Calculating the square footage of a roof is more complex than measuring a floor because of the slope (pitch). A steeper roof covers more surface area than a flat roof covering the same building footprint.
To use this calculator, you need three primary measurements:
House Length and Width: The exterior dimensions of your home's ground footprint.
Overhang: Most roofs extend past the walls (eaves). Typical overhangs are 12 to 24 inches (1 to 2 feet).
Roof Pitch: This is the steepness of the roof, expressed as "rise over run." A 6/12 pitch means the roof rises 6 inches for every 12 inches it horizontal distance.
The Math Behind the Calculation
The formula used by professionals involves a "Pitch Multiplier." Here is the step-by-step logic:
Calculate Footprint: Add the overhang to all sides. (Length + 2×Overhang) × (Width + 2×Overhang).
Apply Pitch Factor: Multiply the footprint area by the specific pitch factor. For example, a 4/12 pitch has a multiplier of 1.083.
Determine Squares: In the roofing industry, 100 square feet is called one "Square." Dividing your total area by 100 tells you how many squares of material to order.
Example Calculation
Imagine a house that is 40 feet long and 30 feet wide with a 1-foot overhang and an 8/12 pitch:
Total Footprint: (40 + 2) × (30 + 2) = 42 × 32 = 1,344 sq. ft.
Pitch Multiplier (8/12): 1.202
Total Roof Area: 1,344 × 1.202 = 1,615.5 sq. ft.
Roofing Squares: 16.15 Squares
Why Waste Percentage Matters
When ordering shingles or metal roofing, you must account for "waste." This includes material cut off at valleys, hips, and edges. Standard practice is to add 10% for simple gable roofs and up to 15-20% for complex roofs with many dormers and valleys.
function calculateRoofArea() {
var length = parseFloat(document.getElementById('baseLength').value);
var width = parseFloat(document.getElementById('baseWidth').value);
var pitchFactor = parseFloat(document.getElementById('roofPitch').value);
var overhang = parseFloat(document.getElementById('overhang').value);
if (isNaN(length) || isNaN(width) || length <= 0 || width <= 0) {
alert("Please enter valid positive numbers for length and width.");
return;
}
if (isNaN(overhang)) {
overhang = 0;
}
// Calculate the flat area including overhangs on both sides
var footprintLength = length + (2 * overhang);
var footprintWidth = width + (2 * overhang);
var flatArea = footprintLength * footprintWidth;
// Apply the pitch multiplier
var totalArea = flatArea * pitchFactor;
// Calculate roofing squares (1 square = 100 sq ft)
var squares = totalArea / 100;
// Calculate with 10% waste
var wasteArea = totalArea * 1.10;
// Display results
document.getElementById('totalSqFt').innerText = totalArea.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('totalSquares').innerText = squares.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('shinglesWithWaste').innerText = wasteArea.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('roofResult').style.display = 'block';
}