How to use it
- Enter the building span from the outside of one wall to the other.
- Enter the pitch, ridge thickness and overhang.
- Enter the building length and rafter spacing for the count.
The formula
Run = span ÷ 2 − ridge thickness ÷ 2
Rafter = (run + overhang) × pitch multiplier
Worked example
| Building span (outside wall to outside wall) | 24 ft |
| Roof pitch (rise per 12 in of run) | 6 /12 |
| Ridge board thickness | 1.5 in |
| Horizontal overhang (eave) | 12 in |
| Building length (for rafter count) | 40 ft |
| Rafter spacing | 16 in o.c. |
Result
| Rafter length (with overhang) | 14 ft 5.625 in |
| Rafter length (decimal) | 14.46 ft |
| Ridge-to-plate length | 13 ft 4.125 in |
| Ridge height above top plate | 5 ft 11.625 in |
| Rafters needed (both sides) | 62 |
| Plumb-cut angle | 26.6 ° |
Tips before you order
- Buy the next standard lumber length up from the calculated rafter, then cut the plumb and birdsmouth.
- Rafter size and spacing depend on span, snow load and lumber grade — use your code's span tables.
Questions
How long is a rafter for a 24 ft span at 6/12?
About 13 ft 4 in from ridge to plate, plus the overhang — about 14 ft 5⅝ in with a 12 in eave.