Spike's Calculators

Roof Framing

Metric Rafter With Roof Slope in Degrees

This calculator uses metric measurements to calculate 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 heel cut.

Metric Rafter Calculator


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 Metric

Building Width m
Ridgeboard Thickness mm
Roof Pitch °
Overhang mm
Ridge Beam Depth mm
Rafter Depth mm
Seat Cut mm


Roof Pitch Rise in 12 : 12
Run m
Run in Millimetres mm
Rise m
Rise in Millimetres mm
Diagonal Length AB m
Diagonal Length AB Millimetres mm
Rafter Length m
Rafter Length in Millimetres mm
Heel Cut mm
HAP mm
Ridge Height m
Post Height m
Post Height in Millimetres mm


  1. enter the width of the building in metres
  2. the rafter overhang in millimetres
  3. the pitch of your roof in degrees
  4. the thickness of your ridge beam or board in millimetres
  5. the depth of the ridge beam in millimetres
  6. the actual rafter depth in millimetres (a 2 x 12 is the nominal measurement, actual the 12 is closer to 11 1/4)
  7. the length of your seat cut in millimetres


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