Estimate the amount of ice and water shield (IWS) needed for your roofing project. This calculator helps you account for eaves, valleys, and roof penetrations, including a waste factor.
Distance roof extends past exterior wall.
Typical building code requirement.
Chimneys, skylights, vents, etc.
Recommended 10-15% for cuts and overlaps.
function calculateIWS() {
var eaveLength = parseFloat(document.getElementById('eaveLength').value);
var eaveOverhang = parseFloat(document.getElementById('eaveOverhang').value);
var minExtension = parseFloat(document.getElementById('minExtension').value);
var valleyLength = parseFloat(document.getElementById('valleyLength').value);
var numPenetrations = parseFloat(document.getElementById('numPenetrations').value);
var rollWidth = parseFloat(document.getElementById('rollWidth').value);
var rollLength = parseFloat(document.getElementById('rollLength').value);
var wasteFactor = parseFloat(document.getElementById('wasteFactor').value);
// Validate inputs
if (isNaN(eaveLength) || isNaN(eaveOverhang) || isNaN(minExtension) || isNaN(valleyLength) ||
isNaN(numPenetrations) || isNaN(rollWidth) || isNaN(rollLength) || isNaN(wasteFactor) ||
eaveLength < 0 || eaveOverhang < 0 || minExtension < 0 || valleyLength < 0 ||
numPenetrations < 0 || rollWidth <= 0 || rollLength <= 0 || wasteFactor 100) {
document.getElementById('result').innerHTML = 'Please enter valid positive numbers for all fields.';
return;
}
// 1. Calculate Eave IWS Coverage
// Total depth IWS needs to extend up the roof from the eave edge
var requiredEaveDepthInches = eaveOverhang + minExtension;
// Number of roll widths needed to cover the required depth
var numStripsForEaves = Math.ceil(requiredEaveDepthInches / rollWidth);
// Linear feet of IWS for eaves
var linearFeetForEaves = eaveLength * numStripsForEaves;
// 2. Calculate Valley IWS Coverage
// Valleys typically use one strip of IWS
var linearFeetForValleys = valleyLength;
// 3. Calculate Penetration IWS Coverage
// Estimate 15 linear feet per penetration (e.g., a 3'x5′ piece or similar for flashing)
var linearFeetForPenetrations = numPenetrations * 15;
// 4. Total Raw Linear Feet of IWS
var totalRawLinearFeet = linearFeetForEaves + linearFeetForValleys + linearFeetForPenetrations;
// 5. Apply Waste Factor
var totalLinearFeetWithWaste = totalRawLinearFeet * (1 + (wasteFactor / 100));
// 6. Calculate Number of Rolls
var numRollsNeeded = Math.ceil(totalLinearFeetWithWaste / rollLength);
var resultHTML = '
Calculation Results:
';
resultHTML += 'Required IWS Linear Feet (raw): ' + totalRawLinearFeet.toFixed(2) + ' feet';
resultHTML += 'Required IWS Linear Feet (with waste): ' + totalLinearFeetWithWaste.toFixed(2) + ' feet';
resultHTML += 'Number of IWS Rolls Needed: ' + numRollsNeeded + ' rolls';
resultHTML += 'Note: This is an estimate. Always consult with a roofing professional and local building codes.';
document.getElementById('result').innerHTML = resultHTML;
}
Understanding Ice and Water Shield
Ice and Water Shield (IWS), also known as self-adhering underlayment, is a crucial component in modern roofing systems, especially in regions prone to cold weather and heavy precipitation. It's a self-sealing, waterproof membrane designed to protect your roof deck from water penetration caused by ice dams, wind-driven rain, and other severe weather conditions.
Why is Ice and Water Shield Important?
Prevents Ice Dams: In cold climates, snow on a roof can melt and refreeze at the colder eaves, forming an ice dam. This dam prevents melting snow from draining off the roof, causing water to back up under shingles and potentially leak into your home. IWS creates a waterproof barrier that prevents this water from penetrating the roof deck.
Protects Against Wind-Driven Rain: During severe storms, strong winds can force rain horizontally or even upwards, allowing it to get under shingles. IWS provides an extra layer of protection against such leaks.
Code Compliance: Many local building codes, particularly in northern climates, mandate the use of IWS along eaves and in valleys to meet minimum waterproofing standards.
Extends Roof Lifespan: By preventing water infiltration, IWS helps protect the structural integrity of your roof deck and attic, reducing the risk of rot, mold, and costly repairs.
Where is Ice and Water Shield Applied?
IWS is not typically applied to the entire roof surface. Instead, it's strategically placed in areas most vulnerable to water penetration:
Eaves: Along the bottom edge of the roof, extending from the fascia up the roof deck. Building codes often specify how far up the roof deck it must extend, usually 24 inches past the interior wall line.
Valleys: The V-shaped channels where two roof planes meet. These areas collect a significant amount of water and are highly susceptible to leaks.
Around Penetrations: Areas where the roof deck is cut for chimneys, skylights, vent pipes, and other protrusions. IWS is wrapped around these elements to create a watertight seal.
Low-Slope Areas: On roofs with a very low pitch where water drainage is slower, increasing the risk of pooling and leaks.
How to Use the Calculator
This calculator simplifies the estimation process for your IWS needs:
Total Eave Length: Measure the total linear feet of all eaves where IWS will be applied.
Eave Overhang: Measure how far your roof extends past the exterior wall.
Min. IWS Extension Past Interior Wall: This is a critical code requirement, typically 24 inches, but always verify with your local building department.
Total Valley Length: Measure the total linear feet of all valleys on your roof.
Number of Roof Penetrations: Count all chimneys, skylights, plumbing vents, and other elements that penetrate the roof.
IWS Roll Width & Length: Input the standard dimensions of the IWS product you plan to use (e.g., 36 inches wide by 75 feet long).
Waste Factor: A percentage (typically 10-15%) to account for cuts, overlaps, and potential errors during installation.
The calculator will then provide an estimate of the total linear feet of IWS required and the number of rolls you'll need to purchase.
Important Considerations
Local Building Codes: Always check with your local building authority for specific requirements regarding IWS application, including minimum extension distances and areas of coverage.
Roof Pitch: While the calculator focuses on linear measurements, steeper roofs might have different application nuances.
Climate: In extremely cold or snowy regions, some professionals might recommend extending IWS coverage beyond the minimum code requirements for added protection.
Professional Advice: This calculator provides an estimate. For precise measurements and installation, it's always best to consult with a qualified roofing contractor.