Rafter Length Calculator
Calculate common rafter length, rise, and overhang
Calculation Summary
Total Rafter Length:
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(Including Overhang)Common Rafter (Hypotenuse):
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(Wall to Ridge)Roof Rise:
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Total Run per Side:
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How to Calculate Rafter Length
Calculating the correct length for your roof rafters is a critical step in framing a shed, garage, or house. To get the most accurate measurement, you must account for the total span of the building, the thickness of the ridge board, and your desired roof pitch.
The Geometry of a Rafter
A standard rafter forms the hypotenuse of a right-angled triangle. The three main components are:
- Run: Half of the building span minus half the thickness of the ridge board.
- Rise: The vertical height of the roof from the top plate to the peak.
- Line Length: The diagonal distance from the outside of the wall plate to the center of the ridge.
Standard Pitch Table
The "Pitch" is the amount of vertical rise for every 12 inches of horizontal run. Common pitches include:
| Pitch (in 12) | Angle (Degrees) | Common Use |
|---|---|---|
| 3/12 | 14.04° | Sheds / Low Slope |
| 4/12 | 18.43° | Standard Residential |
| 6/12 | 26.57° | Steep Residential |
| 12/12 | 45.00° | A-Frame / Very Steep |
Practical Example
If you have a building that is 20 feet wide (240 inches) and you are using a 2x ridge board (1.5 inches thick) with a 4/12 pitch:
- Step 1 (Run): (240 – 1.5) / 2 = 119.25 inches.
- Step 2 (Rise): 119.25 * (4/12) = 39.75 inches.
- Step 3 (Hypotenuse): √(119.25² + 39.75²) = 125.7 inches (approx 10′ 5 11/16″).
Note: Always remember to add the length of your overhang to this measurement to find the total lumber length required.