Rafter Seat Cut, Plumb Cut, and Diagonal Length Calculator

This calculator determines three key measurements for cutting the birdsmouth notch in a roof rafter: the horizontal seat cut, the vertical plumb cut depth, and the diagonal cut length between the seat and plumb lines. By entering the roof pitch and the width of the wall's top plate, framers can lay out precise birdsmouth cuts to fit securely over the plate.

Diagram showing horizontal rafter seat cut on a top plate based on pitch and plate width

It calculates a few more items than the original rafter calculator. It includes the rafter run and rise, length from the top of the 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.

Note:

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.

To calculate the full heel height and seat cut of a rafter, try the Birdsmouth Seat and Heel Height Calculator.
For converting roof pitch into slope degrees, use the Roof Pitch to Angle Calculator.
If you need the total rafter length from run and pitch, see the Common Rafter Length Calculator.

Rafter With Seat Cut Calculator

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

Results:

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

Calculator

  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

Results

  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

Roof Framing

If you have any questions or comments please Contact Us
Privacy Policy
© 1998, VmNet.