Roof sheathing, often made of plywood or OSB (Oriented Strand Board), forms the base layer that structural components like rafters or trusses rest upon. It provides a solid surface for applying roofing materials such as shingles, metal panels, or tiles. Accurately calculating the amount of sheathing needed is crucial for budgeting and ensuring you have enough material without excessive waste.
How the Calculator Works
This calculator simplifies the process of estimating your sheathing requirements. It takes into account the dimensions of your roof, any overhangs, the size of the sheathing sheets you plan to use, and a factor for material waste.
Key Inputs Explained:
Roof Length (ft): The linear measurement of your roof from one end to the other.
Roof Width (ft): The linear measurement of your roof from eave to ridge (or peak to edge).
Overhang (ft): The portion of the roof that extends beyond the exterior walls. This area also needs sheathing.
Sheathing Sheet Size (sq ft): The area of a single sheet of your chosen sheathing material. A standard 4'x8′ sheet covers 32 square feet.
Waste Factor (%): An essential multiplier to account for material lost due to cuts, imperfect angles, accidental damage, or unusable offcuts. A common waste factor for roofing projects is between 5% and 15%.
The Calculation Formula:
The calculation follows these steps:
Calculate Total Roof Area: Total Roof Area = (Roof Length + (2 * Overhang)) * (Roof Width + (2 * Overhang))
*Note: We add twice the overhang to both length and width to account for sheathing on the fascia and rake edges that extend beyond the walls.*
Calculate Area Including Waste: Area with Waste = Total Roof Area * (1 + (Waste Factor / 100))
Determine Number of Sheathing Sheets: Number of Sheets = Area with Waste / Sheathing Sheet Size
The result is typically rounded up to the nearest whole number to ensure you have enough material.
Why Accurate Measurement Matters
Underestimating your sheathing needs can lead to costly delays, last-minute trips to the lumber yard, and potentially a mismatch in sheathing batches if you run out and need to purchase more later. Overestimating too much can lead to unnecessary expense and material waste. This calculator aims to provide a reliable estimate to help you plan efficiently. Always double-check your measurements and consider consulting with a roofing professional for complex roof designs.
function calculateSheathing() {
var roofLength = parseFloat(document.getElementById("roofLength").value);
var roofWidth = parseFloat(document.getElementById("roofWidth").value);
var overhang = parseFloat(document.getElementById("overhang").value);
var sheathingSheetSize = parseFloat(document.getElementById("sheathingSheetSize").value);
var wasteFactor = parseFloat(document.getElementById("wasteFactor").value);
var resultDiv = document.getElementById("result");
// Input validation
if (isNaN(roofLength) || roofLength <= 0) {
resultDiv.innerHTML = "Please enter a valid Roof Length.";
return;
}
if (isNaN(roofWidth) || roofWidth <= 0) {
resultDiv.innerHTML = "Please enter a valid Roof Width.";
return;
}
if (isNaN(overhang) || overhang < 0) {
resultDiv.innerHTML = "Please enter a valid Overhang (0 or greater).";
return;
}
if (isNaN(sheathingSheetSize) || sheathingSheetSize <= 0) {
resultDiv.innerHTML = "Please enter a valid Sheathing Sheet Size.";
return;
}
if (isNaN(wasteFactor) || wasteFactor < 0) {
resultDiv.innerHTML = "Please enter a valid Waste Factor (0 or greater).";
return;
}
// Calculations
var totalRoofLength = roofLength + (2 * overhang);
var totalRoofWidth = roofWidth + (2 * overhang);
var totalRoofArea = totalRoofLength * totalRoofWidth;
var areaWithWaste = totalRoofArea * (1 + (wasteFactor / 100));
var numberOfSheets = Math.ceil(areaWithWaste / sheathingSheetSize);
resultDiv.innerHTML = "You will need approximately: " + numberOfSheets + " sheets of sheathing.";
}