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".
A bird's mouth is a notch cut into the rafter where it sits on the wall plate. It consists of a horizontal seat cut (sits on the plate) and a vertical plumb cut (against the wall). This joint provides a stable connection and prevents the rafter from sliding off the wall.
The plumb cut angle equals the roof pitch angle. For a 6:12 pitch, set your miter saw to 26.57°. For a 4:12 pitch, use 18.43°. For a 12:12 pitch, use 45°. Our calculator shows the exact angle for your pitch.
Typical overhangs range from 12" to 24". Standard is 12-18" for most homes. Larger overhangs (24"+) provide better wall protection and shade but may require additional support. Check local codes and wind load requirements for your area.
Rafter size depends on span, spacing, and snow/wind loads. General guidelines: 2×6 for spans up to 10', 2×8 for 10-15', 2×10 for 15-20', 2×12 for longer spans. Use span tables or consult an engineer for your specific loads and code requirements.
The ridge board sits between opposing rafters at the peak. Its thickness (typically 1.5" for dimensional lumber) reduces each rafter's horizontal run by half the ridge thickness. This ensures rafters meet properly at the ridge.
16" on center is standard for residential roofs and provides good support for most roofing materials. 24" OC can be used for non-structural applications or where larger lumber compensates. Some roof sheathing requires 16" spacing. Check local codes.
Add 6-12" to your calculated rafter length and round up to the next standard lumber length (8', 10', 12', 14', 16', etc.). This allows for cutting waste and any measurement adjustments. Our calculator recommends the appropriate standard length.