Purlins help to form and strengthen the roof framing in a structure, and they support the roof decking or sheathing. They run horizontally, parallel with the ridge of the timber frame. There are three basic types of purlins: purlin plate, principal purlin, and common purlin.
Purlin plates support the rafters near the middle of their span, and are supported by posts. They allow for longer spans and wider structures than the rafters could support without purlins. Principal purlins support common rafters and are connected to principal rafters. Common purlins pass over the top of principal rafters and are intended to carry the roof sheathing.
Where the common purlins cross over principal rafters, we show here a nice technique to secure the connection and lessen the height above the rafter. Called a notched purlin, it is made by cutting out seats on both sides of the rafter. Then the purlin is notched out similar to a lap joint, all the way through to the same depth as the cut on the rafter. This allows it to settle down into the cutouts on the rafter. This joint is further secured with structural screws.