Roof Framing Tools
Rafter Length Calculator
Find the "true" length of your common rafters including any overhang. Enter your building span, roof pitch, and ridge board thickness. The calculator provides the theoretical rafter length, the tail length, and the minimum stock lumber size required.
Rafter Length Calculator
Calculate common rafter length from span/run, pitch, ridge allowance, and overhang
Tip
Ridge allowance
If you enter a ridge thickness, the calculator subtracts half of it from the run (each rafter bears against one side of the ridge).
Results
Enter values above to calculate
Rafter Geometry Explained
The rafter forms the hypotenuse of a right-angled triangle. Note how the run is measured to the center but reduced by half the thickness of the ridge board.
Common Pitch Factors
| Pitch | Angle (°) | Slope Factor |
|---|---|---|
| 3/12 | 14.0° | 1.031 |
| 4/12 | 18.4° | 1.054 |
| 5/12 | 22.6° | 1.083 |
| 6/12 | 26.6° | 1.118 |
| 7/12 | 30.3° | 1.158 |
| 8/12 | 33.7° | 1.202 |
| 12/12 | 45.0° | 1.414 |
* Multiply horizontal run by slope factor to get sloped rafter length quickly.
Step-by-Step Formulas
1. Theoretical Run
The rafter run is exactly half the building span, minus half the thickness of the ridge board. This gives you the horizontal distance the rafter must travel.
2. Theoretical Rafter Length
Using the Pythagorean theorem, we calculate the hypotenuse of the triangle formed by the run and the rise. This is the length from the ridge plumb cut to the outside plate plumb cut.
3. Overhang Length
If you have an overhang (eg 600 mm), the actual material length needed is greater because it follows the roof slope. We divide the horizontal overhang by the cosine of the roof angle.
4. Total Board Length
The sum of the common rafter and the overhang tail gives the total length of the board needed. Always round up to the next standard lumber size (eg 4.8 m or 16 ft).
Worked Example
Imperial: 24 ft span, 6/12 pitch, 1.5 in ridge board, 12 in overhang.
Cutting Rafters Accurately
A rafter is a right triangle problem with three practical adjustments layered on top:
- Run Is Half the Span: For a symmetrical gable, each rafter covers half the building width. Working from the full span instead of the run is the classic way to double your rise by mistake.
- Ridge Thickness Shortens the Run: The rafter stops at the face of the ridge board, not the centreline, so half the ridge thickness comes off the run before the line length is computed.
- Overhang Runs Along the Slope: A 16 in horizontal overhang is longer than 16 in measured up the rafter. It must be multiplied by the pitch factor or every tail will be cut short.
Benefits of Using This Calculator
Example Calculations
Three scenarios worked through step by step:
Example 1 — Common Rafter
Span 24 ft, 6/12 pitch, 1.5 in ridge, 16 in overhang.
Run = 24 ÷ 2 = 12 ft; less half ridge (0.75 in) = 11.9375 ft
Rise = 11.9375 × (6 ÷ 12) = 5.969 ft
Line length = √(11.9375² + 5.969²) = 13.348 ft
Overhang along slope = (16 ÷ 12) × 1.118 = 1.491 ft
Total = 13.348 + 1.491 = 14.84 ft → cut from 16 ft stock
Example 2 — Steep Pitch
Span 20 ft, 10/12 pitch, 1.5 in ridge, 12 in overhang.
Run = 10 ft; less 0.75 in = 9.9375 ft
Rise = 9.9375 × (10 ÷ 12) = 8.281 ft
Line length = √(9.9375² + 8.281²) = 12.936 ft
Multiplier = √(10² + 12²) ÷ 12 = 1.302
Total with overhang = 12.936 + 1.302 = 14.24 ft → 16 ft stock
Example 3 — Shallow Shed Roof
Run 14 ft (single slope), 3/12 pitch, no ridge deduction, 24 in overhang.
Rise = 14 × (3 ÷ 12) = 3.5 ft
Line length = √(14² + 3.5²) = 14.431 ft
Multiplier = √(3² + 12²) ÷ 12 = 1.031
Overhang along slope = 2 × 1.031 = 2.062 ft
Total = 14.431 + 2.062 = 16.49 ft → 18 ft stock
Cutting Tip
Cutting Tip
Cut one rafter, carry it up, and test-fit it at both ends before cutting the rest. Then use that single verified rafter as the physical template for every other one rather than re-measuring. Marking from a pattern eliminates the cumulative drift that creeps in when you measure each piece independently.
Frequently Asked Questions
- How do you calculate rafter length for a gable roof?
- Calculate horizontal run (half-span minus half-ridge), find rise from pitch, and use a² + b² = c² to find the rafter length. Add the sloped overhang length for total board length.
- Do I need to subtract the ridge board?
- Yes. The theoretical run should be reduced by half the thickness of the ridge board so the rafters meet perfectly at the ridge.
- What is the slope factor?
- The multiplier used to convert horizontal distance to sloped distance. For a 6/12 pitch, the factor is 1.118 (√(1 + (6/12)²)).
- How do I calculate rafter length?
- Take the run (half the span for a gable), compute rise as run × pitch ÷ 12, then the line length is √(run² + rise²). Subtract half the ridge thickness from the run first, and add the overhang measured along the slope at the end.
- What is the difference between span and run?
- Span is the full width the roof crosses, wall to wall. Run is the horizontal distance one rafter covers — half the span on a symmetrical gable roof. All rafter maths works from the run, never the span.
- Why subtract half the ridge board thickness?
- Because the rafter bears against the face of the ridge, not its centreline. On a 1.5 in ridge you remove 0.75 in from the run. Skip it and every rafter is fractionally long, pushing the ridge up and out of position.
- How do I find the plumb cut angle?
- The plumb cut angle from vertical equals arctan(rise ÷ run). For a 6/12 pitch that is arctan(6/12) = 26.57°. The seat (level) cut is the complement, 63.43°, and both are taken directly from the pitch.
- What size rafters do I need?
- It depends on span, spacing, species, grade, and snow load — not on pitch. Consult the American Wood Council span tables or your local code. This calculator gives you lengths and angles; it does not size the member.
- How far apart should rafters be spaced?
- Sixteen or 24 inches on centre are standard. Twenty-four is common with a structural ridge and adequate decking thickness, while 16 in centres suit heavier snow loads and thinner sheathing. Spacing must match your decking span rating.
- What is a ridge beam versus a ridge board?
- A ridge board is a non-structural nailer that simply aligns opposing rafters, which then push outward and rely on ceiling joists or collar ties for restraint. A ridge beam is structural and carries load, allowing a vaulted ceiling with no ties.