How many rafters do I need?
Answer
Divide the roof length by the maximum spacing. Increase a partial result to the next whole space. Add one position.
Multiply by two for a simple gable roof.
| Maximum spacing | Spaces | Positions on each side | Gable rafters |
|---|---|---|---|
| 12 in | 30 | 31 | 62 |
| 16 in | 23 | 24 | 48 |
| 24 in | 15 | 16 | 32 |
Calculate the base quantity. Then, add more material.
Quantity formula
- Change the roof length and spacing to the same unit.
- Divide the roof length by the maximum spacing.
- If the result is not whole, increase it to the next whole number.
- Add one rafter position.
Gable and single-slope roofs
A simple gable roof has two common rafters at each position. Multiply the position quantity by two.
A single-slope roof has one rafter at each position. Use the position quantity as the base rafter quantity.
More material
Add material for test cuts, damage, and rejected boards. Apply this percentage after the base rafter quantity.
The rafter calculator also calculates stock length, total linear stock, wood volume, and cost.
Important information
- Count special rafters separately.
- Calculate openings, hips, valleys, and gable-end framing separately.
- Use the approved plans to select the spacing.