Calculating Rafter Length

Rafter Length Calculator

Calculation Results:

Calculated Run: 0 ft 0.00 in

Calculated Rise: 0 ft 0.00 in

Main Rafter Length (Plate to Ridge): 0 ft 0.00 in

Total Rafter Length (including overhang): 0 ft 0.00 in

Please enter valid positive numbers for all fields.
function convertInchesToFeetAndInches(totalInches) { var feet = Math.floor(totalInches / 12); var inches = (totalInches % 12).toFixed(2); return feet + " ft " + inches + " in"; } function calculateRafterLength() { var buildingSpanFt = parseFloat(document.getElementById('buildingSpan').value); var ridgeBoardThicknessIn = parseFloat(document.getElementById('ridgeBoardThickness').value); var rafterOverhangIn = parseFloat(document.getElementById('rafterOverhang').value); var roofPitchX = parseFloat(document.getElementById('roofPitch').value); // Input validation if (isNaN(buildingSpanFt) || isNaN(ridgeBoardThicknessIn) || isNaN(rafterOverhangIn) || isNaN(roofPitchX) || buildingSpanFt <= 0 || ridgeBoardThicknessIn < 0 || rafterOverhangIn < 0 || roofPitchX <= 0) { document.getElementById('rafterError').style.display = 'block'; document.getElementById('rafterResult').style.display = 'none'; return; } document.getElementById('rafterError').style.display = 'none'; document.getElementById('rafterResult').style.display = 'block'; // Convert building span to inches for consistent calculation var buildingSpanIn = buildingSpanFt * 12; // Calculate True Run (horizontal distance from wall plate to ridge) // This is half the span minus half the ridge board thickness var trueRunIn = (buildingSpanIn / 2) – (ridgeBoardThicknessIn / 2); if (trueRunIn <= 0) { document.getElementById('rafterError').innerHTML = 'Building span must be greater than ridge board thickness.'; document.getElementById('rafterError').style.display = 'block'; document.getElementById('rafterResult').style.display = 'none'; return; } // Calculate Rise (vertical distance from wall plate to ridge) // Rise = (Pitch / 12) * Run var riseIn = (roofPitchX / 12) * trueRunIn; // Calculate Main Rafter Length (from wall plate to ridge) using Pythagorean theorem // Rafter Length = sqrt(Run^2 + Rise^2) var mainRafterLengthIn = Math.sqrt(Math.pow(trueRunIn, 2) + Math.pow(riseIn, 2)); // Calculate Total Rafter Length (including overhang) var totalRafterLengthIn = mainRafterLengthIn + rafterOverhangIn; // Display results document.getElementById('calculatedRun').innerText = convertInchesToFeetAndInches(trueRunIn); document.getElementById('calculatedRise').innerText = convertInchesToFeetAndInches(riseIn); document.getElementById('mainRafterLength').innerText = convertInchesToFeetAndInches(mainRafterLengthIn); document.getElementById('totalRafterLength').innerText = convertInchesToFeetAndInches(totalRafterLengthIn); } // Initial calculation on page load with default values window.onload = calculateRafterLength;

Understanding Rafter Length Calculation

Calculating the correct rafter length is a fundamental step in roof framing. Accurate measurements ensure structural integrity, proper drainage, and a visually appealing roofline. This calculator helps you determine the precise length needed for your common rafters, taking into account key architectural elements.

What is a Rafter?

Rafters are the sloping structural members that extend from the ridge (the peak of the roof) to the wall plate (the top of the exterior wall). They support the roof sheathing, insulation, and roofing materials, transferring the load down to the walls and foundation.

Key Terms Explained:

  • Building Span: This is the total horizontal distance from the outside of one exterior wall to the outside of the opposite exterior wall. It represents the full width of the building that the roof covers.
  • Ridge Board Thickness: The ridge board is a non-structural board that runs along the peak of the roof, providing a nailing surface for the top ends of the rafters. Its thickness needs to be accounted for when calculating the rafter's horizontal run.
  • Rafter Overhang: This is the portion of the rafter that extends beyond the exterior wall plate, forming the eaves of the roof. It provides protection for the walls and can be decorative.
  • Roof Pitch (X in 12): Roof pitch describes the steepness of a roof. It's expressed as a ratio, typically "X in 12," meaning for every 12 inches of horizontal run, the roof rises X inches vertically. For example, a 6 in 12 pitch means the roof rises 6 inches for every 12 inches of horizontal distance.
  • Run: The horizontal distance from the outside of the wall plate to the center of the ridge. This forms one leg of the right-angled triangle used in the calculation.
  • Rise: The vertical distance from the top of the wall plate to the top of the ridge board. This forms the other leg of the right-angled triangle.

The Calculation Principle: Pythagorean Theorem

The core of rafter length calculation relies on the Pythagorean theorem: a² + b² = c². In roof framing:

  • a is the Run (horizontal distance).
  • b is the Rise (vertical distance).
  • c is the Main Rafter Length (the hypotenuse, from wall plate to ridge).

Once the main rafter length is found, the rafter overhang is simply added to get the total length of the rafter you need to cut.

How to Use the Calculator:

  1. Enter Building Span: Input the total width of your building in feet.
  2. Enter Ridge Board Thickness: Provide the thickness of your ridge board in inches (e.g., 1.5 for a 2x material).
  3. Enter Rafter Overhang: Specify how far you want your rafters to extend past the wall plate in inches.
  4. Enter Roof Pitch: Input the 'X' value of your roof's pitch (e.g., enter '6' for a 6 in 12 pitch).
  5. Click "Calculate Rafter Length": The calculator will instantly display the calculated run, rise, main rafter length, and the total rafter length including the overhang.

Example Calculation:

Let's say you have:

  • Building Span: 24 feet
  • Ridge Board Thickness: 1.5 inches
  • Rafter Overhang: 12 inches
  • Roof Pitch: 6 in 12

Here's how the calculator processes it:

  1. Building Span in Inches: 24 ft * 12 in/ft = 288 inches
  2. Calculated Run (Plate to Ridge): (288 in / 2) – (1.5 in / 2) = 144 in – 0.75 in = 143.25 inches (11 ft 11.25 in)
  3. Calculated Rise: (6 / 12) * 143.25 in = 0.5 * 143.25 in = 71.625 inches (5 ft 11.63 in)
  4. Main Rafter Length: √(143.25² + 71.625²) = √(20520.5625 + 5130.140625) = √25650.703125 &approx; 160.158 inches (13 ft 4.16 in)
  5. Total Rafter Length: 160.158 in + 12 in (overhang) = 172.158 inches (14 ft 4.16 in)

Using this calculator simplifies these complex steps, providing you with accurate measurements for your construction project.

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