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.
Formulas Used
1. Bending-controlled maximum span:
M = wL²/8 ≤ F'b · S → Lbend = √(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 → Ldefl = ∛(384 · E · I / (5 · w · 360))
Where: E = modulus of elasticity (psi), I = bd³/12 (in⁴)
3. Adjusted bending design value:
F'b = Fb · CD · CM · CF
CD = 1.0 (normal duration), CM = 1.0 (dry service), CF = size factor (1.0–1.3)
4. Load adjustment for non-40 psf loads:
Ladj = Ltable · (40 / wdesign)1/3
5. Tributary load per joist:
w = Total Load (psf) × Spacing (ft)
Assumptions & References
- 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 CD = 1.0 (normal, 10-year occupancy load)
- Wet service factor CM = 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