In Australian commercial construction projects, a "jack rafter" refers to a type of rafter that is shorter than the full length of the roof slope. Jack rafters are used in roof framing and are typically found in hip and valley roof structures. They extend from a hip rafter to the eave or from a valley rafter to the ridge.  

There are two main types of jack rafters:

1. Hip Jack Rafters: These rafters run from the wall plate to the hip rafter, filling in the spaces between common rafters in a hip roof.

2. Valley Jack Rafters: These rafters extend from the valley rafter to the ridge board, filling in the spaces between common rafters in a valley roof.

Jack rafters are essential for providing structural support and completing the roof framework in complex roof designs. They help distribute loads and ensure the roof's stability and integrity.