Roof Truss Calculator
Calculate ridge height, rafter length, and truss dimensions from span and pitch.
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Understanding Roof Truss Dimensions
A roof truss is the structural framework that supports the roof decking, shingles, and any load above. Getting the dimensions right — ridge height, rafter length, and bottom chord — is critical for ordering pre-fabricated trusses or cutting rafters on site.
Truss Calculator Formula
- Half Span (Run) = Total Span ÷ 2
- Ridge Height = Half Span × (Pitch ÷ 12)
- Rafter Length = √(Half Span² + Ridge Height²)
This is the Pythagorean theorem applied to the right triangle formed by the rafter, the ridge height, and the run.
Common Truss Spans and Dimensions
| Span (ft) | 6/12 Ridge Height | 6/12 Rafter Length |
|---|---|---|
| 20 ft | 5.0 ft | 11.18 ft |
| 24 ft | 6.0 ft | 13.42 ft |
| 28 ft | 7.0 ft | 15.65 ft |
| 32 ft | 8.0 ft | 17.89 ft |
| 40 ft | 10.0 ft | 22.36 ft |
Pre-Fab vs. Site-Cut Trusses
Pre-fabricated trusses from a truss manufacturer are faster to install and structurally engineered — use the dimensions from this calculator to order the right size. Site-cut rafters give more flexibility for complex or custom roof shapes but require a skilled framing crew.
Important Disclaimer
This calculator provides geometric dimensions only. Actual structural truss design must be stamped by a licensed structural engineer for building permit purposes. Span, load requirements, snow load, and local code all affect the final engineered design.
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