structural applications such as secondary framing in cladding systems, but should not be substituted for structural purlins without recalculating the load tables. Always confirm the steel grade when ordering, because the section designation alone (200Z15) does not specify grade.
Deflection limits and their practical effect on size
Structural codes limit purlin deflection under service load — commonly span/200 under combined dead and live load, or span/150 under live load alone. For long spans or brittle cladding materials (certain composite panels, plasterboard ceilings), span/300 may be required.
Deflection, not bending stress, often governs the size selection on spans above 6 m. A section that passes the stress check at 7.5 m span may still need to move up one size to satisfy the deflection limit. If the design is governed by deflection rather than strength, a deeper but thinner section (higher second moment of area) is more efficient than a thicker but shallower one.
Anti-sag and bridging requirements
On spans above roughly 5 m, purlins are prone to lateral-torsional buckling (LTB) — the unrestrained compression flange can bow sideways before the section reaches its full bending capacity. Anti-sag rods or bridging channels are fitted between purlins at mid-span or third-points to brace the bottom flange.
The presence or absence of bridging directly affects which load table applies. Load tables are usually published for three conditions: unrestrained, one line of bridging at mid-span, and two lines at third-points. Jumping from an unrestrained to a one-bridgeline table can increase the allowable load by 20–40% for the same section. Confirm with your engineer which table applies to your building design before sizing.