Free estimate — verify against local code before building
Rafter Length Calculator
Turn span and pitch into common-rafter run, rise, and approximate cut length.
What this calculator includes
Lay out common-rafter geometry from the outside-wall span, ridge-board thickness, horizontal overhang, and rise-per-12 pitch. The calculator separates the ridge-face line length from the tail and a user-entered stock allowance.
How to use this rafter length calculator
- 01
Enter the structural span
Use outside bearing-wall to outside bearing-wall distance. The calculator divides it in half for one common rafter.
- 02
Deduct half the ridge
Enter actual ridge-board thickness. Half of that thickness is deducted horizontally so the line ends at the ridge face.
- 03
Set pitch and overhang
Enter rise per 12 inches of run and horizontal overhang. Both horizontal lengths are multiplied by the same slope factor.
- 04
Add a layout allowance
The birdsmouth/layout field is extra stock length, not notch depth. Use it only as a trim allowance before final field layout.
Worked example
Example: 24 ft span at 6-in-12
With a 1.5-inch ridge, the horizontal run is 12 ft minus 0.0625 ft, or 11.9375 ft. Multiplying by the 6-in-12 slope factor of about 1.118 gives a common-rafter line near 13.35 ft before adding the sloped overhang and any trim allowance.
Practical buying and overage guidance
Use the result to screen stock length only. Confirm actual dimensions, crown, grade, species, treatment, moisture, splice restrictions, blocking, connectors, delivery length, and the approved framing plan before ordering.
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Open the project workflow →Calculation sources and review
Primary references and formula assumptions are linked so you can verify them against the selected product, supplier, and adopted local requirements.
Internal formula review completed July 13, 2026. What this review covers
- International Code Council — building safety resources (opens in a new tab)
Structural member sizing, notching, connections, and load paths are code- and design-sensitive.
- American Wood Council — design tools and standards (opens in a new tab)
Engineering resources for wood member design; geometry alone does not size a rafter.
- OSHA — fall protection in residential construction (opens in a new tab)
Roof framing and measurement involve fall hazards.
Frequently asked questions
What span should I enter for a common rafter?
Enter the full horizontal distance from the outside of one bearing wall to the outside of the opposite bearing wall. Do not enter the sloped roof width.
Why is half the ridge-board width deducted?
A common rafter normally meets the face of a centered ridge board rather than its centerline, so half the ridge thickness is removed from the horizontal half-span.
Is overhang measured horizontally or along the rafter?
Enter horizontal projection beyond the wall. The pitch multiplier converts it to added length along the rafter line.
Does this calculator lay out the birdsmouth cut?
No. It adds the entered value as extra stock for field layout. It does not determine seat-cut width, notch depth, heel height, plumb cuts, or code-permitted notching.
Can I use this result to choose rafter size or spacing?
No. Member size and spacing depend on species, grade, loads, span, bracing, connections, deflection, and locally adopted requirements. Use approved plans or qualified design.