Rafter Calculator App

Rafter Calculator

Use this calculator to determine the key dimensions and angles for your roof rafters, ensuring accurate cuts and a structurally sound roof. Input your building's total span, desired roof pitch, rafter overhang, and ridge board thickness to get precise measurements for your rafter run, rise, length, and cut angles.

function calculateRafter() { var totalBuildingSpan = parseFloat(document.getElementById("totalBuildingSpan").value); var roofPitch = parseFloat(document.getElementById("roofPitch").value); var rafterOverhang = parseFloat(document.getElementById("rafterOverhang").value); var ridgeBoardThickness = parseFloat(document.getElementById("ridgeBoardThickness").value); // Input validation if (isNaN(totalBuildingSpan) || totalBuildingSpan <= 0 || isNaN(roofPitch) || roofPitch = 90 || isNaN(rafterOverhang) || rafterOverhang < 0 || isNaN(ridgeBoardThickness) || ridgeBoardThickness < 0) { document.getElementById("rafterResults").innerHTML = "Please enter valid positive numbers for all fields. Roof pitch must be between 1 and 89 degrees."; return; } // Convert inches to feet for consistent calculations var ridgeBoardThicknessFt = ridgeBoardThickness / 12; var rafterOverhangFt = rafterOverhang / 12; // Calculate Half Span var halfSpan = totalBuildingSpan / 2; // Calculate Rafter Run (horizontal distance from wall plate to ridge) var rafterRun = halfSpan – (ridgeBoardThicknessFt / 2); // Convert pitch to radians for trigonometric functions var pitchRadians = roofPitch * (Math.PI / 180); // Calculate Rafter Rise (vertical distance from wall plate to ridge) var rafterRise = rafterRun * Math.tan(pitchRadians); // Calculate Theoretical Rafter Length (from wall plate to ridge) var theoreticalRafterLength = Math.sqrt(Math.pow(rafterRun, 2) + Math.pow(rafterRise, 2)); // Calculate Total Rafter Length (including overhang) var totalRafterLength = theoreticalRafterLength + rafterOverhangFt; // Calculate Cut Angles var plumbCutAngle = roofPitch; var birdsmouthSeatCutAngle = 90 – roofPitch; // Helper function to convert decimal feet to feet and inches function convertFeetToFeetAndInches(decimalFeet) { var feet = Math.floor(decimalFeet); var inches = (decimalFeet – feet) * 12; return { feet: feet, inches: inches.toFixed(2) }; } var runConverted = convertFeetToFeetAndInches(rafterRun); var riseConverted = convertFeetToFeetAndInches(rafterRise); var totalLengthConverted = convertFeetToFeetAndInches(totalRafterLength); // Display results document.getElementById("rafterRunFt").innerText = runConverted.feet; document.getElementById("rafterRunIn").innerText = runConverted.inches; document.getElementById("rafterRiseFt").innerText = riseConverted.feet; document.getElementById("rafterRiseIn").innerText = riseConverted.inches; document.getElementById("totalRafterLengthFt").innerText = totalLengthConverted.feet; document.getElementById("totalRafterLengthIn").innerText = totalLengthConverted.inches; document.getElementById("plumbCutAngle").innerText = plumbCutAngle.toFixed(2); document.getElementById("birdsmouthSeatCutAngle").innerText = birdsmouthSeatCutAngle.toFixed(2); }

Understanding Roof Rafters and Their Calculations

Roof rafters are the structural members that extend from the ridge of a roof down to the wall plate, forming the framework that supports the roof covering. Accurate calculation of rafter dimensions and angles is crucial for the structural integrity, weather resistance, and aesthetic appeal of any building.

Key Rafter Terminology:

• Total Building Span: This is the total width of the building from outside wall to outside wall, perpendicular to the ridge. The rafter run is typically half of this span, minus half the ridge board thickness.
• Roof Pitch (Degrees): The angle or slope of the roof, usually expressed in degrees. A steeper pitch allows for better water runoff and can provide more attic space.
• Rafter Overhang: The portion of the rafter that extends beyond the exterior wall of the building, forming the eaves. This helps protect the walls from rain and sun.
• Ridge Board Thickness: The horizontal board at the very peak of the roof where the tops of the rafters meet. Its thickness needs to be accounted for when calculating the effective rafter run.
• Rafter Run: The horizontal distance from the outside of the wall plate to the center of the ridge board. This is a critical measurement for determining the roof's footprint.
• Rafter Rise: The vertical distance from the top of the wall plate to the top of the ridge board. Together with the run, it defines the roof's slope.
• Total Rafter Length: The actual length of the rafter from its plumb cut at the ridge to the end of its overhang. This is the length of the lumber you'll need to cut.
• Plumb Cut Angle: The angle at which the rafter is cut at the ridge and at the birdsmouth to ensure it sits flush against the ridge board and the wall plate. This angle is the same as the roof pitch.
• Birdsmouth Seat Cut Angle: The horizontal cut at the bottom of the rafter that rests on the wall plate. This angle is 90 degrees minus the roof pitch.

Why Accurate Rafter Calculations Matter:

Precise rafter calculations are fundamental for several reasons:

1. Structural Integrity: Incorrect lengths or angles can lead to an unstable roof structure, potentially causing sagging, leaks, or even collapse.
2. Material Efficiency: Accurate measurements minimize waste, saving both time and money on lumber and roofing materials.
3. Aesthetics: A well-calculated roof ensures a consistent pitch and a professional, finished appearance.
4. Building Code Compliance: Many building codes specify minimum roof pitches and structural requirements that depend on accurate rafter dimensions.

How to Use the Calculator:

Simply input the required values into the fields provided:

• Total Building Span: Measure the total width of your building in feet.
• Roof Pitch: Enter your desired roof pitch in degrees (e.g., 30 for a 30-degree roof).
• Rafter Overhang: Specify how far you want your rafters to extend past the wall in inches.
• Ridge Board Thickness: Input the actual thickness of the ridge board you plan to use in inches.

Click "Calculate Rafter Dimensions" to instantly get your rafter run, rise, total rafter length, and the necessary plumb and birdsmouth seat cut angles.

Example Calculation:

Let's say you have a building with a Total Building Span of 24 feet, you want a Roof Pitch of 30 degrees, a Rafter Overhang of 18 inches, and you're using a Ridge Board Thickness of 1.5 inches.

The calculator would provide the following results:

• Rafter Run: Approximately 11 ft 11.25 in
• Rafter Rise: Approximately 6 ft 10.69 in
• Total Rafter Length: Approximately 15 ft 3.40 in
• Plumb Cut Angle: 30.00 degrees
• Birdsmouth Seat Cut Angle: 60.00 degrees

These measurements give you the precise information needed to mark and cut your rafters accurately for a professional and sturdy roof.