Wood deck framing
A single-level residential deck: joists with an overhang, a built-up beam on posts, the posts, their footings, the ledger and a guard post. Each member is checked by NDS 2024 with ASCE 7-22 loads and compared with the prescriptive tables of AWC's deck guide, DCA 6-2015, which follows the IRC R507. LRFD or ASD.
Inputs of the deck #
- Lumber and loads: Southern Pine, Douglas Fir-Larch, Hem-Fir or Spruce-Pine-Fir, and the grade (DCA 6 assumes No. 2). Wet service and incising are inputs: DCA 6 assumes wet service throughout and incising for Douglas Fir-Larch, Hem-Fir and Spruce-Pine-Fir. The dead and live load on the deck (DCA 6 and IRC: 10 and 40 psf).
- Joists: size, spacing, span L from the face of the ledger to the face of the beam, the overhang LO past the beam, and the concentrated load at the end of the overhang with its deflection limit (DCA 6: 220 lb, LO/180).
- Beam: two or three plies, the ply size, the span LB between posts and an overhang past the end post. The beam carries half the joist span plus the overhang, and the overhang's moment.
- Posts and footings: 6x6 or 8x8, the height from the top of the footing to the underside of the beam, Ke (1.0 pinned both ends), round or square footing size and thickness, depth of its bottom below grade, the frost depth, and the allowable soil bearing (DCA 6: 1,500 psf).
- Ledger and guard: a ledger deck with 1/2 in. lag screws, bolts, or bolts with stacked washers at an on-centre spacing, or a deck on its own beam at the house; a guard post (4x4 or 4x6) with its height and the 200 lb guard load.
- Compare with the DCA 6 tables: on by default. Spans, overhangs, post heights, footing sizes and ledger spacing beyond the tables then fail. Turn it off for an engineered deck and they are shown as notes. That choice is yours.
Auto-design (editor toolbar) varies the joist size and spacing, the beam ply size and plies, the post size and the footing size. Objective: the least wood.
Checks performed #
| Check | Basis |
|---|---|
| Joist bending, shear | NDS 3.3, 3.4 with CM, Ci, CF, Cr and the time factor of each combination; the overhang's moment with CL for its free bottom edge |
| Joist deflection | live load on the span against L/360; the overhang tip under the concentrated load, P LO² (L + LO) / (3 E' I), against LO/180 |
| Beam bending, shear, deflection | NDS 3.3, 3.4, 3.5 on the plies acting together |
| Post | NDS 3.7 with CP over Ke times the height; le/d ≤ 50; the larger of an interior post and an end post with the beam overhang |
| Footing | (P + footing weight) / area against the allowable bearing, D + L; bottom at least 12 in. below grade and below the frost line (IRC R403.1.4) |
| Guard post | 200 lb at the top, bending (IRC Table R301.5); the 1,800 lb hold-down of DCA 6 Figure 26 as a note |
| DCA 6 comparison | Table 2 (joist span, overhang and L/4), Table 3A (beam span, overhang LB/4), Table 4 (post height, footing size and thickness), Table 5 (ledger spacing), 2x8 minimum for ledgers and guard-post joists |
The drawings are the deck section along a joist (ledger, joist, beam, post, footing, guard post) and the beam elevation on its posts.
Limitations #
- Decking, joist hangers, post caps, anchors, lateral load devices (IRC R507.2.4), stairs, and the house band joist are not designed. Design an engineered ledger with the wood ledger type and stairs with the wood stair type.
- Redwood, Western Cedars, Ponderosa Pine and Red Pine are not offered: their NDS values are not built in.
- Simple spans only. A beam continuous over posts, joists from both sides of a beam, hot tubs, concentrated loads and snow over 40 psf are outside the scope.
- DCA 6 tables are read for No. 2 lumber, 40 + 10 psf. Other inputs keep the comparison, with a note that it is a reference only.
Verification examples #
Verification status: 22 values compared in 8 examples (7 published, 1 hand calculations from the cited equations): 22 within tolerance, 0 recorded deviations. Code basis: not yet recorded for this module.
The table values are DCA 6-2015's own, as the type reads them. The member checks of the default deck are hand calculations. DCA 6 publishes rules and not calculations, so they are also read backward in the behaviour tests:
- Table 2's Southern Pine 2x10 at 16 in., 14 ft 0 in., sits at 0.996 of the NDS bending value (wet, repetitive, CD 1.0). The table is bending-controlled at its limit.
- A Douglas Fir-Larch No. 2 4x4 guard post, wet and incised, reaches 918 psi adjusted bending against 1,007 psi under 200 lb at 36 in. It misses the 1,100 psi DCA 6 asks for, and the check fails. A Southern Pine No. 2 4x4 passes at 1,100 psi.
No worked example of a deck by NDS is on disk, so this type should be read as hand verified against published tables.