Rafter Length Calculator
A rafter's actual length is its horizontal run plus overhang, multiplied by the roof factor for your pitch. This calculator does that conversion and rounds up to the nearest length lumber yards stock.
The worksheet
rafter length calculatorActual rafter length ≈ 14.53 ft; buy a 16-ft board.
| Run | 12 ft | |
| Pitch ratio | 6:12 (26.57°) | |
| Roof factor | 1.118× | √(1 + (rise/12)²) |
| Overhang (horizontal) | 12 in (1.00 ft) | |
| Rafter length = (run + overhang) × factor | 14.53 ft | |
| Total rise over the run | 6 ft | |
| Stock board to buy | 16 ft |
Framing hardware for rafters, trusses & joists
Actual rafter length ≈ 14.53 ft; buy a 16-ft board. Connectors, hangers and fasteners sized to the framing you just calculated. A lumber yard or supplier can also cut and engineer trusses to spec.
Simpson Strong-Tie–style joist hangers, rafter ties and hurricane clips for code-compliant framing connections.
Shop framing connectorsMetal truss plates, ridge straps and rafter brackets for built-up or repaired roof framing.
Shop truss hardwareSome links are affiliate links: if you buy through them we may earn a commission at no extra cost to you. These are starting points, not engineering endorsements. Always confirm a product's rated snow load meets the design load above.
Notes
- This gives the rafter's actual (sloped) length along the roof line. The plumb cut at the ridge and the seat/birdsmouth cut at the wall plate still need to be marked and cut separately.
- For an exact ridge layout, subtract half the ridge board's thickness from the run before calculating; most ridge boards are ¾ to 1½ in thick.
- Hip and valley rafters run diagonally, so their run and length multiplier are different from common rafters; use a framing square or a hip/valley table for those members.
Notes & questions
01How do I calculate rafter length?+
Multiply your horizontal run (plus any overhang) by the roof factor for your pitch: factor = √(1 + (rise/12)²). This calculator does the math and rounds up to a stocked board length.
02What length rafter do I need for a 24 ft wide building?+
For a symmetric gable, the run is half the building width (12 ft here). Enter 12 ft as the run with your pitch and overhang above to get the exact rafter length and the board to buy.