Rafter calculator — length, pitch, overhang and cut geometry.
Calculate the geometric relationship between run, rise and rafter length, then connect that rafter to the rest of the roof instead of treating it as an isolated number.
Calculate a common rafter in seconds.
For a symmetrical gable roof with a centered theoretical ridge. Enter the building span, roof pitch and horizontal overhang.
Inputs for a useful rafter calculation
- horizontal run from the support line to the ridge reference,
- roof pitch or rise,
- overhang,
- rafter section,
- ridge detail,
- support position and notch information.
Geometric length vs workshop length
The theoretical line length is not always the final length you mark on timber. A ridge board, actual support position, overhang and end-cut detail can shift the points from which the physical element is measured. That is why a framing workflow needs clear measurement references instead of one unlabeled number.
Pitch and rafter angle
If rise and run are known, the roof angle is:
angle = arctan(rise ÷ run)
The same slope controls the plumb-cut orientation and the relationship between the horizontal bearing surface and the rafter.
Overhang
An overhang measured horizontally is not equal to the sloping length added to the rafter tail. The tail extension follows the roof slope, so its real length must be calculated on that slope.
From one rafter to a complete roof
Once the common-rafter geometry is correct, the same project data can be used for spacing, openings, workshop cards, cutting lists and 3D verification. This is where a full framing application becomes more useful than a standalone calculator.
Rafters in Krokwio
Krokwio connects the rafter with its support points, notches, end geometry, 2D/3D representation and element card. If an opening interrupts the member, the workshop list can work with the real rafter segments rather than an imaginary full-length piece.
Related: Roof Pitch Calculator, Ridge Height Calculator, Birdsmouth Cut Calculator.
Turn roof geometry into workshop-ready information.
Use one project for geometry, 2D/3D views, element cards, cutting data, PDF documentation and QR identification.
Rafter length example: horizontal run versus slope length
A roof pitch of 35° and a horizontal run of 4 m give a theoretical rafter reference length of 4 / cos(35°) ≈ 4.88 m before adding overhang or detailing end cuts. An overhang measured horizontally cannot simply be added as the same number of metres along the slope: for 0.4 m horizontal projection, the corresponding slope extension is approximately 0.488 m at 35°.
Specify whether the input run is measured from the wall face, support centerline or another reference. Real cutting length depends on the ridge detail, seat cut and eaves termination.
More calculators for roof carpenters
Roof pitch calculator · Ridge height · Birdsmouth cuts · Roof framing calculator. Explore Krokwio PRO.