Rafter Length Calculator – Guide & Formulas
Calculate rafter length from run, rise, pitch, and overhang. Includes collar tie and birdsmouth adjustments.
Try the free calculator
Put these formulas into practice with our instant, step-by-step Rafter Length Calculator.
Calculate **precise common rafter lengths** for sloped roofs with our free **rafter length calculator**. Account for **run**, **rise**, **pitch**, **birdsmouth cut** depth, and **overhang** dimensions to plan your roofing materials with high carpentry precision. Whether you are framing a gable roof, replacing old rafters, or building a shed, this calculator provides accurate rafter dimensions including lumber stock length recommendations.
Key Takeaway
Use the free Rafter Length Calculator to calculate rafter length from run, rise, pitch, and overhang. includes collar tie and birdsmouth adjustments. Get instant results with step-by-step explanations.
How to Use the Rafter Length Calculator
- Enter the horizontal run of the roof (half the span of the building).
- Choose whether to specify by vertical rise or roof pitch ratio (e.g., 6/12).
- Input overhang length and birdsmouth cut details to compute full rafter stock length.
- Enter the ridge board thickness if applicable (standard is 1.5 inches for 2x lumber).
- Review the calculated rafter length and recommended lumber stock size.
The Formula
Variable Definitions
- Run: Horizontal distance from wall plate to ridge centerline in feet
- Rise: Vertical distance from top plate to ridge centerline in feet
- OverhangLength: Extra sloped length extended past the outer wall (soffit tail)
- RidgeThickness: Thickness of the ridge board at the roof peak, typically 1.5 inches for 2x lumber
- Pitch: Roof slope ratio expressed as rise over 12 inches of run (e.g., 6/12 means 6 inches rise per 12 inches run)
Common Rafter for 20ft Span House
You are framing a gable roof over a 20-foot wide house. The horizontal run is 10 feet (half span), the roof pitch is 6/12, and you want a 12-inch horizontal overhang.
- Step 1: Calculate rise: Pitch is 6/12, so for a 10 ft run, rise is 10 × 6" = 60 inches or 5 feet.
- Step 2: Find base rafter length: √(10² + 5²) = √125 ≈ 11.18 feet (11 ft 2-3/16 inches).
- Step 3: Calculate slope multiplier for 12-inch horizontal overhang: Multiplier is 1.118, so overhang length on slope = 1.118 × 1 ft = 1.118 ft.
- Step 4: Subtract half the ridge board thickness: 1.5 inches / 12 = 0.125 feet. Adjusted length = 11.18 + 1.118 - 0.125 = 12.17 feet.
- Step 5: Combine lengths and round up to nearest standard lumber size: 12.17 feet → order 14-foot lumber for stock cuts.
Financial Advisory Notice
This calculator provides estimates for rafter length calculations based on standard geometric formulas. Actual structural requirements depend on local building codes, snow loads, wind ratings, and lumber grades. Always consult a licensed structural engineer for load-bearing roof designs.
Frequently Asked Questions
How does a birdsmouth cut affect rafter length?
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.
Should I subtract the ridge beam thickness?
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.
What is the standard spacing for roof rafters?
Most structural codes recommend spacing rafters either 16 inches or 24 inches on-center depending on the lumber size and expected snow/wind load.
How do I calculate rafter length from roof pitch?
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.
What size lumber should I use for rafters?
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.
What is the pitch multiplier for different roof slopes?
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.
How do I account for a birdsmouth cut in rafter length?
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.
What is the difference between a common rafter and a hip rafter?
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.
How much overhang should a roof have?
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.
What is a collar tie and do I need one?
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.
How do I calculate the rafter length for a shed roof?
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.
What is the maximum rafter span for a 2×8?
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.
How does snow load affect rafter sizing?
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.
Should I use pre-cut trusses or site-built rafters?
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.
What is the tail cut on a rafter?
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.
How do I measure for rafter birdsmouth placement?
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.
What is the difference between span and run?
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.
Can I use this calculator for cathedral ceiling rafters?
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.
How do I handle rafters for a valley or hip roof?
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.
What is the standard roof pitch for residential homes?
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.