Roof Truss Calculator
Estimate common truss rafter length from building span and roof rise.
Roof Truss Geometry
For a symmetric simple truss, each rafter forms a right triangle with half the building span and the roof rise.
How Roof Truss Rafter Length Is Estimated
For a simple symmetric roof truss, the rafter length can be estimated from half the building span and the roof rise using the Pythagorean theorem. This gives the straight-line distance from the wall line to the ridge before accounting for overhangs or special truss geometry.
Roof Truss Example
If the building span is 24 ft, half the span is 12 ft. With a 6 ft rise, the basic rafter length is about 13.42 ft because √(12² + 6²) ≈ 13.42.
What the Simple Formula Does Not Include
Real trusses may include overhangs, heel height, bearing details, webs and manufacturer-specific geometry. Snow, wind and dead loads also affect structural design, so this calculator is best used for geometric planning rather than engineering approval.
Frequently Asked Questions
Does roof pitch affect rafter length?
Yes. A steeper roof has more rise and therefore a longer rafter for the same span.
Is this suitable for structural design?
No. Structural sizing and truss design should follow local codes and engineered specifications.
How do I include an overhang?
Add the overhang geometry separately because the simple span-and-rise calculation stops at the wall line.
Calculation Assumptions and Practical Limits
Basic truss or rafter geometry can estimate member length from span and rise, but a roof truss is a structural system. Web configuration, loads, connections, lumber grade and code requirements must be engineered or supplied by an approved truss design.