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Roof Drainage Area Calculator

ANA›Life Services Authority›National Calculator Authority›Roof Drainage Area Calculator

.calc-container { max-width: 640px; margin: 2rem 0; padding: 1.5rem; background: #fff; border: 1px solid #ddd; border-radius: 8px; box-shadow: 0 1px 3px rgba(0,0,0,0.06); font-family: system-ui, -apple-system, sans-serif; } .calc-container h3 { font-family: Georgia, serif; font-size: 1.15rem; color: #1a1a1a; margin-bottom: 1rem; padding-bottom: 0.5rem; border-bottom: 2px solid var(--ac, #3d5a80); } .calc-row { display: flex; align-items: center; gap: 0.75rem; margin-bottom: 0.75rem; flex-wrap: wrap; } .calc-row label { min-width: 160px; font-size: 0.9rem; color: #333; font-weight: 500; } .calc-row input[type="number"], .calc-row select { flex: 1; min-width: 120px; max-width: 200px; padding: 0.5rem 0.6rem; border: 1px solid #ccc; border-radius: 4px; font-size: 0.9rem; font-family: system-ui, sans-serif; color: #1a1a1a; background: #fafaf8; } .calc-row input:focus, .calc-row select:focus { outline: none; border-color: var(--ac, #3d5a80); box-shadow: 0 0 0 2px rgba(26,74,138,0.12); } .calc-row .unit { font-size: 0.82rem; color: #888; min-width: 30px; } .calc-btn { display: inline-block; margin-top: 0.5rem; padding: 0.55rem 1.5rem; background: var(--ac, #3d5a80); color: #fff; border: none; border-radius: 4px; font-size: 0.9rem; font-weight: 600; cursor: pointer; font-family: system-ui, sans-serif; } .calc-btn:hover { opacity: 0.9; } .calc-result { margin-top: 1.25rem; padding: 1rem 1.25rem; background: #f0f6fc; border-left: 3px solid var(--ac, #3d5a80); border-radius: 0 6px 6px 0; display: none; } .calc-result.visible { display: block; } .calc-result-label { font-size: 0.78rem; text-transform: uppercase; letter-spacing: 0.06em; color: #666; margin-bottom: 0.25rem; } .calc-result-value { font-size: 1.6rem; font-weight: 700; color: var(--ac, #3d5a80); } .calc-result-detail { font-size: 0.85rem; color: #555; margin-top: 0.5rem; line-height: 1.5; } .calc-note { margin-top: 1rem; font-size: 0.8rem; color: #888; font-style: italic; } .calc-grid { display: grid; grid-template-columns: 1fr 1fr; gap: 0.75rem; margin-top: 0.75rem; } .calc-grid-item { padding: 0.6rem 0.8rem; background: #f8f9fa; border-radius: 4px; border: 1px solid #eee; } .calc-grid-item .label { font-size: 0.75rem; color: #888; text-transform: uppercase; letter-spacing: 0.04em; } .calc-grid-item .value { font-size: 1.1rem; font-weight: 600; color: #1a1a1a; } @media (max-width: 720px) { .calc-row { flex-direction: column; align-items: flex-start; gap: 0.3rem; } .calc-row label { min-width: auto; } .calc-row input[type="number"], .calc-row select { max-width: 100%; width: 100%; } .calc-grid { grid-template-columns: 1fr; } } .calc-chart { margin: 1rem 0; text-align: center; } .calc-chart svg { max-width: 100%; height: auto; } .calc-chart-legend { display: flex; flex-wrap: wrap; justify-content: center; gap: 0.6rem 1.2rem; margin-top: 0.6rem; font-size: 0.8rem; color: #555; } .calc-chart-legend span { display: inline-flex; align-items: center; gap: 0.3rem; } .calc-chart-legend i { display: inline-block; width: 10px; height: 10px; border-radius: 2px; font-style: normal; } .calc-related { max-width: 640px; margin: 2rem 0 1rem; padding: 1.25rem 1.5rem; background: #f8f9fa; border: 1px solid #e8e8e8; border-radius: 8px; } .calc-related h3 { font-family: Georgia, serif; font-size: 1rem; color: #1a1a1a; margin: 0 0 0.75rem; padding-bottom: 0.4rem; border-bottom: 2px solid var(--ac, #3d5a80); } .calc-related-list { list-style: none; padding: 0; margin: 0 0 0.75rem; display: grid; grid-template-columns: 1fr 1fr; gap: 0.4rem 1.5rem; } .calc-related-list li a { font-size: 0.88rem; color: var(--ac, #3d5a80); text-decoration: none; } .calc-related-list li a:hover { text-decoration: underline; } .calc-browse-all { margin: 0.5rem 0 0; font-size: 0.9rem; font-weight: 600; } .calc-browse-all a { color: var(--ac, #3d5a80); text-decoration: none; } .calc-browse-all a:hover { text-decoration: underline; } @media (max-width: 720px) { .calc-related-list { grid-template-columns: 1fr; } }

Roof Drainage Area Calculator

Calculate the effective horizontal drainage area of a roof section, accounting for roof pitch. Used for sizing gutters, downspouts, and stormwater management systems.

Roof Length (ft)

Roof Width / Horizontal Span (ft)

Roof Rise (in) — vertical rise per 12 in of run

Design Rainfall Intensity (in/hr) — optional, for flow rate

Runoff Coefficient (C) — 0.75–0.95 for roofs

Calculate

function rooCalc() { var L = parseFloat(document.getElementById('roo-length').value); var W = parseFloat(document.getElementById('roo-width').value); var rise = parseFloat(document.getElementById('roo-rise').value); var rainfall = parseFloat(document.getElementById('roo-rainfall').value); var C = parseFloat(document.getElementById('roo-runoff').value);

var errors = []; if (isNaN(L) || L 24) errors.push("Roof Rise must be between 0 and 24 inches."); if (!isNaN(rainfall) && rainfall 1.0)) errors.push("Runoff Coefficient must be between 0.01 and 1.0.");

if (errors.length > 0) { document.getElementById('roo-result').innerHTML = 'Input Error:' + errors.join('') + ''; return; }

// --- Core calculations --- // Pitch factor: slope length / horizontal run = sqrt(rise² + 12²) / 12 var pitchFactor = Math.sqrt(rise * rise + 144) / 12;

// Horizontal (plan) area — used as the effective drainage area per SMACNA / IPC var horizontalArea = L * W; // ft²

// Actual sloped surface area var slopedArea = L * W * pitchFactor; // ft²

// Effective drainage area (horizontal projection is the standard for drainage design) var effectiveArea = horizontalArea; // ft² var effectiveAreaAcres = effectiveArea / 43560; // acres

// Optional: peak flow rate using Rational Method Q = C * i * A (acres) var flowRate = NaN; var flowRateGPM = NaN; if (!isNaN(rainfall) && rainfall > 0 && !isNaN(C) && C > 0) { flowRate = C * rainfall * effectiveAreaAcres; // ft³/s (cfs) — Rational Method flowRateGPM = flowRate * 448.831; // convert cfs to GPM }

// Pitch description var pitchDesc = rise.toFixed(1) + ":12"; var pitchAngle = (Math.atan(rise / 12) * 180 / Math.PI).toFixed(1);

var html = 'Results'; html += ''; html += 'Roof Pitch' + pitchDesc + ' (' + pitchAngle + '°)'; html += 'Pitch Factor' + pitchFactor.toFixed(4) + ''; html += 'Horizontal (Plan) Area' + horizontalArea.toFixed(2) + ' ft² (' + (horizontalArea / 144).toFixed(2) + ' ft² → ' + effectiveAreaAcres.toFixed(5) + ' acres)'; html += 'Actual Sloped Surface Area' + slopedArea.toFixed(2) + ' ft²'; html += 'Effective Drainage Area' + effectiveArea.toFixed(2) + ' ft² (horizontal projection)';

if (!isNaN(flowRate)) { html += 'Peak Flow Rate (Rational)' + flowRate.toFixed(4) + ' cfs (' + flowRateGPM.toFixed(1) + ' GPM)'; } html += '';

// Gutter sizing guidance (SMACNA / IPC Table 1106.2 approximation) var gutterNote = ''; if (effectiveArea Gutter Sizing Guidance: ' + gutterNote + '';

document.getElementById('roo-result').innerHTML = html; }

#### Formulas Used

Pitch Factor: PF = √(rise² + 12²) / 12 where rise is the vertical rise in inches per 12 inches of horizontal run.

Sloped Surface Area: Asloped = Length × Width × PF

Effective Drainage Area (Horizontal Projection): Aeff = Length × Width Per SMACNA and IPC standards, the horizontal projection of the roof is used for drainage design, not the sloped area.

Peak Flow Rate — Rational Method (optional): Q = C × i × A where Q = peak flow (cfs), C = runoff coefficient (dimensionless), i = rainfall intensity (in/hr), A = drainage area (acres).

Unit Conversions: 1 acre = 43,560 ft²  |  1 cfs = 448.831 GPM

#### Assumptions & References

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References

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