Attic trusses, also known as "Room in the Attic" trusses, are engineered structural components designed to create usable living space within the attic of a building. Unlike standard roof trusses, attic trusses have a clear central space, free of web members, that can be finished into a bedroom, office, or storage area. The design and dimensions of these trusses are crucial for both structural integrity and maximizing usable space.
Calculating the basic dimensions for an attic truss involves understanding roof geometry, specifically span, pitch, and the desired internal space. The primary inputs for this calculator are:
Truss Span: The total horizontal distance the truss must cover, typically from exterior wall to exterior wall.
Roof Pitch: The steepness of the roof, expressed as a ratio of vertical rise to horizontal run (e.g., 4/12 means for every 12 inches of horizontal run, the roof rises 4 inches). This is critical for determining the slope of the attic truss's top chords.
Truss Spacing: The distance between each individual truss, usually measured in inches (e.g., 16″ or 24″ on center). This affects the load distribution and the overall number of trusses required.
Ridge Height above Walls: The desired vertical distance from the top of the supporting walls to the peak (ridge) of the attic truss. This directly influences the size of the clear attic space.
The Math Behind the Calculation
The core of attic truss calculation involves trigonometry and geometry. The calculator estimates the approximate dimensions of the attic space and the main structural members based on your inputs.
1. Calculate Total Rise: The total vertical rise of the roof from the wall plate to the ridge is calculated using the span and pitch.
Total Rise = (Truss Span / 2) * (Pitch Rise / Pitch Run)
For example, if the span is 40 feet (480 inches) and the pitch is 4/12:
Total Rise = (480 / 2) * (4 / 12) = 240 * 0.3333 = 80 inches (or 6.67 feet).
2. Calculate Top Chord Lengths: The length of the top chords (the sloping sides of the truss) can be found using the Pythagorean theorem on the right triangles formed by half the span and the rise.
Half Span (Horizontal Run) = Truss Span / 2Slope Length = sqrt( (Half Span)^2 + (Total Rise)^2 )
The length of each top chord is approximately this slope length.
3. Determine Attic Space Dimensions: The clear attic space's width is approximately the Truss Span - (width of two bottom chords/supports). The height of the clear attic space is related to the Ridge Height above Walls and the geometry of the top chords. The calculator provides an estimate of the usable ceiling joist span and the maximum ceiling height.
4. Estimate Number of Trusses: While not directly calculated here, knowing the building's length and the truss spacing allows for estimating the total number of trusses needed.
Number of Trusses = (Building Length in inches / Truss Spacing in inches) + 1
Disclaimer: This calculator provides an *estimation* for informational purposes only. Actual truss design and dimensions must be performed by a qualified structural engineer or truss manufacturer based on specific project requirements, local building codes, and load calculations. The results should not be used for construction without professional verification.
function calculateTrussDimensions() {
var trussSpan = parseFloat(document.getElementById("trussSpan").value);
var roofPitch = document.getElementById("roofPitch").value;
var trussSpacing = parseFloat(document.getElementById("trussSpacing").value);
var ridgeHeight = parseFloat(document.getElementById("ridgeHeight").value);
var resultDiv = document.getElementById("result");
resultDiv.innerHTML = "; // Clear previous results
// Input Validation
if (isNaN(trussSpan) || trussSpan <= 0) {
resultDiv.innerHTML = 'Please enter a valid Truss Span (feet).';
return;
}
if (roofPitch === "") {
resultDiv.innerHTML = 'Please enter a valid Roof Pitch (e.g., 4/12).';
return;
}
if (isNaN(trussSpacing) || trussSpacing <= 0) {
resultDiv.innerHTML = 'Please enter a valid Truss Spacing (inches).';
return;
}
if (isNaN(ridgeHeight) || ridgeHeight <= 0) {
resultDiv.innerHTML = 'Please enter a valid Ridge Height above Walls (feet).';
return;
}
// Parse roof pitch
var pitchParts = roofPitch.split('/');
if (pitchParts.length !== 2) {
resultDiv.innerHTML = 'Invalid Roof Pitch format. Use format like 4/12.';
return;
}
var pitchRise = parseFloat(pitchParts[0]);
var pitchRun = parseFloat(pitchParts[1]);
if (isNaN(pitchRise) || isNaN(pitchRun) || pitchRun === 0) {
resultDiv.innerHTML = 'Invalid Roof Pitch values. Pitch Run cannot be zero.';
return;
}
var halfSpanFeet = trussSpan / 2;
var halfSpanInches = halfSpanFeet * 12;
var pitchRatio = pitchRise / pitchRun;
// Calculate Total Rise based on span and pitch ratio
var calculatedTotalRiseInches = halfSpanInches * pitchRatio;
var calculatedTotalRiseFeet = calculatedTotalRiseInches / 12;
// Calculate top chord length (hypotenuse)
var topChordLengthInches = Math.sqrt(Math.pow(halfSpanInches, 2) + Math.pow(calculatedTotalRiseInches, 2));
var topChordLengthFeet = topChordLengthInches / 12;
// Estimate clear attic space width (approximating bottom chord thickness)
var estimatedClearWidthFeet = trussSpan – (2 * 1.5); // Assuming ~1.5ft for wall structure/bottom chord
if (estimatedClearWidthFeet < 0) estimatedClearWidthFeet = 0;
// Estimate usable ceiling joist span (this is a simplification)
// For attic trusses, the "floor joists" are often the ceiling joists of the space below
// or integrated into the bottom chord. The clear span is the primary feature.
var usableCeilingJoistSpanFeet = estimatedClearWidthFeet; // Approximated as clear width
// Estimate maximum ceiling height within the attic space
// This is a complex calculation depending on truss design, but we can estimate
// based on the ridge height and pitch at the center.
var maxCeilingHeightFeet = ridgeHeight;
// A more refined calculation might consider the clearance needed by the truss members
// near the edges, but for a basic estimation, ridge height is a starting point.
// Estimate number of trusses
var buildingLengthFeet = 30; // Assuming a default building length for estimation if not provided
// In a real scenario, building length would be another input.
// For this example, let's assume a standard length or prompt user.
// For simplicity here, we'll just provide the calculation formula.
var estimatedNumberOfTrusses = "(Building Length in feet / (Truss Spacing in inches / 12)) + 1";
var outputHTML = "
Estimated Max Ceiling Height at Ridge: " + maxCeilingHeightFeet.toFixed(2) + " feet
";
outputHTML += "Number of Trusses = (Building Length in feet / (Truss Spacing in inches / 12)) + 1. Requires Building Length input.";
outputHTML += "Disclaimer: These are estimations. Consult a professional engineer for final designs.";
resultDiv.innerHTML = outputHTML;
}