Roof Rafter Calculator
Calculate simple-gable rafter length, rise, rafter positions, stock pieces, roof surface area and preliminary lumber cost.
Default project preview
17.89 ft
Vertical rise
7.5 ft
Rafter positions
31
Installed rafters
62
Stock pieces
66
Calculator
Build the material order
Your order will appear here
Enter the project details and calculate a rounded material estimate.
Input and output guide
What each value means
The labels and units below are part of this calculator’s visible methodology. Replace every default with measurements or product data from the actual project.
Building width
Enter this project assumption in ft. The current imperial default is 30 ft.
Building length
Enter this project assumption in ft. The current imperial default is 40 ft.
Roof pitch rise
Enter this project assumption in per 12. The current imperial default is 6 per 12. Minimum: 0.
Horizontal overhang
Enter this project assumption in ft. The current imperial default is 1 ft. Minimum: 0.
Rafter spacing
Enter this project assumption in in. The current imperial default is 16 in.
Stock rafter length
Enter this project assumption in ft. The current imperial default is 20 ft.
Extra lumber allowance
Enter this project assumption in %. The current imperial default is 5 %. Minimum: 0. Maximum: 50.
Price per stock piece
Enter this project assumption in a unitless value. The current imperial default is 32. Minimum: 0.
How to read the outputs
Vertical rise
ridge above wall plate. Default preview: 7.5 ft.
Rafter positions
along building length. Default preview: 31.
Installed rafters
both roof sides. Default preview: 62.
Stock pieces
1 per rafter before waste. Default preview: 66.
Roof surface
simple gable geometry. Default preview: 1,431.1 ft².
Stock lumber cost
entered piece price. Default preview: $2,112.00.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Run = building width ÷ 2. Rise = run × pitch ÷ 12. Rafter length = √(run² + rise²) plus the sloped overhang. Positions follow entered spacing.
Reviewed project guidance
How to use this estimate in a real project
Last reviewed: July 30, 2026
Estimate common-rafter quantity and cut length from building geometry, spacing, pitch and overhang while separating takeoff from structural sizing.
Calculation steps
- 1Calculate rafter lines from roof length and spacing.
- 2Use half-span, pitch and overhang for sloped length.
- 3Add ridge, birdsmouth and tail allowances.
- 4Round to full stock lengths and member count.
Worked numerical example
A 32 ft roof length at 24 in on center has 16 spaces, requiring 17 rafter pairs or 34 common rafters before gable lookouts and waste.
Purchasing and rounding
Order the species, grade and size shown by the design; long stock may require engineered alternatives.
Interpret the result
The calculator provides geometry and quantity, not allowable span or connection design.
Page-specific assumptions
- Symmetrical gable roof.
- Uniform spacing.
- Straight common rafters.
Common mistakes
- Counting pairs as individual rafters.
- Ignoring ridge thickness.
- Selecting lumber size from geometry alone.
Primary and technical references
BuildMeter provides planning estimates, not engineering, architectural, electrical, mechanical, plumbing, permit or code approval. Review the methodology and verify final decisions with product documentation and qualified professionals.
BuildMeter Guides
Understand the measurements and assumptions
Use these visible reference guides before changing project inputs or planning allowances.
FAQ
Roof Rafter Calculator questions
Does rafter length include the overhang?+
Yes. The horizontal overhang is converted to sloped length using the pitch factor.
Is rafter quantity twice the number of positions?+
For a simple gable roof, each position has one rafter on each roof side.
Does this select rafter size?+
No. Member size depends on span, species, grade, spacing, loads and code.