Roof Truss Calculator
Calculate truss quantities, peak heights, rafter lengths, and geometric lumber requirements.
Roof Truss Calculator Interactive Tool
Enter your project dimensions below. Results update instantly.
Results
21
2 gable ends + 19 common4 ft
Above wall plate13.65 ft
Includes 12" overhang18.4°
Pitch angle in degrees* Note: Structural loads, connections, and lumber dimensions must be certified by a licensed engineer to meet local wind and snow loads.
Truss Quantity & Rafter Geometry Formulas
Truss Count = ⌈(Building Length × 12) ÷ Spacing⌉ + 1 | Peak Height = (Span ÷ 2) × (Rise ÷ 12) | Rafter Length = √((Span ÷ 2)² + Peak Height²) + OverhangFormula Variables:
- Building Length:Total length of framing wall supporting the trusses.
- Span:Total exterior bearing-to-bearing wall width.
- Spacing:Standard center-to-center spacing (commonly 24" on-center).
This calculator provides geometric layout estimates. Structural load engineering must be certified by a licensed engineer.
Step-by-Step Worked Example
Example: 40-foot building length, 24-foot span, 24-inch on-center spacing, 4:12 pitch
Planning truss orders for a detached garage measuring 24 ft wide by 40 ft long with a 4:12 pitch and 12-inch overhangs.
- Calculate spaces along length: (40 ft × 12 in/ft) ÷ 24 in = 20 spaces.
- Add 1 starting end truss: 20 + 1 = 21 trusses total (2 gable end trusses + 19 common trusses).
- Half span (run): 24 ft ÷ 2 = 12 ft.
- Peak height above wall plate: 12 ft × (4 ÷ 12) = 4.00 ft rise.
- Calculate rafter top-chord length: √(12² + 4²) = √(144 + 16) = √160 = 12.65 ft.
- Add 1 ft overhang: 12.65 + 1.00 = 13.65 ft top chord length.
Input Explanations & Measurement Guidelines
Building Length
Typical: 24 to 60 ftOverall exterior framing length along the eaves.
Building Clear Span
Typical: 20 to 36 ftExterior bearing wall to exterior bearing wall distance.
On-Center Spacing
Typical: 24 inchesDistance between truss centers (typically 24" O.C., occasionally 16" or 19.2").
Assumptions & Limitations
- Assumes uniform on-center spacing across a rectangular building envelope.
- Includes two gable end trusses plus common internal trusses.
- Does not calculate mechanical wind uplift, snow load deflection, or metal gusset plate sizing.
Safety & Professional Notes
- Temporary and permanent diagonal bracing is critical during truss erection to prevent domino-effect progressive collapse before sheathing is fastened.
Frequently Asked Questions
Why is there a +1 in the truss count formula?
Dividing length by spacing counts the number of bays between trusses. You need an additional truss to close the final bay at the end wall.
Can I build my own roof trusses without an engineer?
In nearly all jurisdictions, site-built trusses require stamped engineering calculations to satisfy International Residential Code (IRC) snow and wind uplift requirements.