Rafter Calculator Guide
Understand how roof pitch, span, and rise combine to determine rafter length, and how common, hip, and valley rafters differ in a roof frame.
How Rafter Length Is Worked Out
A rafter calculator turns three basic roof measurements — span, rise, and pitch — into the exact length of timber needed to frame the roof, plus any overhang required at the eaves.
Span, Run, and Rise
Span is the total horizontal width of the roof, run is half the span for a symmetrical roof, and rise is the vertical height from the wall plate up to the ridge. These three measurements form the basis of every rafter calculation.
Roof Pitch
Pitch describes the steepness of the roof, usually expressed as a ratio of rise to run or as a degree angle. A steeper pitch means more rise for the same run, which in turn means a longer rafter.
Calculating Rafter Length
Using the Pythagorean theorem, the run and rise form the two shorter sides of a right-angled triangle, with the rafter itself as the hypotenuse. Any eaves overhang is then added to this core length.
Marking The Cuts
Once length is known, the plumb cut at the ridge, the birdsmouth notch at the wall plate, and the tail cut at the eaves are marked out using the roof’s pitch angle to ensure each rafter sits correctly.
Rafter Quick Facts
Essential insights for roof framing
Rafter Types At A Glance
A quick overview of the main rafter types used in pitched roof construction and how they differ.
| Rafter Type | Runs Between | Cut Complexity | Typical Use |
|---|---|---|---|
| Common Rafter | Ridge board to wall plate, at right angles. | Simple plumb and level cuts. | Standard gable or straight roof sections. |
| Hip Rafter | Ridge end to an external roof corner. | Compound angle cuts. | Hipped roofs with sloped ends. |
| Valley Rafter | Ridge to an internal roof corner. | Compound angle cuts. | Where two roof sections meet, such as an L-shaped building. |
| Jack Rafter | Hip or valley rafter to wall plate. | Simple cuts at one end, angled at the other. | Filling in around hip or valley rafters. |
Rafter Calculator FAQ
Answers to the most frequently asked questions about calculating rafter length and roof framing.
Run is the horizontal distance from the outside wall to the centre ridge point, span is the full horizontal width of the roof (twice the run for a symmetrical roof), and rise is the vertical height gained from the wall plate to the ridge.
Rafter length is generally found using the Pythagorean theorem, treating the run and the rise as the two shorter sides of a right-angled triangle and the rafter itself as the hypotenuse, then adding any overhang for the eaves.
A birdsmouth cut is a notch cut into the underside of a rafter where it crosses the wall plate, allowing the rafter to sit flush against the plate and wall for a secure, stable bearing point.
Common rafters run straight from the ridge to the wall plate at a right angle. Hip rafters run diagonally to an external corner where two roof slopes meet, while valley rafters run diagonally to an internal corner, so both are typically longer than common rafters and cut at compound angles.
Yes. A steeper pitch increases the rise for the same span, which lengthens each rafter and increases the total amount of timber needed compared with a shallower pitch roof of the same footprint.