A jack rafter is a shortened rafter that frames into a hip or valley rafter rather than running the full distance to the ridge, used to complete the roof surface adjacent to these diagonal structural members in a hip or complex roof design.
Because jack rafters vary in length depending on their position along the hip or valley, with each one progressively shorter or longer than its neighbor, calculating and cutting these members accurately requires careful attention to the specific geometric progression involved.
This variation in length is a defining characteristic of jack rafters, distinguishing them from common rafters that are all cut to the same length throughout a simple gable roof.
What Does a Jack Rafter Involve?
- Progressively varying lengths along a hip or valley run
- Compound angle cuts where framing into the hip or valley rafter
- Standard angle cuts at the wall top plate connection
- Consistent spacing matching the roof’s overall framing pattern
Why Are Jack Rafters Important?
Jack rafters complete the roof surface in the areas adjacent to hips and valleys, and their progressively varying lengths, if miscalculated, can create an uneven or improperly aligned roof surface that complicates sheathing installation and compromises the finished roof’s appearance and performance. Getting this calculation right requires understanding the specific geometric relationship between rafter position and length along the hip or valley run.
Given this complexity, many carpenters use specialized framing squares or modern calculation software specifically designed to determine jack rafter lengths and cutting angles accurately, reducing the risk of the errors that manual calculation might otherwise introduce.
Jack Rafter vs. Common Rafter
A jack rafter is shortened and frames into a hip or valley rafter rather than reaching the ridge directly. A common rafter runs the full, consistent distance from wall plate to ridge, found throughout simpler roof sections without hip or valley complications.