Table of Contents
Rafter Length Basics
A rafter is the sloped structural member that runs from the ridge board at the peak of a roof down to the wall plate. Calculating the correct rafter length is essential for proper roof framing. The rafter length depends on the building span (divided by two for the run), the roof pitch, and the overhang distance.
Roof pitch is expressed as rise over run, where the run is always 12 inches. A 6/12 pitch means the roof rises 6 inches for every 12 inches of horizontal run. The rafter length is the hypotenuse of a right triangle formed by the run and the rise.
Rafter Formulas
Pitch Reference Table
| Pitch | Angle | Rise per Foot | Rafter Factor (per ft run) |
|---|---|---|---|
| 3/12 | 14.0° | 3" | 1.031 |
| 4/12 | 18.4° | 4" | 1.054 |
| 5/12 | 22.6° | 5" | 1.083 |
| 6/12 | 26.6° | 6" | 1.118 |
| 7/12 | 30.3° | 7" | 1.158 |
| 8/12 | 33.7° | 8" | 1.202 |
| 9/12 | 36.9° | 9" | 1.250 |
| 10/12 | 39.8° | 10" | 1.302 |
| 12/12 | 45.0° | 12" | 1.414 |
Birdsmouth Cut Guide
The birdsmouth is a notch cut into the rafter where it sits on the wall plate. It consists of two cuts:
- Seat cut (level cut): The horizontal cut that sits flat on the top plate. It should not exceed one-third of the rafter depth to maintain structural integrity.
- Plumb cut (heel cut): The vertical cut that butts against the outside edge of the wall plate.
- The seat cut depth equals the rafter width multiplied by cos(roof angle).
- The plumb cut depth equals the rafter width multiplied by sin(roof angle).
- Maximum birdsmouth depth should be no more than 1/3 of the rafter depth for structural safety.
Frequently Asked Questions
How long should my rafters be for a 24-foot span?
For a 24-foot span with 6/12 pitch: Run = 12 ft, Rise = 6 ft. Rafter length = sqrt(12^2 + 6^2) = sqrt(180) = 13.42 ft. With a 12-inch overhang, the overhang rafter adds about 1.12 ft, making the total approximately 14.54 ft. You would use 16-foot lumber.
What roof pitch is best for my climate?
Steeper pitches (8/12 to 12/12) are better for areas with heavy snow, as snow slides off more easily. Low-slope pitches (3/12 to 4/12) work well in arid climates. A 6/12 pitch is considered a good all-around choice for most residential construction.
What size lumber do I need for rafters?
Rafter size depends on span, spacing, and load requirements. For a 12-foot run with 24-inch spacing: 2x8 for light loads, 2x10 for standard, 2x12 for heavy snow loads. Always consult local building codes and span tables for your specific situation.