Deck Beam and Post Sizing (IRC R507)
Deck beam ply and size, post size (4x4 / 6x6) from an NDS column check, footing size from soil bearing.
Example
You enter
- Joist span / deck depth (ft) 12
- Beam span / post spacing (ft) 8
- Post height (ft) 8
- Live load (psf) 40
- Dead load (psf) 10
- Beam / post species and grade SYP_No2
- Soil bearing clay
- Walking-surface height (in) 36
- Ledger condition attached
You get
- Beam size 2x10
- Beam plies 2
- W (plf) 300
- Post size (axial load) 4x4
- Post load (lb) 2400
- Ledger spacing (in) 15
Checked against published worked examples from: IRC / AWC NDS.
Details, formula, and sources
ScopeDeck beam ply and size, post size (4x4 / 6x6) from an NDS column check, footing size from soil bearing, and the IRC R507.9.1.3 ledger fastener spacing.
Formula or tableTributary width to beam = joist span / 2; beam load w (plf) = (live + dead) psf × tributary. Smallest built-up beam with min(L_bending, L_deflection) ≥ post spacing. Post axial load = w × post spacing; NDS column capacity = F_c × C_P × A with C_P from F_cE = 0.822 E_min / (le/d)². Footing from soil bearing; ledger fastener spacing from IRC Table R507.9.1.3(1).
Source and editionIRC 2024 (International Residential Code). §R507 (decks). AWC NDS-2018 (National Design Specification for Wood Construction). reference values; IRC Table R507.9.1.3(1) ledger fasteners.
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 namesjoist_span_ft, beam_span_ft, post_height_ft, live_load_psf, dead_load_psf, species_grade, soil_class, deck_height_in, ledger, beam_size, beam_plies, w_plf, post_size, post_load_lb, ledger_spacing_in
- Deck loads 40 psf live + 10 psf dead per IRC R507 defaultIRC 2024 §R507 (unchanged from 2021)
- Column stability pinned-pinned (K_e = 1), c = 0.8 for sawn lumber, E_min and F_c for the 4x4 from NDS Supplement Table 4A/4B No.2 dimension lumber and for the 6x6 from Table 4D No.2 Posts and Timbers (5x5 and larger)AWC NDS-2018
- Ledger schedule 1/2 in lag / approved fastener spacing by joist span, attached decks onlyIRC 2024 Table R507.9.1.3(1) (identical in 2021)