A roof truss is a structural framework designed to bridge the space above a room and provide support for a roof. The span of a truss refers to the horizontal distance it covers between its supports (typically exterior walls).
Determining the maximum allowable span for a roof truss is crucial for structural integrity, safety, and compliance with building codes. Several factors influence this: the type of truss, the loads it must bear, and the materials used in its construction.
Key Factors in Truss Span Calculation:
Truss Type: Different truss designs (e.g., W-truss, Attic truss, Gable truss) have varying load-bearing capacities and span limitations. Our calculator considers common wood truss classifications like W-20 and W-30.
Roof Load (psf – pounds per square foot): This includes the weight of roofing materials such as shingles, underlayment, sheathing, and the weight of the truss itself.
Snow Load (psf): In regions with significant snowfall, the weight of accumulated snow on the roof is a critical factor. Building codes specify minimum snow load requirements based on geographic location.
Dead Load (psf): This refers to the permanent weight of the building's components, including the roofing materials and the truss itself. (Note: In common parlance for simple calculators, "Roof Load" often encompasses dead load, but we've separated them for clarity here).
Live Load (psf): This accounts for temporary or moving loads, such as wind pressure, people walking on the roof during construction or maintenance, or the weight of mechanical equipment. (For simplicity in this calculator, we've focused on dead and snow loads).
How the Calculation Works (Simplified):
The maximum span a truss can safely cover is determined by complex engineering principles involving bending moments, shear forces, and material strengths. This calculator uses a simplified, empirical approach based on typical engineering guidelines and manufacturer data for common wood trusses. The combined load (Roof Load + Snow Load + Dead Load) is compared against design limits specific to the selected truss type. A higher combined load will generally reduce the maximum allowable span.
Disclaimer: This calculator is for informational and preliminary estimation purposes only. It does not replace professional engineering calculations or advice. Always consult with a qualified structural engineer and refer to local building codes for any construction project.
function calculateSpan() {
var trussType = document.getElementById("trussType").value;
var roofLoad = parseFloat(document.getElementById("roofLoad").value);
var snowLoad = parseFloat(document.getElementById("snowLoad").value);
var deadLoad = parseFloat(document.getElementById("deadLoad").value);
var combinedLoad = roofLoad + snowLoad + deadLoad;
var maxSpan = 0; // Default to 0 if inputs are invalid or calculation fails
// Basic empirical data and simplified logic for demonstration
// Real-world calculations involve detailed engineering formulas and specific lumber grades/properties.
var baseSpan = 0;
var loadCapacityFactor = 0;
if (trussType === "W-20") {
baseSpan = 30; // Example base span in feet for W-20
loadCapacityFactor = 60; // Example factor (higher means can support more load)
} else if (trussType === "W-30") {
baseSpan = 40; // Example base span in feet for W-30
loadCapacityFactor = 80; // Example factor
} else if (trussType === "LW-20") {
baseSpan = 24; // Example base span in feet for LW-20
loadCapacityFactor = 50; // Example factor
} else if (trussType === "LW-30") {
baseSpan = 32; // Example base span in feet for LW-30
loadCapacityFactor = 70; // Example factor
}
// Input validation
if (isNaN(roofLoad) || isNaN(snowLoad) || isNaN(deadLoad) || combinedLoad <= 0 || baseSpan === 0) {
document.getElementById("result").innerHTML = "Please enter valid numeric values for all loads and select a truss type.";
return;
}
// Simplified span calculation: inversely proportional to load, scaled by base span and capacity factor
// This is a highly simplified model. Real calculations are complex.
// A higher combined load should result in a shorter span.
// We are creating a hypothetical relationship where (baseSpan * loadCapacityFactor) / combinedLoad gives a span value.
// We cap it to the base span and don't allow it to go below a minimum reasonable span.
var calculatedSpan = (baseSpan * loadCapacityFactor) / combinedLoad;
// Ensure span doesn't exceed base span and has a minimum value
maxSpan = Math.min(baseSpan, Math.max(10, calculatedSpan)); // Capping at baseSpan and minimum 10ft
if (isNaN(maxSpan) || maxSpan <= 0) {
document.getElementById("result").innerHTML = "Calculation could not be performed with the given inputs.";
} else {
document.getElementById("result").innerHTML = "Your maximum allowable span is: " + maxSpan.toFixed(2) + " ft";
}
}