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How to calculate rafter length from span and roof pitch
A rafter follows the slope of the roof. Its length is therefore greater than the horizontal distance from the wall to the ridge. You can find that length from the horizontal run and either the pitch angle or the rise. The calculator uses a right triangle; it does not turn a geometric result into a finished cutting list.
How to use the calculator
- Choose a symmetrical gable roof or a mono-pitch roof. Enter the full horizontal span between supports, not the length of the sloping timber.
- For a symmetrical gable, the calculator divides the span by two. For a mono-pitch roof, it uses the entire span as the horizontal run R.
- Choose span and angle if you know the roof pitch in degrees. Choose span and rise if you have measured the vertical height H from support level.
- Enter the horizontal overhang E separately, select a consistent unit, and calculate. Read both the rafter body and total length before marking timber.
Body length = R / cos(α). Total length = (R + E) / cos(α). With rise instead of angle: body length = √(R² + H²).
Worked example
For a symmetrical roof with an 8 m span and a 3 m rise, R = 4 m. The body length is √(4² + 3²) = 5 m. The pitch is about 36.87°. A horizontal overhang of 0.40 m adds 0.50 m along the slope, giving a theoretical total length of 5.50 m.
Open the calculator: Rafter length →
Measurement mistakes to avoid
Do not enter the full span as the run for one side of a symmetrical roof. Do not confuse degrees with percentage pitch: 100% means 45°. An overhang measured along the timber is not the horizontal overhang requested by the form.
Before cutting or ordering
Ridge thickness, birdsmouth geometry, support details and joints can change the cutting dimensions. Verify the construction drawing before cutting. This calculator does not check timber strength or size structural members.
These formulas apply to a straight common rafter and an ideal right-triangle roof section. Hip, valley and curved rafters need a different model.