Deck Joist Span Calculator

Calculate the maximum allowable span for deck joists based on lumber species, size, spacing, and load conditions per IRC Table R507.6.

Lumber Species / Grade

Joist Size

Joist Spacing (inches on center)

Total Design Load (psf)

Actual Joist Span (ft) — optional, for stress check

1. Bending-controlled maximum span:

M = wL²/8 ≤ F'b · S → L_bend = √(8 · F'b · S / w)

Where: w = uniform load per joist (lb/in), F'b = adjusted bending stress (psi), S = bd²/6 (in³)

2. Deflection-controlled maximum span (L/360 live load limit):

Δ = 5wL⁴ / (384EI) ≤ L/360 → L_defl = ∛(384 · E · I / (5 · w · 360))

Where: E = modulus of elasticity (psi), I = bd³/12 (in⁴)

3. Adjusted bending design value:

F'b = Fb · C_D · C_M · C_F

C_D = 1.0 (normal duration), C_M = 1.0 (dry service), C_F = size factor (1.0–1.3)

4. Load adjustment for non-40 psf loads:

L_adj = L_table · (40 / w_design)^1/3

5. Tributary load per joist:

w = Total Load (psf) × Spacing (ft)

Span table values based on IRC Table R507.6 (2018/2021 International Residential Code)
Engineering verification per NDS 2018 (National Design Specification for Wood Construction)
Lumber reference design values from NDS Supplement Table 4A
Deflection limit: L/360 for live load (total load deflection checked at L/240 implicitly via table)
Load duration factor C_D = 1.0 (normal, 10-year occupancy load)
Wet service factor C_M = 1.0 (dry service conditions assumed; apply 0.85 if exposed)
Joists assumed to be simply supported single-span with uniform load
Dead load assumed = 10 psf (decking + framing self-weight)
Standard residential live load = 40 psf per ASCE 7-16 Table 4.3-1
Lateral bracing assumed adequate (no lateral-torsional buckling adjustment applied)
Actual lumber dimensions used (e.g., 2×8 = 1.5" × 7.25" dressed)
Always verify with local building department and a licensed structural engineer for permit applications

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