Rafter Length Calculator

A birdsmouth cut is a notch cut into the rafter so it sits flat on the top wall plate. It slightly lowers the roof line but does not change the centerline mathematical rafter length.

Yes. Since the rafter meets the center ridge board, you must subtract half the thickness of the ridge board (typically 0.75 inches for 2x lumber) from the final length.

Most structural codes recommend spacing rafters either 16 inches or 24 inches on-center depending on the lumber size and expected snow/wind load.

Convert pitch to rise: multiply the run by the pitch ratio (rise/12). Then use the Pythagorean theorem: rafter length = √(run² + rise²). A 6/12 pitch over 10 feet of run gives 5 feet of rise.

Common sizes are 2×6, 2×8, 2×10, or 2×12 depending on span, spacing, and load requirements. A 2×8 at 16-inch spacing can typically span 14 to 18 feet depending on lumber grade.

Common pitch multipliers: 3/12 = 1.031, 4/12 = 1.054, 5/12 = 1.083, 6/12 = 1.118, 8/12 = 1.202, 10/12 = 1.302, 12/12 = 1.414. Multiply horizontal measurements by these to get slope distances.

The birdsmouth cut removes material from the underside of the rafter where it sits on the wall plate. While it does not change the overall centerline length, you need to mark the cut location precisely, typically at the wall's outer edge.

A common rafter runs perpendicular from the wall to the ridge. A hip rafter runs at a 45-degree angle from the building corner to the ridge, forming the hip of the roof. Hip rafters are longer than common rafters for the same run and rise.

Standard residential roof overhangs are 12 to 24 inches. The overhang protects walls from rain and provides shade. Larger overhangs may require fly rafters and lookouts for structural support.

A collar tie (or rafter tie) is a horizontal member connecting opposite rafters, typically in the upper third of the attic space. It prevents rafters from spreading under load. Most codes require collar ties for spans over 16 feet.

A shed roof (single-slope) uses the same common rafter formula. The run is the horizontal distance from the high wall to the low wall. Calculate rise based on the desired slope and apply the Pythagorean theorem.

At 16-inch spacing, a #2 grade 2×8 can typically span 14 to 16 feet. At 24-inch spacing, the maximum span decreases to approximately 12 to 13 feet. Always check local building codes and engineering tables.

Higher snow loads require deeper rafter lumber or closer spacing. Areas with heavy snowfall (40+ psf ground load) may require 2×10 or 2×12 rafters at 16-inch spacing instead of 2×8.

Pre-cut trusses are engineered, factory-built, and typically cheaper for standard roof designs. Site-built rafters offer more design flexibility for complex roofs, dormers, and vaulted ceilings.

The tail cut (or plumb cut) is the angled cut at the lower end of the rafter that creates the soffit fascia line. It is cut at the same angle as the roof pitch for a vertical fascia board.

Measure from the ridge centerline down the rafter to the point where the rafter crosses the outer wall plate. The birdsmouth seat cut should be positioned so the top of the rafter aligns with the top of the wall plate.

Span is the total horizontal distance between supporting walls (the full width of the building). Run is half the span for a symmetrical gable roof, measuring from the wall to the ridge centerline.

Yes. Cathedral ceiling rafters use the same length calculation. The rafter length includes the full slope distance from the wall plate to the ridge, with no collar ties obstructing the open ceiling space.

Valley and hip rafters require specialized compound angle calculations. This calculator is for common rafters only. Use our hip rafter calculator for hip and valley rafter lengths and cut angles.

The most common residential roof pitch is 4/12 to 6/12. Pitches below 4/12 require special waterproofing underlayment. Steeper pitches (8/12 to 12/12) are common in regions with heavy snowfall.

The Rafter Length Calculator uses standard mathematical formulas validated against authoritative references. The specific formula is displayed in the calculator interface with a detailed explanation of each variable.

Each calculator includes a step-by-step worked example showing exactly how the formula is applied. You can follow these steps with pen and paper to verify any result.

The Rafter Length Calculator accepts numeric inputs including integers and decimals. Invalid inputs (letters, special characters) are rejected with clear error messages.