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Commercial Roof Drainage Calculator

ANA›Life Services Authority›National Calculator Authority›Commercial Roof Drainage 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; } }

Commercial Roof Drainage Calculator

Calculate the required roof drainage capacity, number of drains, and pipe sizing for commercial roofing systems using the Rational Method.

Roof Area (ft²)

Design Rainfall Intensity (in/hr)

Use local IDF curves for your region (typically 2–10 in/hr for 10-yr storm)

Runoff Coefficient (C)

Flat Built-Up Roof (C = 0.95) Membrane / TPO Roof (C = 0.90) Green Roof – Shallow (C = 0.75) Green Roof – Deep (C = 0.50) Custom Value…

Custom Runoff Coefficient (0.1 – 1.0)

Capacity per Roof Drain (GPM)

Typical 4″ drain ≈ 100 GPM; 6″ drain ≈ 230 GPM (per manufacturer specs)

Safety Factor

1.0 – No safety factor 1.25 – Standard (recommended) 1.5 – High-risk / critical facility 2.0 – Conservative / ponding risk

Calculate Drainage

document.getElementById('com-runoff').addEventListener('change', function() { var customGroup = document.getElementById('com-custom-runoff-group'); customGroup.style.display = (this.value === 'custom') ? 'block' : 'none'; });

function comCalc() { var resultDiv = document.getElementById('com-result'); resultDiv.style.display = 'none';

// --- Gather inputs --- var areaFt2 = parseFloat(document.getElementById('com-roof-area').value); var rainfall = parseFloat(document.getElementById('com-rainfall').value); var runoffSel = document.getElementById('com-runoff').value; var drainCap = parseFloat(document.getElementById('com-drain-capacity').value); var safety = parseFloat(document.getElementById('com-safety').value);

var C; if (runoffSel === 'custom') { C = parseFloat(document.getElementById('com-custom-runoff').value); } else { C = parseFloat(runoffSel); }

// --- Validation --- var errors = []; if (isNaN(areaFt2) || areaFt2 0 in/hr."); if (isNaN(C) || C 1.0) errors.push("Runoff coefficient must be between 0.10 and 1.00."); if (isNaN(drainCap) || drainCap 0) { resultDiv.style.display = 'block'; resultDiv.innerHTML = 'Please fix the following:' + errors.map(function(e){ return ''; }).join('') + ''; return; }

// --- Core Calculations ---

// 1. Convert roof area from ft² to acres // 1 acre = 43,560 ft² var areaAcres = areaFt2 / 43560;

// 2. Rational Method: Q = C × i × A // Q = peak runoff (ft³/s) — but we need GPM // Q (ft³/s) = C × i (in/hr) × A (acres) / 1 [Rational Method gives ft³/s directly] // Convert ft³/s → GPM: 1 ft³/s = 448.831 GPM var Q_cfs = C * rainfall * areaAcres; // ft³/s (Rational Method) var Q_gpm = Q_cfs * 448.831; // GPM

// 3. Apply safety factor var Q_design_gpm = Q_gpm * safety;

// 4. Number of drains required (round up) var numDrains = Math.ceil(Q_design_gpm / drainCap);

// 5. Effective drainage area per drain var areaPerDrain = areaFt2 / numDrains;

// 6. Recommended leader / downspout pipe diameter // Using IPC Table 1106.2 approximation: // Q (GPM) per leader at 1/8" slope: // 3" pipe ≈ 34 GPM, 4" ≈ 78 GPM, 5" ≈ 139 GPM, 6" ≈ 222 GPM, 8" ≈ 478 GPM var qPerDrain = Q_design_gpm / numDrains; var pipeSize; if (qPerDrain ' + 'ParameterValue' + 'Roof Area' + areaFt2.toLocaleString() + ' ft² (' + areaAcres.toFixed(3) + ' acres)' + 'Design Rainfall Intensity' + rainfall.toFixed(2) + ' in/hr' + 'Runoff Coefficient (C)' + C.toFixed(2) + '' + 'Peak Runoff (Rational Method)' + Q_cfs.toFixed(3) + ' ft³/s (' + Q_gpm.toFixed(1) + ' GPM)' + 'Safety Factor' + safety.toFixed(2) + 'x' + 'Design Flow (with safety factor)' + Q_design_gpm.toFixed(1) + ' GPM' + 'Primary Drains Required' + numDrains + ' drains' + 'Capacity per Drain' + drainCap.toFixed(0) + ' GPM' + 'Roof Area per Drain' + areaPerDrain.toFixed(0) + ' ft²' + 'Recommended Leader Pipe Size' + pipeSize + ' diameter' + 'Overflow / Secondary Drains (IBC 1503.4)' + overflowDrains + ' drains (mirror primary)' + 'Estimated Water Head at Drain' + H_in.toFixed(2) + ' in (' + H_ft.toFixed(3) + ' ft)' + '' + 'Note: Results are for preliminary design only. Final sizing must be verified by a licensed engineer using local IDF data, IPC/IBC code requirements, and manufacturer drain specifications.

'; }

#### Formulas Used

Rational Method (ASCE / IPC):

Q = C × i × A

Unit Conversion: 1 ft³/s = 448.831 GPM

Design Flow: Qdesign = Q × Safety Factor

Number of Drains: N = ⌈ Qdesign / Capacity per Drain ⌉

Weir Flow (head estimate): Q = Cw × L × H1.5 → H = (Q / (Cw × L))2/3

Where Cw = 3.0 (weir coefficient), L = π × drain diameter (ft)

Leader Pipe Sizing: Based on IPC Table 1106.2 at 1/8 in/ft slope

#### Assumptions & References

More Calculators

References

Related Authorities

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