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.
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
- Joists are modeled as simply-supported beams with a uniformly distributed load — the most conservative common assumption.
- Lumber is assumed to be dry-service condition (CM = 1.0) and normal temperature (Ct = 1.0); no additional adjustment factors (size factor CF, repetitive member factor Cr = 1.15) are applied — results are conservative.
- Dead load represents the weight of the floor sheathing, insulation, and joist self-weight (typically 8–15 psf for attic assemblies).
- IRC 2021 Table R802.4.1 requires attic floors used for storage to support a minimum live load of 20 psf; habitable attics require 30 psf.
- Allowable bending stress (Fb) and modulus of elasticity (E) values are from the NDS Supplement: Design Values for Wood Construction (AWC, 2018) for visually graded dimension lumber.
- Deflection limit L/360 is standard for floors; L/240 is commonly accepted for attic storage per IRC.
- This calculator provides an estimate only. Always consult a licensed structural engineer before adding significant loads to an attic.