Calculate Length of Rafters for Roof

Rafter Length Calculator

Calculation Results:

Total Rafter Length: —

Roof Pitch Angle: —

Roof Pitch Ratio: —

function calculateRafterLength() { var totalRunFeet = parseFloat(document.getElementById('totalRun').value); var totalRiseFeet = parseFloat(document.getElementById('totalRise').value); var ridgeThicknessInches = parseFloat(document.getElementById('ridgeThickness').value); var eaveOverhangFeet = parseFloat(document.getElementById('eaveOverhang').value); if (isNaN(totalRunFeet) || isNaN(totalRiseFeet) || isNaN(ridgeThicknessInches) || isNaN(eaveOverhangFeet) || totalRunFeet <= 0 || totalRiseFeet <= 0) { document.getElementById('calculatedRafterLength').innerText = 'Total Rafter Length: Please enter valid positive numbers for Run and Rise.'; document.getElementById('calculatedPitchAngle').innerText = 'Roof Pitch Angle: –'; document.getElementById('calculatedPitchRatio').innerText = 'Roof Pitch Ratio: –'; return; } // Convert ridge thickness to feet var ridgeThicknessFeet = ridgeThicknessInches / 12; // Effective run for one side of the roof (from wall plate to center of ridge) var effectiveRunFeet = totalRunFeet – (ridgeThicknessFeet / 2); // Calculate the main rafter length (hypotenuse of the triangle formed by effective run and rise) var mainRafterLengthFeet = Math.sqrt(Math.pow(effectiveRunFeet, 2) + Math.pow(totalRiseFeet, 2)); // Calculate the overhang portion of the rafter length // This is proportional to the main rafter length based on the overhang run var overhangRafterLengthFeet = (eaveOverhangFeet / effectiveRunFeet) * mainRafterLengthFeet; // Total rafter length including the overhang var totalRafterLength = mainRafterLengthFeet + overhangRafterLengthFeet; // Calculate the roof pitch angle var pitchAngleRadians = Math.atan(totalRiseFeet / effectiveRunFeet); var pitchAngleDegrees = pitchAngleRadians * (180 / Math.PI); // Calculate the roof pitch ratio (rise per 12 inches of run) var risePerFootOfRun = (totalRiseFeet / effectiveRunFeet) * 12; document.getElementById('calculatedRafterLength').innerText = 'Total Rafter Length: ' + totalRafterLength.toFixed(2) + ' feet'; document.getElementById('calculatedPitchAngle').innerText = 'Roof Pitch Angle: ' + pitchAngleDegrees.toFixed(2) + ' degrees'; document.getElementById('calculatedPitchRatio').innerText = 'Roof Pitch Ratio: ' + risePerFootOfRun.toFixed(1) + '/12'; }

Understanding Rafter Length Calculation for Roof Construction

Calculating the correct rafter length is a fundamental step in roof framing. An accurate measurement ensures structural integrity, proper drainage, and a visually appealing roofline. This calculator helps you determine the precise length of your common rafters, taking into account the main roof dimensions, ridge board, and eave overhang.

Key Components of Rafter Length:

• Total Horizontal Run: This is the horizontal distance from the outside of the wall plate to the center of the ridge board. For a symmetrical gable roof, it's typically half the total span of the building.
• Total Vertical Rise: This is the vertical distance from the top of the wall plate to the top of the ridge board. It dictates the steepness of your roof.
• Ridge Board Thickness: The ridge board is the horizontal timber at the peak of the roof where the rafters meet. Its thickness needs to be accounted for because the rafter's effective run is measured to the center of this board, not its face.
• Eave Overhang: This is the portion of the rafter that extends horizontally beyond the exterior wall plate, forming the eaves. It provides protection for the walls and can add architectural detail.

The Calculation Explained:

The core of rafter length calculation relies on the Pythagorean theorem (a² + b² = c²), which applies to right-angled triangles. In roof framing, the 'run' and 'rise' form the two shorter sides of a right triangle, and the rafter itself is the hypotenuse.

1. Effective Run: First, we adjust the total horizontal run by subtracting half the thickness of the ridge board. This gives us the true horizontal distance for one rafter from the wall plate to the point where it meets the center of the ridge.
2. Main Rafter Length: Using the effective run and the total vertical rise, we calculate the length of the rafter from the wall plate to the ridge. This is sqrt(Effective Run² + Rise²).
3. Overhang Length: The eave overhang is then added. This isn't simply adding the horizontal overhang distance; instead, we calculate the proportional length of the rafter that extends past the wall plate, maintaining the roof's pitch. This is done by multiplying the horizontal overhang by the ratio of the main rafter length to the effective run.
4. Total Rafter Length: The main rafter length and the overhang rafter length are summed to give you the total length of the rafter you need to cut.

Understanding Roof Pitch:

The calculator also provides two important pitch measurements:

• Roof Pitch Angle: This is the angle, in degrees, that the roof surface makes with the horizontal plane. It's crucial for understanding the roof's steepness and for various construction tasks.
• Roof Pitch Ratio: Often expressed as "X/12" (e.g., 6/12, 8/12), this ratio indicates how many inches the roof rises vertically for every 12 inches of horizontal run. This is a common way for builders to describe roof steepness.

Example Scenario:

Let's say you have a building with a total horizontal run of 12 feet (meaning the span is 24 feet), and you want a roof with a total vertical rise of 6 feet. You're using a standard 1.5-inch thick ridge board and desire a 1-foot eave overhang.

• Total Horizontal Run: 12 ft
• Total Vertical Rise: 6 ft
• Ridge Board Thickness: 1.5 inches
• Eave Overhang: 1 ft

Using the calculator:

• Effective Run = 12 ft – (1.5 inches / 12 inches/ft / 2) = 12 ft – 0.0625 ft = 11.9375 ft
• Main Rafter Length = sqrt(11.9375² + 6²) = sqrt(142.50 + 36) = sqrt(178.50) ≈ 13.36 ft
• Overhang Rafter Length = (1 ft / 11.9375 ft) * 13.36 ft ≈ 1.12 ft
• Total Rafter Length: 13.36 ft + 1.12 ft = 14.48 feet
• Roof Pitch Angle: Approximately 26.6 degrees
• Roof Pitch Ratio: Approximately 6.0/12

This calculator simplifies these complex calculations, providing you with the precise measurements needed for your roofing project.