Unrestrained Purlin System
The Unrestrained Purlin System caters for the wide range of standing seam and secret-fix cladding systems now available that do not provide adequate lateral restraint to the top flange of the purlins, it is necessary to replace standard sag bars with angle strut braces. Used in conjunction with rigid apex ties in duo-pitched roofs and appropriate diagonal bracing in mono-pitch roofs, allowable loads for all purlin systems are given in the load tables. Where a liner tray capable of providing adequate lateral restraint to the purlin top flange is used in conjunction with standing-seam cladding the standard sleeved system can be used.
With non-restraining cladding sag bars must be replaced with rigid-fix struts in order to restrain the top and bottom flange.