Calculate the approximate dimensions and material needs for a scissor truss.
Understanding Scissor Trusses and This Calculator
Scissor trusses, also known as cathedral trusses, are a type of roof truss that creates an interior vaulted ceiling effect while still providing structural support for the roof. They are characterized by two sloped bottom chords that meet at a central peak, resembling an inverted gable truss. This design is popular in residential architecture for creating dramatic interior spaces, often found in great rooms, living rooms, and churches.
This calculator is designed to provide approximate estimations for key dimensions and material quantities for a single scissor truss based on your input parameters. It's crucial to understand that these are estimations for preliminary planning and design. Actual construction should always be based on detailed engineering drawings prepared by a qualified structural engineer, taking into account local building codes, specific load requirements (snow, wind, dead loads), and material strengths.
How the Calculations Work (Simplified):
The calculations for a scissor truss involve trigonometry and basic geometry. Here's a simplified overview of what this calculator estimates:
Truss Height at Peak: Determined by the span width and the difference between the top and bottom chord pitches.
Bottom Chord Length: Calculated based on the span width and the bottom chord pitch.
Top Chord Length: Calculated based on the span width and the top chord pitch.
Web Members: Approximate lengths are estimated based on the geometry of the truss.
Total Lumber Needed (Linear Feet): Sum of the lengths of all major components (top chords, bottom chords, and a simplified estimate for web members).
Estimated Number of Trusses: Calculated by dividing the total roof length (span + overhang on both sides) by the truss spacing.
Approximate Lumber per Truss: Total lumber needed divided by the number of trusses.
Formulas (Conceptual):
The calculations rely on Pythagorean theorem and trigonometric functions (like tangent and arctangent) to determine lengths and heights based on pitches and span. A pitch of "X in 12" means for every 12 inches of horizontal run, there is an X-inch rise.
Horizontal Run per side = Span Width / 2
Vertical Rise (Top Chord) = Horizontal Run * (Pitch Top / 12)
Length of Chord = sqrt(Horizontal Run^2 + Vertical Rise^2)
Web member calculations are more complex and often involve breaking the truss into smaller triangles.
Key Inputs Explained:
Span Width (ft): The total horizontal distance the truss will cover from outer wall to outer wall.
Top Chord Pitch (in 12): The steepness of the upper roof slope. Expressed as rise (inches) over run (12 inches). A higher number means a steeper slope.
Bottom Chord Pitch (in 12): The steepness of the lower, interior sloped ceiling. A lower number means a gentler slope, creating a wider vault.
Overhang (ft): The distance the roof extends beyond the exterior walls for weather protection and aesthetic appeal.
Truss Spacing (ft): The distance between each parallel truss. Common spacing is 24 inches (2 ft) or 16 inches (1.33 ft) on center.
Disclaimer:
This calculator is a tool for estimation purposes only. It does not account for all structural considerations, load requirements, or specific building codes. Always consult with a qualified structural engineer and architect before commencing any construction project. The accuracy of the output depends on the accuracy of the input data and the simplified nature of the models used.
function calculateScissorTruss() {
var spanWidth = parseFloat(document.getElementById("spanWidth").value);
var pitchTop = parseFloat(document.getElementById("pitchTop").value);
var pitchBottom = parseFloat(document.getElementById("pitchBottom").value);
var overhang = parseFloat(document.getElementById("overhang").value);
var trussSpacing = parseFloat(document.getElementById("trussSpacing").value);
var resultDiv = document.getElementById("result");
resultDiv.innerHTML = "; // Clear previous results
// Input validation
if (isNaN(spanWidth) || spanWidth <= 0 ||
isNaN(pitchTop) || pitchTop <= 0 ||
isNaN(pitchBottom) || pitchBottom < 0 || // Bottom pitch can be 0 or very low
isNaN(overhang) || overhang < 0 || // Overhang can be 0
isNaN(trussSpacing) || trussSpacing <= 0) {
resultDiv.innerHTML = "Please enter valid positive numbers for all fields.";
return;
}
// Calculations
var halfSpan = spanWidth / 2;
var run = halfSpan; // Horizontal run for each chord side
// Calculate vertical rises
var riseTop = run * (pitchTop / 12);
var riseBottom = run * (pitchBottom / 12);
// Calculate chord lengths using Pythagorean theorem
var lengthTopChordSide = Math.sqrt(Math.pow(run, 2) + Math.pow(riseTop, 2));
var lengthBottomChordSide = Math.sqrt(Math.pow(run, 2) + Math.pow(riseBottom, 2));
var totalTopChord = lengthTopChordSide * 2;
var totalBottomChord = lengthBottomChordSide * 2;
// Approximate web member lengths (simplified – this is a complex part of truss design)
// For a basic scissor truss, we have a central vertical and sloping webs.
// We'll simplify by estimating a few key members. A real design uses many more.
var centralVerticalHeight = Math.abs(riseTop – riseBottom);
var peakToWallPointLength = Math.sqrt(Math.pow(halfSpan, 2) + Math.pow(centralVerticalHeight, 2)); // Approximates diagonal webs
// Sum of major components for lumber estimation
// This is a gross simplification. A real estimate requires detailed framing plan.
// We'll add a factor for common web members (e.g., central post, some diagonals).
// Factor is an estimate; actual web members vary greatly by design.
var estimatedWebLumber = (totalTopChord + totalBottomChord) * 0.75; // Rough estimate
var totalLumberLinearFeet = totalTopChord + totalBottomChord + estimatedWebLumber;
// Calculate total roof length for number of trusses
var totalRoofLength = spanWidth + (overhang * 2);
var numberOfTrusses = Math.ceil(totalRoofLength / trussSpacing);
// Ensure we have at least one truss if calculation results in zero or less
if (numberOfTrusses <= 0) {
numberOfTrusses = 1;
}
var lumberPerTruss = totalLumberLinearFeet; // Simplified: total lumber / number of trusses assumes uniform material distribution which isn't perfectly true but provides a baseline
// Display Results
var resultHtml = "