Roof Slope and Coating Thickness Calculator

ANA›Life Services Authority›National Calculator Authority›Roof Slope and Coating Thickness Calculator

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Roof Slope and Coating Thickness Calculator

Calculate your roof's slope (pitch ratio and angle) and determine the recommended coating thickness based on slope and coating type.

### Roof Dimensions

Vertical Rise (inches):

Horizontal Run (inches, typically 12):

Roof Area (sq ft):

### Coating Parameters

Coating Type:

Elastomeric / Acrylic Silicone Polyurethane Modified Bitumen / Asphalt Epoxy

Number of Coats:

Coverage Rate (sq ft / gallon):

Leave blank to use coating-type default

Calculate

function rooCalc() { // --- Inputs --- const rise = parseFloat(document.getElementById('roo-rise').value); const run = parseFloat(document.getElementById('roo-run').value); const area = parseFloat(document.getElementById('roo-area').value); const coating = document.getElementById('roo-coating').value; const coats = parseInt(document.getElementById('roo-coats').value); const covInput = document.getElementById('roo-coverage').value.trim();

// --- Validation --- const errors = []; if (isNaN(rise) || rise 5) errors.push("Number of Coats must be between 1 and 5."); if (covInput !== "" && (isNaN(parseFloat(covInput)) || parseFloat(covInput) 0) { resultDiv.style.display = 'block'; resultDiv.innerHTML = 'Please fix the following:' + errors.map(e => '').join('') + ''; return; }

// --------------------------------------------------------------- // 1. SLOPE CALCULATIONS // --------------------------------------------------------------- // Pitch ratio = rise / run (e.g. 6:12 → 0.5) const pitchRatio = rise / run;

// Slope angle in degrees: θ = arctan(rise / run) const angleRad = Math.atan(pitchRatio); const angleDeg = angleRad * (180 / Math.PI);

// Slope factor (multiplier to convert horizontal area → actual roof area) // slopeFactor = sqrt(rise² + run²) / run const slopeFactor = Math.sqrt(rise * rise + run * run) / run;

// Actual (sloped) roof area const actualArea = area * slopeFactor;

// Classify slope let slopeClass = ""; if (pitchRatio 10:12)";

// --------------------------------------------------------------- // 2. COATING THICKNESS RECOMMENDATIONS // Industry standard wet-film thickness (WFT) per coat in mils // and default coverage rates (sq ft / gallon) per coating type. // Sources: ASTM D6083, ASTM C1127, manufacturer TDS averages. // --------------------------------------------------------------- const coatingData = { elastomeric: { name: "Elastomeric / Acrylic", wftMilPerCoat: 20, defaultCoverage: 100, dftMilPerCoat: 14 }, silicone: { name: "Silicone", wftMilPerCoat: 20, defaultCoverage: 80, dftMilPerCoat: 16 }, polyurethane: { name: "Polyurethane", wftMilPerCoat: 25, defaultCoverage: 70, dftMilPerCoat: 20 }, bitumen: { name: "Modified Bitumen / Asphalt", wftMilPerCoat: 30, defaultCoverage: 50, dftMilPerCoat: 24 }, epoxy: { name: "Epoxy", wftMilPerCoat: 15, defaultCoverage: 120, dftMilPerCoat: 12 } };

const cd = coatingData[coating];

// Slope adjustment factor for coating thickness: // Steeper roofs require thicker coatings to compensate for drainage/sag. // Adjustment: +5% per degree above 14° (conventional threshold), capped at +25%. const baseAngleThreshold = 14; // degrees let slopeAdjFactor = 1.0; if (angleDeg > baseAngleThreshold) { slopeAdjFactor = Math.min(1.0 + 0.005 * (angleDeg - baseAngleThreshold), 1.25); }

// Adjusted WFT and DFT per coat (mils) const adjWFTPerCoat = cd.wftMilPerCoat * slopeAdjFactor; const adjDFTPerCoat = cd.dftMilPerCoat * slopeAdjFactor;

// Total WFT and DFT (mils) const totalWFT = adjWFTPerCoat * coats; const totalDFT = adjDFTPerCoat * coats;

// Coverage rate (sq ft / gallon) // 1 gallon = 231 cubic inches; 1 mil = 0.001 inch // Theoretical coverage (sq ft/gal) = 1604.17 / WFT_mils // (1604.17 = 231 in³/gal ÷ 0.001 in/mil ÷ 144 in²/ft²) const theoreticalCoverage = 1604.17 / adjWFTPerCoat;

// Use user-supplied coverage or coating default const coverageRate = covInput !== "" ? parseFloat(covInput) : cd.defaultCoverage;

// Gallons needed per coat = actual area / coverage rate const gallonsPerCoat = actualArea / coverageRate;

// Total gallons for all coats const totalGallons = gallonsPerCoat * coats;

// Add 10% waste factor const totalGallonsWithWaste = totalGallons * 1.10;

// --------------------------------------------------------------- // 3. OUTPUT // --------------------------------------------------------------- const fmt = (v, d=2) => v.toFixed(d);

resultDiv.style.display = 'block'; resultDiv.innerHTML = ` ### Results

#### 📐 Roof Slope

Pitch Ratio${fmt(rise,2)} : ${fmt(run,2)} Decimal Pitch${fmt(pitchRatio,4)} Slope Angle${fmt(angleDeg,2)}° Slope Classification${slopeClass} Slope Factor${fmt(slopeFactor,4)} Horizontal (Plan) Area${fmt(area,1)} sq ft Actual (Sloped) Roof Area${fmt(actualArea,1)} sq ft

#### 🎨 Coating Thickness — ${cd.name}

Slope Adjustment Factor${fmt(slopeAdjFactor,3)}× WFT per Coat (adjusted)${fmt(adjWFTPerCoat,1)} mils DFT per Coat (adjusted)${fmt(adjDFTPerCoat,1)} mils Number of Coats${coats} Total WFT (all coats)${fmt(totalWFT,1)} mils Total DFT (all coats)${fmt(totalDFT,1)} mils Total DFT (mm)${fmt(totalDFT * 0.0254,3)} mm

#### 🪣 Material Estimate

Coverage Rate Used${fmt(coverageRate,1)} sq ft / gal Theoretical Coverage @ WFT${fmt(theoreticalCoverage,1)} sq ft / gal Gallons per Coat${fmt(gallonsPerCoat,2)} gal Total Gallons (no waste)${fmt(totalGallons,2)} gal Total Gallons (+10% waste)${fmt(totalGallonsWithWaste,2)} gal

`; }

#### Formulas Used

Pitch Ratio: Pitch = Rise ÷ Run

Slope Angle: θ = arctan(Rise ÷ Run) × (180 ÷ π)

Slope Factor: SF = √(Rise² + Run²) ÷ Run

Actual Roof Area: Aactual = Aplan × SF

Slope Adjustment Factor: SAF = min(1 + 0.005 × max(θ − 14°, 0), 1.25)

Adjusted WFT per Coat: WFTadj = WFTbase × SAF

Total DFT: DFTtotal = DFTadj/coat × Number of Coats

Theoretical Coverage: Cov = 1604.17 ÷ WFTadj (sq ft/gal)

Total Gallons: G = (Aactual ÷ Coverage Rate) × Coats × 1.10

#### Assumptions & References

Roof Slope and Coating Thickness Calculator

Roof Slope and Coating Thickness Calculator

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