Attic Storage Load Capacity Calculator

ANA›Life Services Authority›National Calculator Authority›Attic Storage Load Capacity Calculator

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Attic Storage Load Capacity Calculator

Estimate the safe distributed load capacity (psf) and total storage weight your attic floor joists can support based on lumber dimensions, span, spacing, and wood species/grade.

Joist Width (b) — inches (e.g., 2 for a 2×8)

Joist Depth (d) — inches (e.g., 8 for a 2×8)

Clear Span (L) — feet

Joist Spacing (s) — inches on center (e.g., 16 or 24)

Allowable Bending Stress (Fb) — psi

Douglas Fir-Larch #1 — 1,500 psi Douglas Fir-Larch #2 — 1,200 psi Douglas Fir-Larch #3 — 900 psi Southern Pine #1 — 1,300 psi Southern Pine #2 — 1,100 psi Hem-Fir #2 — 850 psi Spruce-Pine-Fir #2 — 700 psi Custom value…

Custom Fb — psi

Dead Load (self-weight of floor assembly) — psf

Deflection Limit

L/360 (typical floor) L/240 (storage/attic) L/180 (roof/non-plaster)

Modulus of Elasticity (E) — psi

Douglas Fir-Larch — 1,800,000 psi Southern Pine / Hem-Fir — 1,600,000 psi Spruce-Pine-Fir — 1,400,000 psi Custom value…

Custom E — psi

Calculate

document.getElementById('att-fb').addEventListener('change', function() { document.getElementById('att-fb-custom-group').style.display = this.value === 'custom' ? 'block' : 'none'; }); document.getElementById('att-e-mod').addEventListener('change', function() { document.getElementById('att-e-custom-group').style.display = this.value === 'custom' ? 'block' : 'none'; });

function attCalc() { var errors = [];

var b = parseFloat(document.getElementById('att-joist-width').value); var d = parseFloat(document.getElementById('att-joist-depth').value); var L_ft = parseFloat(document.getElementById('att-span').value); var s_in = parseFloat(document.getElementById('att-spacing').value); var deadLoad = parseFloat(document.getElementById('att-dead-load').value); var deflLimit = parseFloat(document.getElementById('att-deflection-limit').value);

var fbSel = document.getElementById('att-fb').value;
var Fb = fbSel === 'custom'
? parseFloat(document.getElementById('att-fb-custom').value)
parseFloat(fbSel);
var eSel = document.getElementById('att-e-mod').value;
var E = eSel === 'custom'
? parseFloat(document.getElementById('att-e-custom').value)
parseFloat(eSel);

if (isNaN(b) || b 0) { resultDiv.innerHTML = 'Please fix the following:' + errors.map(function(e){ return ''; }).join('') + ''; return; }

// ── Unit conversions ────────────────────────────────────────────────────── var L_in = L_ft * 12; // span in inches var s_ft = s_in / 12; // spacing in feet (tributary width per joist)

// ── Section properties ──────────────────────────────────────────────────── // Section modulus S = b * d² / 6 (in³) var S = (b * d * d) / 6; // Moment of inertia I = b * d³ / 12 (in⁴) var I = (b * Math.pow(d, 3)) / 12;

// ── Bending-governed allowable uniform load ─────────────────────────────── // For a simply-supported beam with uniform load w (lb/in): // M_max = w * L² / 8 // Allowable moment M_allow = Fb * S // → w_allow_bend = 8 * Fb * S / L² (lb/in) var w_bend_lb_in = (8 * Fb * S) / (L_in * L_in); var w_bend_lb_ft = w_bend_lb_in * 12; // lb/ft of joist

// Convert to psf (divide by tributary width in feet) var w_bend_psf = w_bend_lb_ft / s_ft;

// ── Deflection-governed allowable uniform load ──────────────────────────── // Max deflection for uniform load: δ_max = 5 * w * L⁴ / (384 * E * I) // Allowable deflection: δ_allow = L / deflLimit // Solving for w: w_defl = (384 * E * I * δ_allow) / (5 * L⁴) (lb/in) var delta_allow = L_in / deflLimit; var w_defl_lb_in = (384 * E * I * delta_allow) / (5 * Math.pow(L_in, 4)); var w_defl_lb_ft = w_defl_lb_in * 12; var w_defl_psf = w_defl_lb_ft / s_ft;

// ── Governing total allowable load (psf) ───────────────────────────────── var w_total_psf = Math.min(w_bend_psf, w_defl_psf); var governs = w_bend_psf ⚠️ The dead load alone exceeds the joist capacity. ' + 'The attic cannot safely carry any additional storage load with these parameters.'; } else if (liveLoad_psf ⚠️ Available live load capacity is very low (< 10 psf). ' + 'Consider reinforcing joists before storing items.'; }

// ── Format output ───────────────────────────────────────────────────────── var html = '### Results '; html += safetyNote; html += ''; html += 'ParameterValue'; html += 'Section Modulus (S)' + S.toFixed(3) + ' in³'; html += 'Moment of Inertia (I)' + I.toFixed(3) + ' in⁴'; html += 'Bending-limited total load' + w_bend_psf.toFixed(1) + ' psf'; html += 'Deflection-limited total load (L/' + deflLimit + ')' + w_defl_psf.toFixed(1) + ' psf'; html += 'Governing total allowable load' + w_total_psf.toFixed(1) + ' psf (governed by ' + governs + ')'; html += 'Dead load (assumed)' + deadLoad.toFixed(1) + ' psf'; html += 'Available live / storage load' + (liveLoad_psf > 0 ? liveLoad_psf.toFixed(1) : "0.0") + ' psf'; html += 'Max storage weight per joist bay (' + s_ft.toFixed(2) + ' ft × ' + L_ft + ' ft)' + (liveLoad_per_joist_bay_lb > 0 ? liveLoad_per_joist_bay_lb.toFixed(0) : "0") + ' lb'; html += '';

if (liveLoad_psf >= 10) { html += 'Interpretation: Each square foot of attic floor can safely support up to ' + liveLoad_psf.toFixed(1) + ' lb of stored items (live load). ' + 'For example, a 10 ft × 12 ft storage area could hold up to ' + (liveLoad_psf * 10 * 12).toFixed(0) + ' lb.

'; }

resultDiv.innerHTML = html; }

#### Formulas Used

Section Modulus: S = b·d² / 6

Moment of Inertia: I = b·d³ / 12

Bending-governed allowable uniform load (lb/in): wbend = 8·Fb·S / L² derived from Mmax = w·L²/8 ≤ Fb·S for a simply-supported beam.

Deflection-governed allowable uniform load (lb/in): wdefl = 384·E·I·(L/Δlimit) / (5·L⁴) derived from δmax = 5·w·L⁴/(384·E·I) ≤ L/Δlimit.

Allowable load per unit area (psf): wpsf = wlb/ft / sft, where sft is joist spacing in feet (tributary width).

Available live load: wlive = min(wbend, wdefl) − Dead Load

#### Assumptions & References

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