Rafter Length
Rafter length from span, pitch, overhang.
Example
You enter
- Horizontal run (ft; half the building span on a gable) 12
- Pitch (rise per 12) 6
- Overhang (ft) 1
You get
- Multiplier 1.118
- Rafter length 14.534 ft
Checked against published worked examples from: CertainTeed; Naval Education and Training Professional Development and Technology Center.
Details, formula, and sources
Formula or tableRafter length = (horizontal run + overhang) x sqrt(P^2 + 144)/12; no ridge or plumb-cut allowance. Birdsmouth (seat-cut) depth: IRC R802.7.1 applies the R502.8.1 notch limits, so a notch at the bearing end may not exceed 1/4 of the rafter depth (R802.7.1.1 allows a deeper notch on a cantilevered tail that keeps 3-1/2 in); the 1/3 often quoted is a rule of thumb, not the IRC, and this tile computes length only.
Source and editionIRC 2024 (International Residential Code). §R802.4 (rafters) and §R802.7.1 (notching), unchanged from 2021. Until 2026-10-17 this cited R802.5, which is ceiling joists.
Free accessFree read-only access at codes.iccsafe.org.
What governsEstimate. AHJ and licensed structural engineer govern.
API field nameshorizontal_span_ft, pitch_rise_per_12, overhang_ft, multiplier, rafter_length_ft
- Heel-cut depth limit end notch ≤ 1/4 of rafter depth (≤ 1/3 is a rule of thumb, not the IRC)IRC 2024 §R802.7.1 / R502.8.1