Common Rafter Length Calculator
Framing a roof involves many calculations, one of which is determining the length of a common rafter. The common rafter spans from the top of the wall (plate) to the ridge board, supporting the roof decking. Knowing the correct length is crucial for accurate cuts and solid framing.
This Common Rafter Length Calculator uses the horizontal run and roof pitch to give you a precise rafter length. It's a handy tool for builders, contractors, carpenters, and even DIYers tackling shed or home framing projects.
Formula
To calculate the length of a common rafter:
Rafter Length = √(Run² + Rise²)
But rise is determined by pitch:
- Pitch = rise in inches per 12 inches (1 foot) of run
- So, Rise = (Pitch / 12) × Run
Substitute into the main formula: Rafter Length=Run2+(Run×Pitch12)2\text{Rafter Length} = \sqrt{\text{Run}^2 + \left(\text{Run} \times \frac{\text{Pitch}}{12}\right)^2}Rafter Length=Run2+(Run×12Pitch)2
How to Use
- Enter the horizontal run of one side of the roof (in feet).
- Input the roof pitch — typically described in inches of rise per 12 inches of run (e.g., 6 for 6:12 pitch).
- Click “Calculate” — The tool returns the length of one common rafter.
- Use the result for accurate rafter cuts in construction.
Example
Input:
- Run: 10 feet
- Pitch: 6 (meaning 6:12 roof pitch)
Calculation:
- Rise = (6 / 12) × 10 = 5 feet
- Rafter Length = √(10² + 5²) = √(100 + 25) = √125 ≈ 11.18 feet
Output:
Common Rafter Length: 11.18 feet
FAQs
- What is a common rafter?
It’s a diagonal rafter spanning from the top wall plate to the roof ridge, used in standard gable roofs. - What units does this calculator use?
Inputs and results are in feet for length and inches per foot for pitch. - What is roof pitch?
It’s the number of inches the roof rises for every 12 inches of horizontal run. - Can I use this for metric units?
This version uses feet/inches. A metric version can be built if needed. - What if I enter a 12:12 pitch?
That’s a 45-degree roof. The rise = run, and the rafter length will be √2 × run. - What’s the typical pitch for residential roofs?
Between 4:12 and 9:12 is common for homes. - Does this include rafter overhang?
No. It only calculates the structural rafter length to the ridge. Add overhang separately. - Is the calculator accurate for hip or valley rafters?
No, those require different calculations (diagonal runs). This is for common rafters only. - Can this calculator help with truss design?
Yes, for verifying measurements, but full truss design requires more details. - Is the pitch required?
Yes—it determines the slope and rise of the roof. - What if I know rise and run already?
Then use the Pythagorean theorem directly: √(run² + rise²) - Can I use this for shed roofs or lean-to roofs?
Yes, as long as you have a sloped side with a defined pitch. - Do I input the total span or half of it?
Input half of the total span (just one side’s run). - How do I measure the run?
From the outside wall to the center of the ridge board (horizontally). - Does the calculator assume a right triangle?
Yes—rafter, rise, and run form a right triangle. - How precise is the result?
The result is rounded to two decimal places. - Can I use this for a cathedral ceiling?
Yes, as long as you're measuring to the ridge beam and the structure matches a right triangle. - Does it account for roof thickness or material?
No, this gives raw length. Adjust for ridge thickness or cuts manually. - Is the result usable for lumber selection?
Yes—it helps choose the correct rafter length and board size. - Is this calculator free?
Yes, completely free for DIYers, contractors, and educators.
Conclusion
Accurate roof framing starts with solid measurements, and knowing your common rafter length is one of the first steps in constructing a well-supported roof. This Common Rafter Length Calculator simplifies the process by turning run and pitch into exact lengths in seconds.