QuickCheckToolsRafter calculator
Work out common rafter length, rise and cut angles from the building span and roof pitch, including the ridge board and overhang.
Rafter multipliers by pitch
Rafter length per foot of level run.
| Pitch | Multiplier | Plumb cut angle |
|---|---|---|
| 3/12 | 1.031 | 14.04° |
| 4/12 | 1.054 | 18.43° |
| 5/12 | 1.083 | 22.62° |
| 6/12 | 1.118 | 26.57° |
| 7/12 | 1.158 | 30.26° |
| 8/12 | 1.202 | 33.69° |
| 9/12 | 1.250 | 36.87° |
| 10/12 | 1.302 | 39.81° |
| 12/12 | 1.414 | 45.00° |
Need the pitch from a measurement or angle? Use the roof pitch calculator.
Questions
How do I calculate rafter length?
Find the run (half the building span, minus half the ridge board), multiply by the rafter multiplier for your pitch (√(1 + (pitch ÷ 12)²)), then add the overhang measured along the rafter. A 24 ft span at 6/12 with no ridge board gives a 12 ft run × 1.118 = 13.42 ft.
Do I subtract the ridge board?
Yes. Each rafter stops at the face of the ridge board, so take half the ridge thickness off each run: ¾ in for a 1½ in ridge board.
What are the plumb and seat cuts?
The plumb cut is the vertical cut at the ridge end; the seat cut (bird's mouth) sits on the wall plate. At 6/12 the plumb cut is 26.57° from square and the seat cut 63.43°.
How long a board do I need?
We round the total rafter length (including overhang) up to the next standard lumber length: 8, 10, 12, 14, 16, 18, 20, 22 or 24 ft.
What is the rafter multiplier?
The length of rafter per foot of horizontal run. It's 1.118 for a 6/12 roof and 1.414 for 12/12. The table on this page lists common pitches.
Does this size the rafters?
No. It gives lengths and angles only. The rafter size and spacing depend on span, loads and your local building code, so check span tables or ask a structural engineer.
Lengths and angles for common rafters on a simple gable roof. Rafter size, spacing and fixings must follow your local building code.