Free Rafter Calculator

Free Rafter Calculator

Use this calculator to determine the essential dimensions for your roof rafters, including rafter length, rise, pitch angle, and birdsmouth cuts. All inputs should be in inches unless specified.

The total horizontal distance from the outside of one wall plate to the outside of the opposite wall plate.

The vertical rise in inches for every 12 inches of horizontal run (e.g., enter '6' for a 6/12 pitch).

The horizontal run component of the roof pitch (standard is 12 inches).

The horizontal distance the rafter extends past the outside of the wall plate.

The thickness of the ridge board (e.g., 1.5″ for standard 2x lumber).

The actual width of the wall plate (e.g., 3.5″ for a 2×4, 5.5″ for a 2×6).

function convertInchesToFeetAndInches(totalInches) { if (isNaN(totalInches) || totalInches < 0) { return "N/A"; } var feet = Math.floor(totalInches / 12); var inches = totalInches % 12; return feet + "' " + inches.toFixed(2) + "\""; } function calculateRafter() { var errorDiv = document.getElementById("errorMessages"); errorDiv.innerHTML = ""; // Clear previous errors // Get input values var totalSpanInches = parseFloat(document.getElementById("totalSpanInches").value); var pitchRiseInches = parseFloat(document.getElementById("pitchRiseInches").value); var pitchRunInches = parseFloat(document.getElementById("pitchRunInches").value); var overhangRunInches = parseFloat(document.getElementById("overhangRunInches").value); var ridgeThicknessInches = parseFloat(document.getElementById("ridgeThicknessInches").value); var wallPlateWidthInches = parseFloat(document.getElementById("wallPlateWidthInches").value); // Validate inputs if (isNaN(totalSpanInches) || totalSpanInches <= 0) { errorDiv.innerHTML += "Please enter a valid positive number for Total Roof Span."; return; } if (isNaN(pitchRiseInches) || pitchRiseInches <= 0) { errorDiv.innerHTML += "Please enter a valid positive number for Roof Pitch Rise."; return; } if (isNaN(pitchRunInches) || pitchRunInches <= 0) { errorDiv.innerHTML += "Please enter a valid positive number for Roof Pitch Run."; return; } if (isNaN(overhangRunInches) || overhangRunInches < 0) { errorDiv.innerHTML += "Please enter a valid non-negative number for Horizontal Overhang."; return; } if (isNaN(ridgeThicknessInches) || ridgeThicknessInches < 0) { errorDiv.innerHTML += "Please enter a valid non-negative number for Ridge Board Thickness."; return; } if (isNaN(wallPlateWidthInches) || wallPlateWidthInches <= 0) { errorDiv.innerHTML += "Please enter a valid positive number for Wall Plate Width."; return; } // Calculations var effectiveRun = (totalSpanInches / 2) – (ridgeThicknessInches / 2); if (effectiveRun <= 0) { errorDiv.innerHTML += "Effective Rafter Run cannot be zero or negative. Check your Span and Ridge Board Thickness."; return; } var pitchAngleRadians = Math.atan(pitchRiseInches / pitchRunInches); var pitchAngleDegrees = pitchAngleRadians * (180 / Math.PI); var rise = effectiveRun * (pitchRiseInches / pitchRunInches); var rafterLineLength = Math.sqrt(Math.pow(effectiveRun, 2) + Math.pow(rise, 2)); var overhangLineLength = overhangRunInches / Math.cos(pitchAngleRadians); var totalRafterLength = rafterLineLength + overhangLineLength; var birdsmouthSeatLength = wallPlateWidthInches; var birdsmouthPlumbDepth = birdsmouthSeatLength * Math.tan(pitchAngleRadians); var fasciaCutAngleDegrees = pitchAngleDegrees; // Plumb cut angle is the same as the roof pitch angle // Display results document.getElementById("effectiveRunFtIn").innerText = convertInchesToFeetAndInches(effectiveRun); document.getElementById("riseFtIn").innerText = convertInchesToFeetAndInches(rise); document.getElementById("pitchAngleDegrees").innerText = pitchAngleDegrees.toFixed(2); document.getElementById("rafterLineLengthFtIn").innerText = convertInchesToFeetAndInches(rafterLineLength); document.getElementById("totalRafterLengthFtIn").innerText = convertInchesToFeetAndInches(totalRafterLength); document.getElementById("birdsmouthPlumbDepthInches").innerText = birdsmouthPlumbDepth.toFixed(2); document.getElementById("birdsmouthSeatLengthInches").innerText = birdsmouthSeatLength.toFixed(2); document.getElementById("overhangLineLengthInches").innerText = overhangLineLength.toFixed(2); document.getElementById("fasciaCutAngleDegrees").innerText = fasciaCutAngleDegrees.toFixed(2); }

Understanding Roof Rafters and Their Calculation

Roof rafters are the structural members that extend from the ridge or hip of the roof to the wall plate, forming the framework that supports the roof covering. Accurate rafter calculation is crucial for ensuring the structural integrity, proper drainage, and aesthetic appeal of any roof.

Key Rafter Terminology:

• Total Roof Span: The horizontal distance measured from the outside of one wall plate to the outside of the opposite wall plate. This defines the overall width of the roof structure.
• Effective Rafter Run: For a common rafter, this is the horizontal distance from the outside face of the wall plate to the centerline of the ridge board. It's typically half the total span minus half the ridge board thickness.
• Roof Pitch (Rise/Run): Describes the steepness of the roof. It's expressed as a ratio, typically "X in 12," where 'X' is the vertical rise in inches for every 12 inches of horizontal run.
• Roof Rise: The total vertical distance from the top of the wall plate to the top of the ridge board.
• Rafter Line Length: The true length of the rafter from the plumb cut at the ridge to the plumb cut at the wall plate (birdsmouth). This is the hypotenuse of the right triangle formed by the run and rise.
• Overhang: The portion of the rafter that extends horizontally past the exterior wall plate, providing shade and protecting the walls from rain.
• Ridge Board: The horizontal board at the peak of the roof to which the tops of the rafters are fastened.
• Wall Plate (Top Plate): The horizontal member at the top of the wall frame that supports the rafters.
• Birdsmouth Cut: A notch cut into the rafter where it rests on the wall plate. It consists of a horizontal "seat cut" and a vertical "plumb cut" to ensure a stable bearing surface.
• Fascia Plumb Cut: The vertical cut at the end of the rafter overhang, to which the fascia board is attached.

Why Accurate Rafter Calculation Matters:

Incorrect rafter dimensions can lead to a host of problems, including:

• Structural Weakness: Rafters that are too short or too long can compromise the roof's ability to withstand loads like snow, wind, and the weight of roofing materials.
• Drainage Issues: An improperly pitched roof can lead to water pooling, leaks, and premature deterioration of roofing materials.
• Aesthetic Problems: Uneven roof lines are visually unappealing and can detract from the overall look of a building.
• Material Waste: Errors in cutting lead to wasted lumber and increased project costs.
• Code Compliance: Building codes often specify minimum requirements for rafter sizing and roof pitch, which must be met for safety and legality.

How to Use This Rafter Calculator:

1. Measure Your Total Roof Span: Accurately measure the distance from the outside edge of one top wall plate to the outside edge of the opposite top wall plate. Enter this value in inches.
2. Determine Your Roof Pitch: Decide on the desired steepness of your roof. This is typically expressed as a "rise in 12" ratio (e.g., 6/12, 8/12). Enter the 'rise' value (e.g., 6) and keep the 'run' at 12.
3. Specify Horizontal Overhang: Enter the desired horizontal distance your rafters will extend beyond the wall plate.
4. Input Ridge Board Thickness: Measure the actual thickness of your ridge board (e.g., a 2x material is typically 1.5 inches thick).
5. Input Wall Plate Width: Measure the actual width of your wall plate (e.g., a 2×4 is 3.5 inches wide, a 2×6 is 5.5 inches wide).
6. Click "Calculate": The calculator will instantly provide all the critical dimensions you need for cutting your rafters.

Understanding the Results:

• Effective Rafter Run: This is the horizontal distance used in the right-angle triangle calculation for a single rafter.
• Total Roof Rise: The vertical height of your roof from the wall plate to the ridge.
• Roof Pitch Angle: The angle of your roof slope in degrees, useful for setting up miter saws or angle finders.
• Rafter Line Length (Wall Plate to Ridge): This is the main length of the rafter body, measured along its top edge from the ridge plumb cut to the birdsmouth plumb cut.
• Total Rafter Length (including overhang): The full length of the rafter from the ridge plumb cut to the end of the overhang fascia cut.
• Birdsmouth Plumb Cut Depth: The vertical depth of the notch where the rafter sits on the wall plate. This ensures the rafter sits flush and securely.
• Birdsmouth Seat Cut Length: The horizontal length of the notch that rests on the wall plate. This should match your wall plate width for full bearing.
• Overhang Line Length (along rafter): The length of the rafter that extends past the wall plate, measured along the rafter's top edge.
• Fascia Plumb Cut Angle: The angle at which the end of the rafter overhang should be cut to allow for a vertical fascia board. This angle is the same as the roof pitch angle.

Example Calculation:

Let's say you have a roof with a:

• Total Roof Span: 24 feet (288 inches)
• Roof Pitch: 6/12 (Rise: 6 inches, Run: 12 inches)
• Horizontal Overhang: 18 inches
• Ridge Board Thickness: 1.5 inches
• Wall Plate Width: 3.5 inches (for a 2×4 wall)

Using the calculator with these inputs, you would get results similar to:

• Effective Rafter Run: 11′ 11.25″
• Total Roof Rise: 5′ 11.63″
• Roof Pitch Angle: 26.57 degrees
• Rafter Line Length (Wall Plate to Ridge): 13′ 4.16″
• Total Rafter Length (including overhang): 15′ 0.28″
• Birdsmouth Plumb Cut Depth: 1.75 inches
• Birdsmouth Seat Cut Length: 3.50 inches
• Overhang Line Length (along rafter): 20.13 inches
• Fascia Plumb Cut Angle: 26.57 degrees

This calculator provides the precise measurements needed to cut your rafters accurately, saving time and material while ensuring a professional-quality roof structure.