Rafter Lengths Calculator

Calculate the length of a rafter based on the width of the building (room width), the width of the beam and the pitch of roof or height of ridge above plate.

This calculator assumes a gable roof with a specified ridge height above the top plate of the exterior walls.

Follow the next easy steps and get your result:

  • Find the room width by measuring the horizontal distance across the building from one outer wall to the opposite outer wall. This is also known as the building width.
  • Insert the width of timber used for the ridge.
  • Roof Pitch: The pitch is the slope of the roof and is given as an angle (e.g., 30 degrees).
  • If you don’t know the roof pitch then enter the height of ridge above plate. To determine this just measure the vertical distance from the top plate of the exterior wall to the ridge of the roof.
Room width
Ridge width
Pitch of Roof
Height of Ridge Above Plate
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