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Rafter Calculator
Common, hip, and valley rafters. Get line length, total length with overhang, plumb cut angle, and birdsmouth max depth.
Building width
e.g. 6 = 6/12 pitch
Tail length
2×6 = 5.5″ · 2×8 = 7.25″ · 2×10 = 9.25″ · 2×12 = 11.25″
Common Rafter · 6/12 Pitch
14′ 6 7/16″
Total length (line length + overhang)
Line Length
13′ 5″
Run
12′ 0″
Rise
6′ 0″
Plumb Cut Angle
26.57°
Overhang Length
1′ 1 7/16″
Max Seat Cut
2 7/16″
Birdsmouth seat-cut rule of thumb: 1/3 of rafter depth max (per IRC 2018). For hip/valley rafters, run is multiplied by √2 ≈ 1.414.
How to Use This Calculator
Enter your building span (full width — wall to wall), the pitch (rise per 12 of run), and your overhang. The calculator handles the math: line length, total length, plumb cut angle, and the maximum birdsmouth depth allowed by IRC 2018.
Common Rafter
The standard sloping rafter that runs perpendicular from a wall plate to the ridge. Run = span / 2. Rise = run × (rise/12). Line length = √(rise² + run²).
Hip Rafter
Runs diagonally from a corner of the building up to the ridge. Because the rafter travels at a 45° angle relative to the building, run is multiplied by √2 (≈ 1.414). Pitch is expressed per 17 instead of per 12 for hip-specific tables.
Valley Rafter
Runs along the inside corner where two roof planes meet. Same √2 multiplier as a hip rafter — just installed in a valley instead of on a hip. Often the longest single piece of dimensional lumber on a roof.
Frequently Asked Questions
How do I calculate rafter length?
Rafter line length = √(rise² + run²). For a 6/12 pitch on a 24-foot span: run = 12′, rise = 6′, line length = √(144+36) = √180 ≈ 13.42′. Add overhang × pitch multiplier for total rafter length.
What is the difference between common, hip, and valley rafters?
Common rafters run perpendicular from wall plate to ridge. Hip rafters run from a corner of the building diagonally up to the ridge. Valley rafters run in the inside corner where two roof planes meet. Hip and valley rafter runs are √2 ≈ 1.414 times longer than common rafter runs because they travel diagonally.
What is a birdsmouth cut?
The birdsmouth (or seat cut) is a notch cut into the underside of a rafter where it sits on the wall plate. It has two parts: the plumb cut (vertical, against the wall) and the seat cut (horizontal, on the plate). The 2018 IRC limits seat depth to 1/3 of rafter depth for structural safety.
How deep can I make a birdsmouth seat cut?
Per IRC 2018, the seat cut cannot exceed 1/3 the depth of the rafter. For a 2×8 (7.25″ actual): max 2.42″. For a 2×10 (9.25″): max 3.08″. For a 2×12 (11.25″): max 3.75″. Cutting deeper compromises rafter strength and may fail inspection.
What pitch should I use for hip and valley rafters?
Hip and valley rafters use a unit run of 17 (instead of 12) on the rafter table. This accounts for the √2 diagonal. So a 6/12 common rafter becomes 6/17 on a hip — but the angle relative to the wall plate is the same. The Rapid Rafter has both 12 and 17 unit-run scales built in.
How does a rafter multiplier work?
The rafter multiplier is √(rise² + 144) / 12. It tells you how many inches of rafter you get per inch of horizontal run. For 6/12 pitch: √(36+144)/12 = 1.1180. So 12 inches of run = 13.42 inches of rafter. Multiply your run by the multiplier to get line length.
Stop Doing This Math by Hand
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