Deck Joist with Wet Service (CM) per NDS 2024

Example

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.

How StructSuite solves this

StructSuite's free wood beam design calculator NDS: select Floor Joist, Design per NDS. 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.

Steps

1. Step 1: Select member type

   Design consideration: 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.

   In StructSuite: In the "Select member type" section at the top, click the Floor Joist card.

2. Step 2: Select design method

   Design consideration: 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.

   In StructSuite: In the "Select design method" section, click the "Design per 2024 NDS (National Design Specification for Wood Construction)" card.

3. Step 1: Geometry & Configuration

   Design consideration: 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%.

   In StructSuite: Open the Geometry section. In the beam schematic, enter span lengths: 10. For each support, use the support type dropdown to select: Pinned, Pinned. NDS 2024 Eq 3.3-1, 3.4-2 for flexure and shear.

4. Step 2: Load Definition (ASCE 7-22 / NDS 2024)

   Design consideration: Dead: 5–10 plf (decking + joist). Live 40 psf × 1.33 = 53 plf. CD=1.0 for D+L. Concentrated (grill, planter) may require separate check. Guardrail load often negligible for joist.

   In StructSuite: Open the Loads section. Add load items with these exact values: D 13 plf; L 53 plf. For each uniform load: set Type (D, L, Lr, S), enter the plf value in Uniform (plf). For point loads: set category Point, enter magnitude (lb) and position (ft). Include self-weight. NDS 2024 Table 2.3.2 sets CD.

5. Step 3: Material & Section

   Design consideration: CM=Yes for wet. Incising=Yes if PT incised (most PT is). L/240 deck; L/360 if ceiling below. Southern Pine common for deck—Southeast; DF-L, SPF also used. KDAT (kiln-dried after treatment) has lower MC, smaller CM reduction.

   In StructSuite: Open the Materials section. In Product category select the type; in Species select Southern Pine; in Grade select No. 2; in Size select 2×8. Open Design Parameters: set Wet service (CM) to Yes; set Repetitive member to No; select Deflection limit. NDS 2024 §4.3.

Live design (pre-filled)

The form below is the real StructSuite module with example data loaded. Display only—values cannot be changed.