Rafter Cutting Sheet
Work the line length out, apply the deductions, cut ONE as a pattern. Then check it on the building before cutting the rest.
BFCBrilliance · bfcbrilliance.com/tools/rafter-length-calculator
The roof
- Job
- Span
- Run (half span)
- Pitch
- Rise
- Line length
- Overhang along slope
- TOTAL RAFTER
- Ridge thickness
- Deduct half
Common pitches — slope factor and rise per foot of run
| Pitch | Slope factor | Rise per ft of run | Roof area vs footprint |
|---|---|---|---|
The arithmetic
- Run
- span / 2 (standard gable)
- Rise
- run x pitch / 12
- Line length
- square root of (run² + rise²)
- Slope factor
- square root of (1 + (pitch/12)²)
- Horizontal to sloped
- multiply by the slope factor
- Roof area
- footprint area x slope factor
- 4/12 slope factor
- 1.054 — about 5% more area
- 12/12 slope factor
- 1.414 — over 40% more area
- ⚠ Ridge deduction
- half the ridge board thickness, per rafter
Before cutting the whole stack
- SPAN entered, not the run — the run is half of it
- Half the ridge board thickness deducted
- Birdsmouth seat cut marked and its effect understood
- Overhang converted to slope length if you wanted it horizontal
- ONE rafter cut and test-fitted on the building as a pattern
- Pattern used to mark all the others — never re-measure each one
- Timber size, grade and spacing checked against local code
- Crown of each board marked and turned up before cutting
This is a line length, not a cut length
The figure runs from the CENTRE of the ridge to the outside of the wall. Two rafters and a ridge board cannot share the point where the theoretical lines meet, so half the ridge thickness comes off each side — 0.75 inches for a 1.5 inch ridge. Cut one, fit it, and only then cut the rest.
Roof framing is code work
Span, timber size, grade and spacing are all governed by load and local code, and none of that is arithmetic this or any calculator can supply. This gives you the geometry. Whether a given rafter is strong enough for the span, the snow load and the timber you are holding is a different question with a legal answer.