Rafter Length Calculator

Frequently Asked Questions

How long is a rafter for a 12-foot run at 6:12 pitch?

Length = 12 x sqrt(1 + (6/12)^2) = 12 x 1.118 = 13.42 ft. With a 2-ft overhang: (12+2) x 1.118 = 15.65 ft.

What is the roof angle for a 6:12 pitch?

atan(6/12) = atan(0.5) = 26.6 degrees.

How many rafters for a 40-foot building at 16 OC?

ceil(40 / (16/12)) + 1 = ceil(30) + 1 = 31 rafter positions per side.

What is the pitch multiplier for a 5:12 roof?

sqrt(1 + (5/12)^2) = sqrt(1.1736) = 1.083. Multiply the run by 1.083 to get rafter length.

Provided by AllCalculators.io
Free online calculators for everyday. No registration required.

Estimates for informational purposes only.

Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

JavaScript is required to use the interactive calculator above. The questions and answers below remain readable without JavaScript.

How This Calculator Works

A common rafter forms the hypotenuse of a right triangle whose legs are the horizontal run (from the outside wall to directly below the ridge) and the vertical rise. For a 6:12 pitch, the rise is 6 inches for every 12 inches of run. The Pythagorean theorem gives the rafter length.

Rafter length (no overhang) = run_ft x sqrt(1 + (pitch/12)^2). Total rafter length = (run_ft + overhang_ft) x sqrt(1 + (pitch/12)^2). Roof angle in degrees = atan(pitch/12) x (180/pi). Rafter count = ceil(building_length_ft / (spacing_in/12)) + 1.

Common Roof Pitches

Pitch is expressed as rise over 12 inches of run. Common residential pitches and their properties:

3:12 (14 degrees) - low slope, fast draining, good for asphalt shingles if properly installed

4:12 (18 degrees) - minimum slope recommended by most shingle manufacturers

6:12 (27 degrees) - very common residential pitch, good balance of attic space and wind resistance

8:12 (34 degrees) - steep colonial style, larger attic, higher wind uplift

12:12 (45 degrees) - equal rise and run, classic cottage look

Pitches below 2:12 are considered flat or low-slope and require modified bitumen, TPO, or EPDM membrane roofing rather than asphalt shingles.

Overhang and Soffit

The overhang (eave extension) is the portion of the rafter that extends past the exterior wall. It protects the wall from rain, shades windows in summer, and gives the roof its characteristic look. Standard residential overhangs range from 12 to 24 inches. The overhang run adds to the rafter length using the same pitch multiplier as the main rafter, so total rafter = (run + overhang) x pitch factor.

The ridge board runs the length of the building at the peak. Its length equals the building length (ridge boards are typically one size thinner than the rafters - use a 1x ridge for 2x rafters).

FAQ

What is the run versus the span?

Span is the full width of the building from outside wall to outside wall. Run is half the span (for a simple gable with a centered ridge). Enter the run, not the span, in this calculator.

How many rafters for a 24-foot building at 16 OC?

Two sets of rafters meet at the ridge: ceil(24 / (16/12)) + 1 = 19 rafter positions per side, so 19 pairs (38 rafters total) plus a ridge board.

Do hip and valley roofs use the same formula?

No. Hip rafters and valley rafters have different geometry (they run diagonally across the plan). This calculator is for common (perpendicular to ridge) rafters only.

What length lumber should I buy?

Round the total rafter length up to the nearest standard lumber length (8, 10, 12, 14, 16, 18, 20 ft). Add 5-10% waste factor for bird's-mouth cuts and trim.

Rafter Length Calculator

Frequently Asked Questions

Related Construction Calculators