Jack rafter calculator — lengths, spacing and hip-roof layout.
Jack rafters shorten progressively as they meet the hip. Their calculation should follow the actual spacing and hip geometry rather than a disconnected list of guessed lengths.
What determines jack-rafter length?
- roof pitch,
- distance from the corner or ridge reference,
- rafter spacing,
- hip-rafter position,
- overhang and tail geometry,
- the exact cut reference at the hip.
Why spacing matters
Each jack rafter sits at a known position along the wall or ridge. If those positions are generated from a spacing rule, the plan run of every jack can be calculated from its intersection with the hip line. The sloping length then follows from the roof slope.
Do not treat all jacks as one generic element
Every jack can have a different length even when the section and roof pitch are identical. For workshop preparation the element IDs should therefore distinguish each unique member or clearly group only truly identical pieces.
Jack rafters and openings
On a four-slope roof, an opening can intersect common rafters or jack rafters depending on its position. The framing model should determine the actual collision instead of assuming the opening always affects the same member type.
Jack rafters in Krokwio
Krokwio generates jack rafters as part of the hip-roof geometry and keeps them connected to the 2D/3D model, element list and workshop workflow.
Related: Hip Rafter Calculator, Hip Roof Calculator.
Turn roof geometry into workshop-ready information.
Use one project for geometry, 2D/3D views, element cards, cutting data, PDF documentation and QR identification.