Rafter Length Calculator

Rafter length is calculated using the Pythagorean theorem. A rafter forms the hypotenuse of a right triangle where the horizontal leg is the run and the vertical leg is the rise.

The Formula

The basic formula is:

Rafter Length = √(Run² + Rise²) + Overhang

Where:

• Run = Span ÷ 2 (distance from exterior wall to center of ridge)
• Rise = Run × (Pitch ÷ 12)
• Overhang = eave projection in feet (typically 1 ft)

Worked Example

For a building with a 30 ft span, 6/12 pitch, and 12" overhang:

1. Run = 30 ÷ 2 = 15 ft
2. Rise = 15 × (6 ÷ 12) = 7.5 ft
3. Rafter = √(15² + 7.5²) = √(225 + 56.25) = √281.25 = 16.77 ft
4. With overhang = 16.77 + 1.0 = 17.77 ft

You would order 18 ft or 20 ft lumber to accommodate the birdsmouth cut and any trimming needed.

The International Residential Code (IRC) sets minimum standards for rafter design. Here are the key requirements most jurisdictions follow:

• Minimum rafter depth: Nominal 2x4 for spans up to 5 ft; 2x6 or larger for typical residential use
• Spacing: Maximum 24" on center; 16" O.C. is standard for most applications
• Load requirements: Must support dead loads (roofing materials, ~10-15 psf) plus live loads (snow, workers, ~20 psf minimum, higher in snow regions)
• Bearing: Minimum 1.5" bearing on the wall plate; birdsmouth cuts must leave at least 2/3 of the rafter depth intact
• Ridge connections: Rafters must be connected at the ridge with a ridge board at least 1" nominal thickness, or use an engineered ridge beam
• Collar ties: Required in the upper third of the rafter span to prevent roof spread, typically at every other rafter pair minimum

These are general guidelines based on the IRC. Local jurisdictions may have stricter requirements, especially in high-wind, seismic, or heavy-snow areas. Always consult your local building department and consider hiring a structural engineer for non-standard designs.

Traditional stick-framed rafters and factory-built engineered trusses each have advantages depending on the project. Rafters are ideal for simple gable and shed roofs, additions, and situations where you want usable attic space beneath the roof. They are cut and assembled on site, giving the framer flexibility to adjust dimensions. A rafter length calculator like the one above makes the math quick and accurate.

Engineered trusses, on the other hand, are manufactured in a factory to exact specifications and delivered to the job site ready to install. They are the preferred choice for complex roof designs, wide spans exceeding 20 feet, and production home building where speed and consistency matter. Trusses typically cost 10–20% less than equivalent stick-framed roof rafter systems when labor is factored in, because installation is significantly faster. However, trusses create a web of internal members that eliminates usable attic space.