Roof Truss Design Calculator

Roof Truss Design Parameter Calculator

Truss Span (feet):

Roof Pitch Rise (inches, e.g., 4 for 4/12 pitch):

Roof Pitch Run (inches, typically 12):

Eave Overhang (feet):

Truss Spacing (feet, for area estimation):

Calculation Results:

Roof Pitch Angle:

Total Truss Height (Peak to Bottom Chord):

Main Rafter Length (Wall Plate to Peak):

Overhang Rafter Length (Sloped):

Total Top Chord Length (One Side):

Bottom Chord Length:

Estimated Roof Surface Area per Truss:

function calculateTrussDesign() { var trussSpan = parseFloat(document.getElementById('trussSpan').value); var roofRise = parseFloat(document.getElementById('roofRise').value); var roofRun = parseFloat(document.getElementById('roofRun').value); var eaveOverhang = parseFloat(document.getElementById('eaveOverhang').value); var trussSpacing = parseFloat(document.getElementById('trussSpacing').value); if (isNaN(trussSpan) || isNaN(roofRise) || isNaN(roofRun) || isNaN(eaveOverhang) || isNaN(trussSpacing) || trussSpan <= 0 || roofRise <= 0 || roofRun <= 0 || eaveOverhang < 0 || trussSpacing <= 0) { document.getElementById('trussResults').innerHTML = '

Please enter valid positive numbers for all fields. Overhang can be zero.

'; return; } // Convert rise/run to a ratio for angle calculation var pitchRatio = roofRise / roofRun; // e.g., 4/12 = 0.333 var pitchAngleRad = Math.atan(pitchRatio); var pitchAngleDeg = pitchAngleRad * (180 / Math.PI); // Half span for calculations var halfSpan = trussSpan / 2; // 1. Total Truss Height (from bottom chord to peak) // Height = (Half Span) * tan(Pitch Angle) var totalTrussHeight = halfSpan * Math.tan(pitchAngleRad); // 2. Main Rafter Length (from wall plate to peak) // This is the hypotenuse of a right triangle with halfSpan and totalTrussHeight var mainRafterLength = Math.sqrt(Math.pow(halfSpan, 2) + Math.pow(totalTrussHeight, 2)); // 3. Overhang Rafter Length (the sloped part of the overhang) // Overhang along slope = Overhang (horizontal) / cos(Pitch Angle) var overhangRafterLength = eaveOverhang / Math.cos(pitchAngleRad); // 4. Total Top Chord Length (one side) var totalTopChordLength = mainRafterLength + overhangRafterLength; // 5. Bottom Chord Length var bottomChordLength = trussSpan; // 6. Estimated Roof Surface Area per Truss (one side * 2 * truss spacing) // This is a simplified estimation. Actual area depends on sheathing layout. var estimatedSurfaceAreaPerTruss = (totalTopChordLength * trussSpacing) * 2; // Two sides of the roof document.getElementById('resultPitchAngle').innerHTML = 'Roof Pitch Angle: ' + pitchAngleDeg.toFixed(2) + ' degrees'; document.getElementById('resultTrussHeight').innerHTML = 'Total Truss Height (Peak to Bottom Chord): ' + totalTrussHeight.toFixed(2) + ' feet'; document.getElementById('resultMainRafterLength').innerHTML = 'Main Rafter Length (Wall Plate to Peak): ' + mainRafterLength.toFixed(2) + ' feet'; document.getElementById('resultOverhangRafterLength').innerHTML = 'Overhang Rafter Length (Sloped): ' + overhangRafterLength.toFixed(2) + ' feet'; document.getElementById('resultTotalTopChordLength').innerHTML = 'Total Top Chord Length (One Side): ' + totalTopChordLength.toFixed(2) + ' feet'; document.getElementById('resultBottomChordLength').innerHTML = 'Bottom Chord Length: ' + bottomChordLength.toFixed(2) + ' feet'; document.getElementById('resultSurfaceArea').innerHTML = 'Estimated Roof Surface Area per Truss: ' + estimatedSurfaceAreaPerTruss.toFixed(2) + ' sq ft'; } // Calculate on page load with default values window.onload = calculateTrussDesign;

Understanding Roof Truss Design Parameters

Roof trusses are engineered structural components that form the framework of a roof, supporting the roof deck, insulation, and finishes. They are designed to efficiently transfer loads from the roof to the building's walls and foundation. While a complete truss design involves complex structural analysis by engineers, understanding the fundamental geometric parameters is crucial for planning and preliminary design.

What is a Roof Truss?

A roof truss is essentially a pre-fabricated triangular structure made from timber or light-gauge steel, connected by metal plates or gussets. The triangular shape provides inherent strength and stability, allowing trusses to span large distances without intermediate supports, unlike traditional stick-framed roofs.

Key Design Parameters Explained

Our Roof Truss Design Parameter Calculator helps you determine essential dimensions based on your project's specifications. Here's a breakdown of the inputs and outputs:

Input Fields:

Truss Span (feet): This is the horizontal distance from the outside of one exterior wall to the outside of the opposite exterior wall that the truss will cover. It's the total width of the building the roof will span.
Roof Pitch Rise (inches): The vertical rise of the roof for every 12 inches of horizontal run. For example, a "4/12 pitch" means the roof rises 4 inches for every 12 inches of horizontal distance. This input takes the '4' part.
Roof Pitch Run (inches): The horizontal distance over which the 'rise' is measured, typically 12 inches in standard pitch notation (e.g., 4/12). This input takes the '12' part.
Eave Overhang (feet): The horizontal distance that the roof extends beyond the exterior wall. This provides protection for the walls and can contribute to architectural aesthetics.
Truss Spacing (feet): The center-to-center distance between individual trusses. Common spacings are 16 inches (1.33 ft) or 24 inches (2 ft). This input is used to estimate the roof surface area covered by a single truss.

Output Results:

Roof Pitch Angle (degrees): The actual angle of the roof slope measured from the horizontal. This is derived from your rise/run input.
Total Truss Height (Peak to Bottom Chord): The vertical distance from the bottom edge of the truss (where it rests on the wall) to the highest point (the peak). This is crucial for determining overall building height and attic space.
Main Rafter Length (Wall Plate to Peak): The length of the top chord of the truss from where it sits on the wall plate up to the peak of the roof.
Overhang Rafter Length (Sloped): The actual sloped length of the top chord that extends past the wall, forming the eave overhang.
Total Top Chord Length (One Side): The combined length of the main rafter and the overhang rafter for one side of the roof. This represents the total length of the sloped member from the very end of the overhang to the peak.
Bottom Chord Length: The horizontal length of the bottom member of the truss, which typically matches the truss span.
Estimated Roof Surface Area per Truss: A simplified estimation of the roof surface area that a single truss supports, based on its total top chord length and the specified truss spacing. This can be useful for material estimations like sheathing or roofing felt per truss.

Important Considerations:

This calculator provides geometric parameters for preliminary planning. It does NOT perform structural analysis or account for specific loading conditions (snow, wind, dead loads), material strengths, or connection details. Always consult with a qualified structural engineer and truss manufacturer for final truss designs that comply with local building codes and ensure the safety and integrity of your structure.

Factors like truss type (e.g., King Post, Fink, Howe), web member configurations, and lumber dimensions significantly impact a truss's load-bearing capacity and are beyond the scope of this geometric calculator.