Lap Siding Overlap and Course Spacing Calculator
ANA›Life Services Authority›National Calculator Authority›Lap Siding Overlap and Course Spacing Calculator
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Lap Siding Overlap and Course Spacing Calculator
Calculate the number of siding courses, course spacing (exposure), and overlap for horizontal lap siding installations.
Wall Height (inches)
Siding Board Width (inches)
Minimum Overlap (inches)
Typical minimum: 1¼″ (1.25 in)
Starter Course Height (inches)
Extra drop below sheathing bottom (typically 0.75″–1″)
Calculate
function lapCalc() { var resultDiv = document.getElementById('lap-result'); resultDiv.style.display = 'block';
var wallHeight = parseFloat(document.getElementById('lap-wall-height').value); var boardWidth = parseFloat(document.getElementById('lap-board-width').value); var minOverlap = parseFloat(document.getElementById('lap-min-overlap').value); var starterH = parseFloat(document.getElementById('lap-starter-height').value);
// --- Validation --- var errors = []; if (isNaN(wallHeight) || wallHeight = boardWidth) { errors.push('Minimum Overlap must be less than the Board Width.'); } if (!isNaN(wallHeight) && !isNaN(starterH) && starterH >= wallHeight) { errors.push('Starter Course Height must be less than Wall Height.'); }
if (errors.length > 0) { resultDiv.innerHTML = 'Please fix the following:' + errors.map(function(e){ return ''; }).join('') + ''; return; }
// --- Core Formulas --- // Maximum allowable exposure per course var maxExposure = boardWidth - minOverlap;
// Net wall height available for full courses (subtract starter drop) var netHeight = wallHeight - starterH;
// Minimum number of courses (ceiling so exposure never exceeds maxExposure) var numCourses = Math.ceil(netHeight / maxExposure);
// Actual exposure (evenly distributed) var exposure = netHeight / numCourses;
// Actual overlap achieved var actualOverlap = boardWidth - exposure;
// Total linear feet of siding needed per linear foot of wall width // Each course uses one full board width; overlap is shared between courses // Boards per vertical foot of wall = numCourses / (wallHeight/12) var wallHeightFt = wallHeight / 12; var coursesPerFt = numCourses / wallHeightFt;
// Coverage per board (exposure in feet) var exposureFt = exposure / 12;
// --- Format helpers --- function fmt(n, d) { return parseFloat(n.toFixed(d !== undefined ? d : 4)); } function frac(inches) { // Express decimal inches as nearest 1/8" var whole = Math.floor(inches); var dec = inches - whole; var eighths = Math.round(dec * 8); if (eighths === 0) return whole + '″'; if (eighths === 8) return (whole + 1) + '″'; var gcd = function(a,b){ return b ? gcd(b, a%b) : a; }; var g = gcd(eighths, 8); return (whole > 0 ? whole : '') + ' ' + (eighths/g) + '⁄' + (8/g) + '″'; }
resultDiv.innerHTML = '### Results ' + '' + 'ParameterValue' + 'Number of Courses' + numCourses + ' courses' + 'Course Exposure (Spacing)' + fmt(exposure, 4) + '″ (≈ ' + frac(exposure) + ')' + 'Actual Overlap' + fmt(actualOverlap, 4) + '″ (≈ ' + frac(actualOverlap) + ')' + 'Maximum Allowable Exposure' + fmt(maxExposure, 4) + '″' + 'Net Wall Height (excl. starter)' + fmt(netHeight, 4) + '″' + 'Courses per Vertical Foot' + fmt(coursesPerFt, 3) + ' courses/ft' + 'Exposure per Course (ft)' + fmt(exposureFt, 4) + ' ft' + '' + 'Tip: Mark your story pole every ' + fmt(exposure, 4) + '″ starting at ' + fmt(starterH, 4) + '″ below the bottom of the sheathing.
'; }
#### Formulas Used
Maximum Exposure Max Exposure = Board Width − Minimum Overlap
Number of Courses N = ⌈ (Wall Height − Starter Height) ÷ Max Exposure ⌉ (ceiling ensures exposure never exceeds the maximum)
Actual Exposure (Course Spacing) Exposure = (Wall Height − Starter Height) ÷ N
Actual Overlap Overlap = Board Width − Exposure
Courses per Vertical Foot Courses/ft = N ÷ (Wall Height ÷ 12)
#### Assumptions & References
- All inputs are in inches; wall height is the full sheathing height from bottom plate to top plate.
- The starter course drops below the bottom of the sheathing by the specified starter height to direct water away from the foundation.
- The minimum overlap of 1¼″ (1.25″) is the industry standard per ASTM D3679 and most siding manufacturer guidelines (e.g., James Hardie, LP SmartSide).
- Courses are evenly distributed across the net wall height so no single course looks noticeably different from the others.
- Reference: Residential Construction Academy: Carpentry, 4th ed., Chapter 22 (Exterior Siding).
- Reference: IRC 2021 Section R703 — Exterior Covering.
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