Because of these factors, the unbraced C and Z sections tend to twist and to become unstable even under gravity loading on a perfectly horizontal roof. In sloped roofs, the purlin web is tilted from ...

Cold-formed C and Z purlins are the workhorses of the industry ... Purlin spacing is dictated by the load-carrying capacity of the roof panels; a 5-ft spacing is common. Appendix B includes section ...