Calculate rafter length, ridge height, plumb cut angle, and bird's mouth cut for roof framing
Total width of building (wall to wall)
Common residential pitches from shallow to steep
Typical range: 12-24 inches (enter 0 for no overhang)
Standard: 3.5" (2×4 wall), max 1/3 of rafter depth
Rafter Length = √(Run² + Rise²)
Example: 10 ft run, 5 ft rise → √(10² + 5²) = √125 = 11.18 ft
Rise = Run × (Pitch Rise / 12)
Example: 10 ft run with 6/12 pitch → 10 × (6/12) = 5 ft rise
Angle = arctan(Rise / Run) in degrees
Example: 5 ft rise, 10 ft run → arctan(0.5) = 26.57°
Important Notes:
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Rafter length is calculated using the Pythagorean theorem: √(run² + rise²). The run is half the building span, and the rise is determined by the roof pitch. For example, with a 20-foot span (10-foot run) and 6/12 pitch (6-inch rise per foot), the rise is 5 feet (10 ft × 6/12), and rafter length = √(10² + 5²) = 11.18 feet.
The bird's mouth (or seat cut) is a notch cut into the rafter where it sits on the top plate of the wall. It typically consists of a horizontal seat cut and a vertical plumb cut. The seat cut should not exceed 1/3 of the rafter depth to maintain structural integrity. Standard depth is usually 3.5 inches for a 2×6 or larger rafter.
The plumb cut angle is the vertical cut at the top of the rafter where it meets the ridge board. It's calculated as arctan(rise/run) in degrees. For a 6/12 pitch, this equals 26.57 degrees. This angle ensures the rafter fits properly against the ridge and allows for proper alignment of the roof plane.
Typical rafter overhangs range from 12 to 24 inches, with 16-18 inches being most common for residential construction. The overhang protects walls from rain and provides shade. Longer overhangs may require additional support or larger rafter sizes. Always check local building codes for specific requirements in your area.
"This calculator is a lifesaver on the job site. I can quickly verify my rafter calculations and cut angles without pulling out the Speed Square every time. The bird's mouth calculations are especially helpful for training new crew members. Accurate and reliable!"
"Building my own shed was daunting until I found this calculator. It broke down every measurement I needed including the cuts I had never done before. The visual breakdown of rise, run, and angles made everything clear. My rafters fit perfectly on the first try!"
"Great tool for preliminary framing calculations. I use it to quickly check rafter dimensions during the design phase. The ability to test different pitches helps clients visualize options."
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