Purlins are horizontal members that are supported by struts. The purpose of purlins is to provide intermediate support for the roof. The struts transfer the load from the struts to either a load-bearing wall or a specially designed joist.
Diagram D43 - Purlin location
Purlin spans are to be perpendicular to the plane of the rafter; it is incorrect for the purlin to be placed vertically. Usually, collars or ceiling ties are included also to contribute to the performance of the roof structure. If purlins are placed in the vertical, they should be appropriately designed by an engineer. The engineer should specify the size, span and strength class for the purlin along with any fixings, support, and horizontal ties.
Diagram D44 - Detail A - Rafters to purlin connection birdsmouth detail
If the purlin is being fixed vertically, the rafter should be birdsmouthed as detailed above.
Diagram D45 - Purlin connections
Joint must be supported directly beneath; the purlins should be cut to suit strut positions. Ensure purlins are propped at regular intervals.
The span of a rafter is its length on the slope. Unless a ceiling joist is specifically designed to carry the load, you should never prop a purlin off a ceiling joist.
Diagram D46 - Propping purlins
The diagram above shows two options for a propping timber purlins:
Detail showing a purlin load being transferred through a strut to a load-bearing wall. Hangers not included in sketch for clarity. Note that as the purlin is fixed vertically, the rafters are birdsmouthed.
A specially designed timber beam used to support struts. This could also be propped onto a steel beam.Please note the 25mm clearance between the timber beam and the ceiling , this is done by packing at the wall plate and at any load bearing partitions.
Note: Hangers not included in sketch for clarity.