Disclaimer

Every rafter length, plumb-cut angle, seat-cut angle, and birdsmouth dimension on rafter-calculator.com is computed from standard geometric formulas — the Pythagorean relationship between run and rise for a given pitch. These are planning estimates to help you lay out and budget a roof, not a stamped set of construction drawings.

This tool does not replace a licensed structural engineer, architect, truss designer, or your local building department. For load-bearing roofs, engineered trusses, complex hips and valleys, or anything that needs a permit submittal, hire a qualified professional who can sign off on the design.

Span tables, snow and wind load rules, fire-rated assemblies, and permit requirements differ from one city to the next. You are responsible for confirming that your design complies with your local building code — including any IRC or IBC amendments adopted in your area — before you build.

A pitch entered as 6/12 instead of 6.5/12, a span measured to the wrong face of the wall, or confusing nominal lumber sizes (a “2x8” that is actually 1.5” × 7.25”) changes every downstream number. Re-measure, re-enter the values, and sanity-check the result against the real framing.

The calculator solves rafter geometry. It does not size rafters for snow, wind, dead, or live loads, and it does not check deflection, bearing, or connection details. Use published span tables or a structural engineer for member sizing on anything beyond a simple shed or porch roof.

rafter-calculator.com is provided “as is” without warranties of accuracy, fitness for a particular project, or merchantability. Always verify calculations against printed span tables and your local code, and follow the carpenter’s rule: measure twice, cut once.