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Calculate roof rafter dimensions, ridge height, and cuts needed for accurate framing. Get precise measurements for common and hip rafters.
Span from outer wall to outer wall
Extend beyond wall plate
Typical: 1.5" (2×nominal)
Default: 2.5" (⅓ of rafter depth)
Enter values to calculate
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| Pitch Ratio | Angle (degrees) | Multiplier | Use Case |
|---|---|---|---|
| 2/12 | 9.46° | 1.014 | Low slope |
| 3/12 | 14.04° | 1.031 | Low slope |
| 4/12 | 18.43° | 1.054 | Low slope |
| 5/12 | 22.62° | 1.083 | Standard residential |
| 6/12 | 26.57° | 1.118 | Standard residential |
| 7/12 | 30.26° | 1.158 | Standard residential |
| 8/12 | 33.69° | 1.202 | Steep/Mountain |
| 9/12 | 36.87° | 1.250 | Steep/Mountain |
| 10/12 | 39.81° | 1.302 | Steep/Mountain |
| 12/12 | 45.00° | 1.414 | Steep/Mountain |
Multiplier: Use to calculate rafter length from run, or roof area from horizontal projection.
The main diagonal member from ridge to wall plate. Calculated using the Pythagorean theorem with run and rise measurements.
Vertical distance from wall plate to ridge board. Determines roof slope and interior ceiling height in attics.
Notch at the wall plate where the rafter sits. Has a plumb cut (vertical) and a seat cut (horizontal) for solid bearing.
Extension beyond the wall plate creating eaves for weather protection and shade. Typically 12-24 inches.
Common rafters run from the ridge perpendicular to the walls. Hip rafters run diagonally from the ridge to the corner of the building, requiring angle cuts on both ends. Use this calculator for common rafters; hip rafters need additional calculations.
Pitch selection depends on climate, aesthetics, and building code. 4:12 to 6:12 is typical for wet climates (drainage), 8:12+ for steep/aesthetic looks, 2:12 to 4:12 for low-slope/modern. Local codes may specify minimums for your area.
The birdsmouth is a notch in the rafter that allows it to sit flat on the wall plate. It has two parts: the plumb cut (vertical) and seat cut (horizontal). It prevents rafter rotation and provides solid bearing, which is critical for structural integrity and code compliance.
The ridge board thickness slightly reduces the rafter run length. Standard dimensions: 2×6 nominal = 1.5" actual, 2×8 = 1.5", 2×10 = 1.5", 2×12 = 1.5". Always measure your actual lumber as widths can vary by grade and species.
Rafter size depends on span, spacing (typically 16" or 24"), snow load, and local codes. 2×6 for short spans, 2×8 for 16-20' spans, 2×10 for 20-24' spans, 2×12 for 24-30' spans. Always consult local building codes and consider snow load requirements.
Standard spacing is 16" or 24" on center (O.C.). Divide building width by spacing: (30' × 12") ÷ 16" = 22.5, round up to 23 pairs of rafters. At 24" O.C.: (30' × 12") ÷ 24" = 15 pairs. Add one for the ridge and multiply by 2 for both sides.
The multiplier converts horizontal distances to sloped distances. For 6:12 pitch (multiplier 1.118): if the run is 15 feet, rafter length = 15' × 1.118 = 16.77 feet. Also used for roofing materials: if area is 2,000 sq ft horizontal, multiply by multiplier for actual roof coverage needed.
Use a framing square with the rafter table printed on it. Set the square to the pitch ratio (rise on tongue, run on body). Mark the plumb cut at the ridge, seat cut at the wall, and tail cut for overhang. Cut one rafter as a pattern and test-fit before cutting all others. Double-check measurements before cutting expensive lumber.