The principal rafter and common purlin system
The first in a series of articles exploring some common types of timber frame configurations. Considered a modern style of timber framing, the principal rafter and common purlin system has several advantages that beginner timber framers will appreciate. The joinery is, on a whole, simpler than in a common rafter system. Purlins are typically shorter than the common rafters required and are consequently easier to acquire from small sawmills. Likewise shorter timbers are typically less expensive per board-foot. Though professional framers will typically assemble and fly-in large bents with a crane it is possible to create "drop-in" joints more suitable to hand raising.
- Bents can be fit-up on the ground rather than in the air
- Uses shorter timbers.
- Purlin joinery is simpler than birds-mouth joinery typical in a common rafter system
- Some believe it provides and a more "open feeling"
- Large bents require a crane to raise
- Lap joints cut in the top of principal rafter reduces rafter strength
Parts of the Frame
In this simple frame there are two bents, that form a single bay in between. Principal posts are held together by by a tie beam whose joint is in tension*1. Principal Rafters connect directly to Principal Post and support common purlins. Girts connect the posts within along exterior wall-line.
- Bent – a cross sectional assembly of timbers that are perpendicular to the ridge.
- Bay – the area between the two bents.
- Principal post – Main posts that connect to principal rafters. Seen here in the corners.
- Principal Rafter – Large angled roof timbers that support the common purlins.
- Tie Beam – Horizontal beam that ties the outer posts together and resists the tendency of the posts to spread caused by the outward thrust created at the rafters
- Purlin – Horizontal roof timbers that connect the two bents.
- Ridge Purlin – Horizon roof timber that runs horizontally along the peak. Seen here with two chamfered edges.
- Girt – a horizontal timber along an external wall. See here with one edge chamfered to match the roof pitch.