Spike's Calculators

Roof Framing

Rafter With Seat Cut

This calculator calculates a few more items than the original rafter calculator. It includes the rafter run and rise, length from top of rafter to the plumb line, overall rafter length, height above the plate (HAP), height of ridge above the top plate, ridge board (beam) support, and to complete the birdsmouth, the length of the seat cut.

Rafter With Seat Cut


Diagonal length A to B is measured from where the rafter attaches to the ridge board to outside of the wall, the location of your plumb line used for the seat and heel cuts.

Rafter With Seat Cut Calculator

Building Width ft in n
Overhang in n
Roof Pitch : 12
Ridgeboard Thickness in n
Ridgeboard Depth in n
Rafter Depth in n
Seat Cut in n


Slope Factor: #
Run ft in
Run in Inches in
Rise ft in
Rise in Inches in
Diagonal Length AB ft in
Diagonal Length AB in Inches in
Rafter Length ft in
Rafter Length in Inches in
Heel Cut in
HAP in
Ridge Height ft in
Post Heightft in
Post Height Inches in


  1. enter the width of the building in feet and inches
  2. the rafter overhang in inches
  3. the pitch of your roof
  4. the thickness of your ridge beam or board in inches
  5. the depth of your ridge beam or board in inches
  6. the actual rafter depth in inches ( as in, a 2 x 12 being 11 1/4")
  7. the length of your seat cut in inches


  1. the slope factor
  2. the run in feet and inches
  3. the run in inches
  4. the rise in feet and inches
  5. the rise in inches
  6. A to B length (length of rafter minus overhang) in feet and inches
  7. A to B in inches
  8. length of rafter including overhang in feet and inches
  9. rafter length in inches
  10. the heel cut length based on the seat cut in inches
  11. the height above plate measurement in millimetres (HAP)
  12. the ridge height (top of plate to top of the ridge) in feet and inches
  13. post height (ridge beam support) in feet and inches
  14. post height in inches
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