How do I choose the right purlin size for a metal roof?
Design & Analysis
Purlin sizing depends on four things: the purlin spacing, the span between rafters, the design loads (dead + live + snow + wind), and the deflection limit. For typical industrial metal roofs with 1.5-2.0 m purlin spacing and 5-6 m spans, a cold-formed C-profile like 150×50×15×2.0 mm or 200×75×20×2.5 mm is common. Start by calculating the tributary load on each purlin (load per m² × spacing), then compute the bending moment (wL²/8 for simply supported), and check the profile's moment capacity per NBR 14762 / AISI S100. In CalcSteel, you can model the purlin as a single beam, apply the distributed load, run analysis, and let the software pick the lightest profile that passes verification.
Key takeaways
- Modern purlin sizing rests on cold-formed steel theory pioneered by George Winter at Cornell, codified in the first AISI specification in 1946.
- A purlin must pass several distinct checks - bending, the wind-uplift case, distortional and lateral-torsional buckling, shear, and deflection - not a single 'strength' number.
- Roof sheeting partially restrains the purlin; standards capture this with R-factors (AISI S908) or rotational-restraint procedures (Eurocode 3, EN 1993-1-3 clause 7.3).
- Software like CalcSteel automates the iterative effective-width and buckling math, but the engineer still owns the load case and serviceability limits.
Full guide
How to Size a Purlin for a Metal Roof
Step-by-step with figures and sources — 8 min read
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