Window / Door Header Sizing (IRC R602.7)
Smallest built-up dimension-lumber header (double / triple 2x6-2x12) from tributary load, span, snow, and species.
Example
You enter
- Header clear span (ft) 6
- Tributary width (ft; half the building width) 14
- Ground snow load (psf) 30
- Species and grade SPF_No2
You get
- Member size 2x10
- Plies 2
- Design load on header 630
- Jack (trimmer) studs each end 1
- Bending ok Yes
Checked against published worked examples from: IRC / AWC NDS.
Details, formula, and sources
ScopeWith the AWC NDS bending / L-360 deflection check and IRC R602.7.5 jack-stud count.
Formula or tableTributary uniform load w (plf) = total area load (psf) × supported width (ft); roof = the greater of ground snow and the 20 psf roof live load, + 15 psf dead, each floor adds 50 psf. Smallest built-up member with min(L_bending, L_deflection) ≥ span, where Fb is adjusted by load-duration C_D and size factor C_F. Jack studs each end = ceil(end reaction / (F_c-perp × 5.25 in²)).
Source and editionIRC 2024 (International Residential Code). §R602.7 (headers). AWC NDS-2018 (National Design Specification for Wood Construction). reference design values, load-duration C_D and size factor C_F.
Free accessFree read-only access at codes.iccsafe.org. AWC NDS free at awc.org.
What governsEstimate. AHJ and licensed structural engineer govern.
API field namesheader_span_ft, tributary_width_ft, ground_snow_psf, species_grade, member_size, plies, w_plf, jack_studs_each_end, bending_ok
- Load assembly roof = max(ground snow, 20 psf roof live) + 15 psf dead; each floor above = 40 psf live + 10 psf deadIRC 2024 Tables R301.5 / R301.6 live loads (unchanged from 2021); dead loads typical
- Load-duration factor C_D 1.15 snow (roof-only) / 1.0 occupancy (floors above)AWC NDS-2018 Table 2.3.2
- Cross-check allowable spans verified against IRC Table R602.7(1) by physics; discrepancies flaggedIRC 2024 Table R602.7(1) (same number as 2021)