Deck Joist with Wet Service (CM) per NDS 2024

Summary: Design an exterior deck joist with wet service factor CM per NDS 2024 Table 4.3.1—moisture content >19% reduces capacity. Pressure-treated lumber in…

Overview

Design an exterior deck joist with wet service factor CM per NDS 2024 Table 4.3.1—moisture content >19% reduces capacity. Pressure-treated lumber in ground contact or exposed uses CM < 1.

Problem statement

A pressure-treated 2×8 Southern Pine No. 2 deck joist, 10 ft span, 16 in. o.c. Dead 10 plf (decking + joist), live 40 psf (deck). Moisture content >19%—apply CM per NDS Table 4.3.1. CD = 1.0 for D+L. Check capacity.

Workflow in StructSuite

StructSuite's free wood beam design calculator NDS: open the Wood Beam module. Enter span [10], pinned supports. Add uniform load: tributary width 16/12 ≈ 1.33 ft → 13 plf D, 53 plf L. In Design Parameters, set Wet service (CM) = Yes. StructSuite applies CM from NDS Table 4.3.1 (Fb, Fv, Fc, E, Emin reduced). Incising for pressure treatment may further reduce—select if applicable.

Design considerations (excerpt)

Deck joists exposed to weather—MC > 19% typical. NDS CM reduces Fb, Fv ~10–20% when wet. PT lumber for decay resistance; incising (drill holes for treatment) further reduces—check PT stamp. IBC tables assume dry; decks need NDS. 10 ft typical deck span. 16 in. o.c. = tributary 1.33 ft. Longer spans (12–14 ft) need larger joist or tighter spacing. PT 2×8 SP #2: wet service cuts dry span by ~10–15%. NDS with CM=wet: Southern Pine CM ~0.85–0.90 for Fb. Incising factor 0.80 for most PT. Combined: capacity ~70% of dry. 40 psf live per IBC; heavier for hot tubs, gatherings.

Dead: 5–10 plf (decking + joist). Live 40 psf × 1.33 = 53 plf. CD=1.0 for D+L. Concentrated (grill, planter) may require…

Related terms

• StructSuite
• structural engineering
• design example
• wood beam
• NDS 2024
• nds wet service

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