An example of Principal rafter and common purlin timber frame configuration with timbers and parts labelled.

Anatomy of a simple timber frame

An example of Principal rafter and common purlin timber frame configuration with timbers and parts labelled.
Principal rafter and common purlin timber frame configuration

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.

Advantages

  • 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"

Disadvantages

  • 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.

Frame parts
  1. Bent – a cross sectional assembly of timbers that are perpendicular to the ridge.
  2. Bay – the area between the two bents.
Timbers List
  1. Principal post – Main posts that connect to principal rafters. Seen here in the corners.
  2. Principal Rafter – Large angled roof timbers that support the common purlins.
  3. 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
  4. Purlin – Horizontal roof timbers that connect the two bents.
  5. Ridge Purlin – Horizon roof timber that runs horizontally along the peak. Seen here with two chamfered edges.
  6. Girt – a horizontal timber along an external wall. See here with one edge chamfered to match the roof pitch.