Lumber Spans
Maximum span from species, grade, size, load.
Example
You enter
- Species and grade DF-L_No2
- Nominal size 2x10
- Total load (psf) 50
- Tributary width (in) 16
- Deflection limit (L/x) 360
You get
- Allowable span 15.21 ft
- By bending 15.61 ft
- By deflection 15.21 ft
- Governing deflection
Checked against published worked examples from: American Wood Council; Southern Forest Products Association.
Details, formula, and sources
Formula or tableAllowable simple-beam span: deflection-limited L = cbrt(384 × E × I / (5 × w × n)) (5wL⁴/384EI = L/n), w the live load when entered, else the total; strength-limited from M_allow = Fb' × S, with Fb' = Fb × C_F × C_r (C_F the size factor, 1.0 for Southern Pine; C_r = 1.15 at 24 in on center or closer). F_b and E from AWC NDS-2018 design values; deflection limit L/360 (live) or L/240 (total) per IRC §R301.7.
Source and editionAWC NDS-2018 (National Design Specification for Wood Construction). IRC 2024 (International Residential Code). §R301.7, R502.3, R802.4 (unchanged from 2021).
Free accessAWC NDS free at awc.org/codes-standards. Free read-only access at codes.iccsafe.org.
What governsEstimate. AHJ and licensed structural engineer govern.
API field namesspecies_grade, nominal_size, total_load_psf, tributary_width_in, deflection_limit, allowable_span_ft, by_bending_ft, by_deflection_ft, governing
- Species / grade design values data/construction/lumber-properties.json keyed to species / gradeAWC NDS / lumber-grading agency
- Default deflection limit L/360 live, L/240 totalIRC 2024 §R301.7 (unchanged from 2021)