How do you calculate rafter length for a gable roof?

Calculate the horizontal run (half-span minus half-ridge), determine the rise from the pitch, and use the Pythagorean theorem (a² + b² = c²) to find the rafter length. Add the sloped length of any overhang to get the 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 (typically 19 mm or 0.75 in for a 2x ridge) so the rafters meet perfectly at the ridge.

What is the slope factor?

The slope factor is the multiplier used to convert horizontal distance to sloped distance. For example, a 6/12 pitch has a factor of 1.118; for every 10 inches of horizontal run, the rafter is 11.18 inches long.

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

Input type

Waste allowance

Building span

Overhang (horizontal)

Ridge thickness

Roof pitch

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 dimensions 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

Run = (Span ÷ 2) − (Ridge Thickness ÷ 2)

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

Common Rafter = √(Run² + Rise²)

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

Tail Length = Plan Overhang / cos(θ)

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

Total = Common Rafter + Tail 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.

Theoretical run = (24 ft ÷ 2) - (1.5 in ÷ 2) = 12 ft - 0.75 in = 11 ft 11.25 in (11.9375 ft)

Rise = 11.9375 ft × (6/12) = 5.96875 ft

Common rafter = √(11.9375² + 5.96875²) ≈ 13.346 ft (13 ft 4⅛ in)

Tail length (sloped overhang) = 1 ft × 1.118 ≈ 1.118 ft (1 ft 1⅜ in)

Total length = 13.346 + 1.118 = 14.464 ft ≈ 14 ft 5⁹⁄₁₆ in

Minimum order: 16 ft board

💡 Pro tip

When layout out rafters, always remember to subtract the ridge thickness from your measurement. Forgetting the ridge means your rafters will be too long and the ridge board won't fit between them, or your roof pitch will slightly change.

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)).