How do I choose the right purlin size for a metal roof?

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.