Traditional Cut Purlin Roof

For example an 8 x 4 under purlin would support the center of a row of 6 x 2 rafters that in turn would support 3 x 2 roof purlins to which the roof cladding was fixed.

Traditional cut purlin roof.

A common arrangement is a rafter and purlin system which combines purlins and rafters. The rafters are the main load bearing elements of the roof. A traditional cut timber roof incorporating a collar for lateral restraint purlins for rigidity and struts to distribute some of the load to an internal load bearing wall. From the 1970s onwards most low rise domestic pitched roofs have been formed with trussed rafters.

The roof support system is a ridgepole and either rafters which run perpendicular from the top of the walls to the ridge or purlins which run parallel to the ridge. This roof shown on a previous page is a good example of a traditional cut roof with dormers. They span from the wallplate to the ridge board providing a platform for the underlay battens and tiles. It has a timber purlin on each slope built into the gable ends.

In traditional timber truss construction purlins rest on the principal rafters of the truss. In the left hand image you can see the overall layout of the roof.