Calculate rafter length, cut angles, bird's mouth dimensions, and lumber requirements for roof framing. Uses roof span and pitch to determine common rafter measurements.
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Building a roof? Our rafter length calculator determines the exact length of common rafters based on your roof span and pitch. Get precise cut angles for plumb cuts, seat cuts, and bird's mouth dimensions. Calculate lumber requirements for your entire roof with material lists and recommended board lengths.
Rafters are the sloped structural members that support the roof deck and transfer loads to the walls. Rafter length is calculated using the Pythagorean theorem: the rafter is the hypotenuse of a right triangle where the run (horizontal distance from wall to ridge) and rise (vertical height) are the legs. The total rafter length includes the overhang (tail) that extends past the wall.
Rafter Length Formula
L = √(Run² + Rise²) + Tail LengthGet exact rafter lengths to avoid costly cutting mistakes and wasted lumber.
Know the exact plumb cut and seat cut angles for your speed square or miter saw.
Calculate proper bird's mouth cuts for secure rafter-to-plate connections.
Determine total rafters needed and recommended lumber lengths for ordering.
Include eave overhang in total length for proper fascia and soffit planning.
Automatically accounts for ridge board thickness in run calculation.
Calculate common rafters for traditional gable roofs with ridge beam support.
Determine rafter lengths for single-slope shed roofs and lean-to additions.
Plan rafters for detached garages and carports with proper overhang.
Size rafters for covered porches and outdoor living spaces.
Match existing roof pitch and calculate rafters for home additions.
Calculate rafters for agricultural buildings and storage structures.
Use the formula: Rafter Length = √(Run² + Rise²). The run is half the span minus half the ridge board thickness. The rise = (Pitch ÷ 12) × Run. For example, a 24' span with 6:12 pitch: Run = 12' - 0.75" = 143.25", Rise = 71.625". Rafter = √(143.25² + 71.625²) = 160.16" or about 13'4".