Construction

Rafter Length Calculator

Calculate the length of a roof rafter from run, pitch and overhang.

Run (half span)

Pitch rise (in 12) (/12)

Overhang

Calculated Output

Rafter length 17.771

Total rise 7.5

Formula: rafter = run × √(1 + (rise/12)²) + overhang

Step-by-Step Breakdown

1. Values used:

2. Run (half span) = 15

3. Pitch rise (in 12) = 6 /12

4. Overhang = 1

6. Rafter length = 17.771

7. Total rise = 7.5

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The math behind it

The rafter is the hypotenuse of a right triangle. Length = run × √(1 + (rise/12)²).

Add the overhang (measured along the slope) for the total cut length.

15 ft run at 6/12 → 15 × √(1 + 0.25) = 16.77 ft + 1 ft overhang = 17.77 ft.

Is run the full span?

No — run is half the building width (from outer wall to ridge), for a symmetric gable roof.