The Prow Rafter Roof Framing Angles calculator will calculate the rafter length and angles for the prow rafter plan.

Input variables

- Roof Pitch or Angle
- Prow Rafter Run
- Prow Ridge

Roof Pitch or Angle

Select the input type from the input menu, Imperial/English Inch base 12 —— default, enter roof pitches from 1—24
like 8 for an 8:12 pitched roof, Metric 300mm base, enter millimeters like 200 for a 33.69007° roof pitch angle,
Decimal Inch base 10 —— enter base 10 decimal numbers like 5.7735 for a 30° roof pitch angle,
Degrees input —— use any roof angle in degrees like 33.69007°

- Length/Dimensions Input Menu Mode
- Imperial/English Inch base 12 —— default.. use American standard pitches over 12.
- Metric 300mm base —— 300mm is 11.81102 inches, similar usage to the American framing squares printed in inches.
- Decimal Inch base 10 —— use this option when working with tangents or using the Chappell Master Framing Square multiply the tangent of the angle by 10

The output mode selection can be completely different from the input mode

- Length/Dimensions Output Menu Mode
- Imperial/English Inch base 12 —— default.. use American standard pitches over 12.
- Metric 300mm base —— 300mm is 11.81102 inches, similar usage to the American framing squares.
- Decimal Inch base 10 —— use this option when working with tangents or using the Chappell Master Framing Square

Prow Rafter Run

This menu option allows you to set the prow rafter run used to calculate the angles and dimensions of the
roof.

Prow Ridge Length

This menu option allows you to set the length of the prow ridge. See the second drawing below for an
example of the prow ridge length calculated from the common rafter run and the common rafter overhang.

- Math to calculate the prow rafter angle
- prow plan angle = arctan(common rafter run ÷ ridge)
- prow eave angle = 90° + arctan(ridge ÷ rafter run)
- prow rafter angle = arctan(tan(common rafter angle) * sin(prow plan angle))
- prow rafter run = common rafter_run ÷ sin(prow plan angle)
- prow rafter length = prow rafter run ÷ cos(prow rafter angle)
- prow rafter miter angle = prow rafter angle
- prow bevel angle = arctan(ridge ÷ common rafter run)