Purlins are usually connected to the main portal frame with cleats. In cold rolled steel sheds the Cleats are bolted to the Frame, with purlins bolted to the cleats.

The cleats and purlins holes or punching are generally done to the AISC Standardised Structural Connections with most fabricators/manufacturers working within the standards. Bolts connecting purlins to cleats and frames are usually M12 Grade 4.6 done up to a snug tightening. The supplier of your purlins can usually supply you with the appropriate bolts as described in your engineering.

Holes punched or drilled in your purlins, cleats, columns & rafters are generally oversized to give clearance to make for easier assembly. Typically you may find 18mm holes for your 12mm diameter bolt. It’s common for manufacturers to produce 18mm by 22mm slots. These tolerances benefit assembly but do not affect structural performance. These extra clearances are especially useful when lapping Z purlins Zeds.

When placing Cee purlins into a roof the purlin should be bolted to the top side face of the cleat with the open C facing up the hill.

lysaght cee c purlin girt roof sample layout

Image shows C purlins in a roof installed with the cleat to the down slope side and the open c section facing up slope


With Zeds or Z section the purlin should be bolted to the lower side of the cleat with the upper flange facing up the hill. This is to minimise rotation of purlins between portals and bridging.

lysaght zed z purlin girts sample roof layout

Drawing shows the purlin fixed to the lower side of the cleat with the top flange directed up the slope.


Purlin fixing is really a quite straight forward process however you should note… that they are quite flexible as can be the frame as a whole until bracing, bridging and cladding are added. Ideally, these should be added progressively as the purlins are added. loading up of the structure should be avoided until these are in place other wise distortion or even failure can occur. Bundles of roof sheeting should not be placed on unsheeted purlins, as this can cause overloading and result in permanent deformation of the sections.