Rafter Calculator
Calculate common rafter length, cut length, spacing count, and board feet.
Rafter Layout
Common roofs
Ready to calculate
Enter span, length, pitch, and spacing.
Last updated June 3, 2026 by our expert review team
Common Rafter Planning Checks
| Planning item | Calculator uses | Verify locally |
|---|---|---|
| Run | half the building span | actual wall-to-ridge layout |
| Pitch | rise per 12 in run | roof plan or field measurement |
| Rafter count | building length and spacing | layout at gable ends and openings |
| Lumber size | board-foot takeoff only | species, grade, load, and span tables |
Expert Contributors
Ehsan Ghazanfari
Licensed Structural Engineer
FISE-certified structural engineer with 11+ years designing bridges, retaining walls, and foundations. MSc from Aalto University.
See full profileHawkin
Certified Cost & Estimating Professional
AACE-certified estimator working with 20+ insulation companies including the two largest franchises in America.
See full profileUpdated June 2026
How we verify our calculatorsRafter Layout Planning Guide
A common rafter starts with span, turns half of that into run, then uses pitch to find rise and slope length.
This visual explains the takeoff path. It does not replace span tables, engineering, or local roof-framing requirements.
Pro Tips
Use this as a layout and takeoff helper. Span tables, snow load, wind load, species, grade, and local code still control rafter sizing.
Run is half the building span on a simple gable roof. Using the full span is the classic first-cut mistake.
The overhang input is horizontal eave projection. The calculator converts it to the extra rafter length along the slope.
After framing, use the roofing calculator for shingles, underlayment, and ridge caps.
Use the roof pitch calculator if you need to convert rise and run into an X/12 pitch.
How the Calculator Works
The calculator models a simple common rafter for a gable roof. It finds the right-triangle length to the ridge, subtracts half the ridge board thickness, then adds the overhang tail along the roof slope.
Run
span / 2
Rise
run x pitch / 12
Cut length
sqrt(run2 + rise2) - ridge deduction + tail
Quick Reference
- Run
- half span
- Default spacing
- 16 in OC
- Ridge deduction
- half ridge thickness
- Overhang
- converted to slope
- Sizing
- use span tables
Rafter Length by Pitch
Real-World Rafter Examples
Small shed
12 ft span, 4/12, 12 ft long, 16 in OC
7.32 ft cut, 20 rafters
Short common rafters with a 12 in overhang.
Ranch roof
24 ft span, 6/12, 40 ft long, 16 in OC
14.47 ft cut, 62 rafters
Default example with 2x8 board-foot takeoff.
Long garage
24 ft span, 4/12, 28 ft long, 24 in OC
13.64 ft cut, 30 rafters
Wider spacing lowers rafter count, but span tables still control sizing.
Metric roof
7.3 m span, 12 m long, 6/12, 16 in OC
4.39 m cut, 62 rafters
Metric dimensions with a 30 cm overhang.
Rafter Calculator FAQ
How do I calculate common rafter length?
Use run and rise as a right triangle: rafter length = square root of run squared plus rise squared.
Why is run half the span?
On a simple gable roof, each common rafter runs from an outside wall to the ridge, which sits near the centerline.
Does this size the lumber?
No. The page estimates length and board feet. Use AWC span tables or local design documents for sizing.
What is the ridge deduction?
Each opposing rafter meets a ridge board. The calculator subtracts half the ridge thickness from each rafter.
Why does overhang add more than its horizontal distance?
The tail follows the roof slope, so the sloped length is longer than the horizontal projection.
Can I use this for hips or valleys?
Use it for common rafters only. Hip, valley, jack, and compound cuts need a separate layout.
Rafter Layout Mistakes
Using full span as the run
Run is half the span on a simple gable roof.
Skipping the ridge deduction
Subtract half the ridge board thickness from each common rafter.
Treating board feet as structural approval
Board feet estimate lumber volume. It does not validate species, grade, or span.
Forgetting layout interruptions
Gable ends, openings, outlookers, and framing details can change the count.
Important Disclaimer
These estimates are for planning purposes only. Actual costs vary by location, material availability, and project complexity. Always get at least 3 local quotes. This calculator does not replace professional advice.