How do you calculate rafter length for a 20 foot span?

Carpentry & FramingIRC Roof Framing Standards

Rafter Length Calculator (Rise, Run & Birdsmouth)

Calculate exact common rafter length, plumb cut angle, birdsmouth cut, and ridge beam thickness deductions based on roof span and pitch.

Carpentry & Framing Field StandardsIRC Roof Framing Standards

Engineering formulations calibrated to IPC, UPC & NEC specificationsSub-Zero Cold Climate Tolerances

Quick Summary & Key Answer

To calculate rafter length, use the Pythagorean theorem: Rafter Length = √(Run² + Rise²). For a 24-foot building span with a 6:12 pitch, the adjusted run is 11.94 feet (accounting for a 1.5" ridge), giving an 13.35-foot line length, plus a 1.12-foot overhang, totaling 14.47 feet (order 16-foot lumber).

Plumb cut angle at ridge = 90° - Pitch Angle.
Birdsmouth seat cut depth must not exceed 1/3 of the rafter width per IRC R802.7.1.
A 6:12 pitch has an incline angle of 26.57 degrees.

Interactive Calculator

Adjust inputs for real-time results

Building Span (Wall to Wall)feet

feet

Roof Pitch (/12)

Horizontal Eave Overhanginches

inches

Ridge Board Thickness

Calculated OutputRafter Geometry Verified

Total Rafter Cut Length

14.46'feet

Line Length (Plumb to Heel): 13.35 ft (160.2")
Tail Overhang Length: 13.4" (1.12 ft)
Recommended Stock Lumber Board: 16 ft Board (2x6/2x8)
Plumb Cut Angle: 63.4° (Level cut: 26.6°)

Note: Remember to deduct half the ridge board thickness from the building run before laying out the birdsmouth seat cut.

IRC Birdsmouth Notching Code

The horizontal seat cut bearing on the 2x4 or 2x6 wall plate must be at least 1-1/2 inches wide and must not notch deeper than one-third rafter depth.

Frequently Asked Questions

Common questions regarding this calculation and code compliance

At a 10-foot run with a standard 6:12 pitch, the rafter line length is 11.18 feet, plus overhang.