Roof run vs rafter length: measure different lines
Run is a horizontal distance. Rafter length follows the sloping line. They share endpoints in the simplified triangle above, but they are not interchangeable inputs. On any positive slope, the diagonal is longer than the horizontal run.
If you enter the sloped board length in a field labeled horizontal run, the calculator will use the wrong triangle. Keep the reference points consistent: outside wall to ridge centerline in this example, without an overhang or ridge deduction.
Three measurements in the diagram
- Run: 12 feet. The horizontal distance from the outside wall to the ridge centerline.
- Rise: 6 feet. The vertical difference between the two ends of that run. This is not the height of the building from the ground.
- Theoretical rafter length: about 13.416 feet. The diagonal joining those reference points, before construction adjustments.
The ratio 6 ÷ 12 is 0.5, so this roof has a 6:12 pitch, a 50% slope and an angle of about 26.57°. You can load this pitch in the calculator, then open Estimate rafter length and enter 12 feet for the full roof run.
Does run mean half the building span?
Only use half the span when your geometry supports it. For a symmetric gable roof with a centered ridge, the idealized wall-to-centerline run is half the building width. A 24-foot span then gives a 12-foot run on each side.
For a single-slope roof or an off-center ridge, measure the horizontal distance for the specific roof section instead. The rafter calculator offers both Building span and Run inputs; building span mode simply divides the entered width by two.
A metric example
A horizontal run of 3.6 m and a vertical rise of 1.8 m also produce a 6:12 pitch. The diagonal is:
√(3.6² + 1.8²) = √16.2 ≈ 4.025 m.
To measure just the slope over a short distance, 150 mm of rise over 300 mm of horizontal run is enough. Those small measurements establish the ratio; the separate 3.6 m roof run scales it to the full roof section.
Where overhang and cut details fit
An overhang goes beyond the outside wall. A horizontal overhang extends the run, while its sloped length is longer whenever the roof has a positive pitch. Do not add a sloped overhang measurement to a horizontal run.
The triangle is a reference for measurement, not a finished cutting layout. Ridge dimensions, birdsmouth geometry, bearing and end cuts change what must be marked on the timber. Confirm those details for the actual plans before using a theoretical length.
References and next steps
The Johnson Level rafter-square guide illustrates the distinction between framing dimensions and layout lines. Our diagram is an original geometric example, not a reproduction of a construction plan.
Use the printable roof pitch chart for angle and slope conversions, or follow the measurement guide to find your rise and run. Our methods page explains rounding and limitations. Contact us if an example or label needs correcting.