Rafter Length CalculatorEstimate Rafter Length and Timber Quantity
Use this free rafter length calculator to find the exact roof rafter cut length from horizontal run and vertical rise. Optionally add an overhang to get the full cut length, and enter the number of rafter pairs to calculate total timber needed for an entire roof.
Getting started
How to use this rafter length calculator
1
Select unit system
Imperial (feet) or Metric (meters).
2
Enter horizontal run
The flat horizontal distance from the outside wall face to the ridge centerline.
3
Enter vertical rise
The height from the wall plate top to the ridge top.
4
Add options
Optionally enter the eave overhang distance or number of pairs to get full timber requirements.
The calculation
Step-by-step: how your rafter length was calculated
1
Square run and rise
Run² = 12² = 144
Rise² = 6² = 36
Sum = 180
2
Calculate structural rafter length (Pythagorean theorem)
Rafter = sqrt(Run² + Rise²) = sqrt(180)
3
Roof angle from rise and run
Angle = arctan(Rise / Run) = arctan(6 / 12)
Formulas
Rafter length formula explained
The core calculation is the Pythagorean theorem applied to the right triangle formed by the rafter, the horizontal run, and the vertical rise:
Structural rafter = sqrt( Run² + Rise² )
Overhang rise = Overhang x (Rise / Run)
Overhang slope = sqrt( Overhang² + Overhang rise² )
Full cut length = Structural rafter + Overhang slope
Total timber = Full cut length x (Number of pairs x 2)
Roof angle = arctan(Rise / Run) x (180 / PI)
The overhang calculation uses the same pitch ratio (Rise / Run) as the main roof to compute the rise over the overhang distance. This ensures the rafter tail continues at an identical slope to the main rafter.
Background Information
Why precise rafter cut length matters
When framing a gable roof, every common rafter is cut from the same template. A rafter that is even half an inch too long or too short will throw the ridge out of level, create uneven eave lines, and cause the roof deck to be wavy along the top edge. Precision at the layout stage prevents expensive corrections later.
The rafter length calculated here is the sloped length - how long the rafter actually is when it lies in place on the roof. This is the measurement you mark on the rafter stock to locate the ridge plumb cut at the top and the bird's mouth (seat cut) at the bottom.
When you add an overhang, the rafter tail extends past the wall and hangs in free air. That tail is part of the same sloped line as the main rafter.
Context
Interpreting the rafter length results
Structural rafter length
This is the rafter length from the seat cut center to the ridge plumb cut. It does not include any overhang. Use this when the overhang is cut from a separate rafter tail board.
Full cut length (with overhang)
This is the total length of rafter stock to cut - from the fascia cut on the tail through the structural portion to the ridge plumb cut.
Total timber
This is the total linear footage needed for the full roof. Divide by standard lumber lengths to determine how many boards to purchase. Add 10 to 15% for waste.
Examples
Rafter length calculation examples
Example 1 - Simple gable roof, no overhang
Run: 12 ft, Rise: 6 ft
Rafter length = sqrt(144 + 36) = sqrt(180) = 13.42 ft. Angle = arctan(6/12) = 26.6 deg.
Example 2 - With 1.5 ft eave overhang
Run: 12 ft, Rise: 6 ft, Overhang: 1.5 ft
Structural: 13.42 ft. Overhang rise = 1.5 x (6/12) = 0.75 ft. Overhang slope = sqrt(2.25 + 0.5625) = 1.68 ft. Total cut = 15.10 ft.
Example 3 - Full roof timber estimate, 10 rafter pairs
Full cut length 15.10 ft, 10 pairs = 20 rafters
Total timber = 15.10 x 20 = 302 ft. With 12% waste: order approximately 338 ft of rafter stock.
Mistakes
Common mistakes when calculating rafter length
Using span instead of run
The span is the full building width. The run is half the span (for a symmetrical gable). Using the full span as the run produces rafters that are far too long.
Ignoring the overhang slope when ordering stock
Because the rafter is sloped, the actual extra stock needed is always longer than the horizontal projection of the overhang.
Forgetting the ridge board deduction
The calculated rafter length is measured to the center of the ridge. Deduct half the ridge board thickness from the plumb cut end.
FAQ
Frequently asked questions
Q
What is rafter length and how is it measured?
Rafter length is the sloped measurement of a roof rafter from the seat cut at the wall plate to the plumb cut at the ridge. It is calculated from the horizontal run and vertical rise using the Pythagorean theorem: rafter = sqrt(run^2 + rise^2). This is different from the horizontal run or the vertical height alone.
Q
What is the difference between run and rise for a rafter?
The run is the horizontal distance from the outside wall face to the centerline of the ridge. The rise is the vertical height from the top of the wall plate to the top of the ridge. Together, they form the two legs of a right triangle, with the rafter as the hypotenuse.
Q
How does the overhang affect rafter length?
The overhang (or eave projection) is the horizontal distance the rafter extends beyond the outside wall. The extra sloped length is calculated by treating the overhang as a small additional run with the same pitch as the main roof. This calculator adds the converted overhang length to the structural rafter length to give you the total rafter cut length.
Q
What does 'total timber' mean in this calculator?
If you enter a number of rafter pairs, the calculator multiplies the total cut length of a single rafter by twice that number (one rafter on each side of the ridge per pair). This gives you the total linear feet or meters of rafter stock to order for the entire roof.
Q
How do I find the rise if I only know the pitch?
Rise = Run x (Pitch / 12) for imperial (e.g., a 6/12 pitch roof with 12 ft run has rise = 12 x 6/12 = 6 ft). Use the Rafter Calculator on this site to compute rise from pitch and run, then enter that rise value here if needed.
Q
Should I add extra to the rafter length for waste?
For material ordering, add 10 to 15% to the total timber figure to account for miscuts, knots, and end waste. For the actual cut length of individual rafters, the calculator gives you the exact theoretical length - add no extra there, just measure and cut precisely.
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