🏠 Rafter Length Calculator
Calculate rafter length and roof pitch
Horizontal distance from wall to ridge (half the building width)
Vertical distance from wall to ridge
How to Use This Calculator
Enter Run Distance
Input the run in feet. This is the horizontal distance from the outside wall to the ridge (center of roof). For a 24-foot wide building, the run is 12 feet (half the width).
Enter Rise or Pitch
Choose to enter either rise (vertical distance) or pitch (inches per 12 inches). Common pitches are 4/12, 6/12, 8/12, or 12/12. Check the box to use pitch instead of rise.
Calculate Rafter Length
Click "Calculate Rafter Length" to see the rafter length, pitch, and angle. The calculator uses the Pythagorean theorem to calculate the diagonal rafter length.
Cut Rafters
Use the rafter length to cut your rafters. Add extra length for overhang (typically 12-18 inches) and account for birdsmouth cuts at the wall and ridge connections.
Formula
Rafter Length = √(Run² + Rise²)
Pitch = (Rise ÷ Run) × 12 (inches per 12 inches)
Pitch Angle = arctan(Rise ÷ Run) (degrees)
Rise from Pitch = (Run × Pitch) ÷ 12
Example 1: Standard Roof (12 ft run, 3 ft rise)
Step 1: Calculate pitch: (3 ÷ 12) × 12 = 3/12 pitch
Step 2: Calculate rafter length: √(12² + 3²) = √(144 + 9) = √153 = 12.37 feet
Step 3: Calculate angle: arctan(3 ÷ 12) = arctan(0.25) = 14.0°
Example 2: Steeper Roof (12 ft run, 6/12 pitch)
Step 1: Calculate rise: (12 × 6) ÷ 12 = 6 feet
Step 2: Calculate rafter length: √(12² + 6²) = √(144 + 36) = √180 = 13.42 feet
Step 3: Calculate angle: arctan(6 ÷ 12) = arctan(0.5) = 26.6°
About Rafter Length Calculator
The Rafter Length Calculator is an essential tool for contractors, roofers, carpenters, and DIY enthusiasts who need to accurately calculate rafter length for their roofing projects. This calculator eliminates guesswork by providing precise rafter length calculations using the Pythagorean theorem, helping you cut rafters to the correct length without waste or shortages.
When to Use This Calculator
- New Roof Construction: Calculate rafter lengths for new roofs, additions, or buildings
- Roof Replacement: Determine rafter lengths for replacing existing roof framing
- Deck Roofs: Estimate rafter dimensions for deck covers or patio roofs
- Shed Construction: Calculate rafter lengths for shed roofs or outbuildings
- Pitch Verification: Verify roof pitch and rafter dimensions for existing roofs
Why Use Our Calculator?
- ✅ Accurate Length: Precise calculations prevent cutting rafters too short or too long
- ✅ Pitch Calculations: Converts between pitch (inches per 12") and rise automatically
- ✅ Pitch Angle: Shows roof pitch angle in degrees for reference
- ✅ Multiple Inputs: Supports both rise and pitch inputs for convenience
- ✅ Cost Savings: Cut rafters accurately on first try, reducing waste
- ✅ Time Savings: Instant calculations eliminate manual math and measurements
Common Applications
Residential Roofing: Homeowners and contractors planning new roofs or roof replacements use this calculator to determine rafter lengths. Standard residential roofs use 6/12 pitch (6 inches rise per 12 inches run), which this calculator helps determine accurately.
Deck Roofs: Contractors and homeowners planning deck covers use this tool to estimate rafter dimensions needed. Deck roofs typically use similar pitches to residential roofs but may have different dimensions.
Shed Construction: DIY enthusiasts and contractors planning shed roofs use this calculator to determine rafter lengths needed, ensuring rafters are cut accurately before installation.
Tips for Best Results
- Run Calculation: Run is half the building width (from wall to ridge center)
- Add Overhang: Add 12-18 inches to rafter length for roof overhang beyond the wall
- Birdsmouth Cuts: Account for birdsmouth cuts (notches at wall and ridge) when cutting
- Common Pitches: Standard pitches: 4/12 (low), 6/12 (standard), 8/12 (moderate), 12/12 (45° angle)
- Verify Measurements: Always verify run and rise measurements before cutting rafters
Frequently Asked Questions
How do I calculate rafter length for a 24-foot wide building with 6/12 pitch?
For a 24-foot wide building, the run is 12 feet (half the width). With 6/12 pitch, the rise is (12 × 6) ÷ 12 = 6 feet. Rafter length = √(12² + 6²) = √180 = 13.42 feet. Add 12-18 inches for overhang, so cut rafters at approximately 14.5-15 feet long.
What's the difference between run and rise?
Run is the horizontal distance from the wall to the ridge (center of roof). Rise is the vertical distance from the wall to the ridge. For a 24-foot wide building, run is 12 feet (half the width). Rise depends on the roof pitch.
What is roof pitch?
Roof pitch is expressed as inches of rise per 12 inches of run. A 6/12 pitch means the roof rises 6 inches for every 12 inches of horizontal run. Common pitches: 4/12 (low), 6/12 (standard), 8/12 (moderate), 12/12 (45° angle).
Do I need to add extra length for overhang?
Yes! Always add 12-18 inches to the rafter length for roof overhang beyond the wall. Overhang protects walls from water and provides better drainage. Standard overhang is 12-18 inches, but check local building codes for requirements.
What's a birdsmouth cut?
A birdsmouth is a notch cut in the rafter where it sits on the wall plate. It consists of a horizontal cut (seat) and a vertical cut (heel) that allows the rafter to sit flat on the wall plate. Account for this when cutting rafters.
How do I find the run for my building?
Run is half the building width. For a 24-foot wide building, run is 12 feet. For a 30-foot wide building, run is 15 feet. Measure the total building width, then divide by 2 to get the run.