Wood ledger

A deck or floor ledger fastened to a wood rim (band) joist, a concrete wall or a fully grouted masonry wall with through bolts, lag screws or anchors: the adjusted lateral design value of one fastener (NDS 2024 Chapters 11 and 12, with the gap of the sheathing and washers by the general dowel equations of AWC Technical Report 12), the required on-centre spacing along the ledger under every ASCE 7-22 combination, the check of the spacing used, the fastener pattern (edge, end and row distances) and, as a comparison, the prescriptive IRC deck ledger table.

Source note. The NDS provisions were checked against ANSI/AWC NDS-2024 (scanned, with Commentary); the steps cite the section, table or equation and the printed page. The gap equations are those of AWC TR12 (2015), Table 1-1. The masonry anchor bolt follows TMS 402-22 as TMS's cross-reference, 2022 handouts and sample Guide examples state it (the code text is not on hand); its masonry equations were checked against TMS 402-13. One value is unconfirmed: φ of the anchor steel in shear (see the checks). The IRC table values are those of AWC DCA 6-09 Table 5 (the IRC 2009 to 2021 ledger table); the IRC 2021 text itself is not freely downloadable as a file (ICC read-only access) and the placement table was read from state-adopted copies (to confirm against the code text: IRC 2021 Table R507.9.1.3(2)).

Inputs of the wood ledger #

• Support and fastener: a wood rim joist (through bolts or lag screws), a concrete wall or a fully grouted masonry wall (anchor bolts or post-installed anchors). Diameters 1/2, 5/8 and 3/4 in (bolts and anchors) and 1/2 and 5/8 in (lag screws, reduced body: D = Dr in the yield equations, NDS 12.3.7.1 c, with the thread length of Table L2 or entered). Fyb = 45,000 psi (Table I1). A ledger shall not be attached to brick, stone or masonry veneer (IRC R507.9.1.1); lag shields and lead anchors are not covered.
• Concrete wall: f'c and the anchor embedment. The dowel bearing strength of concrete is Fe = 3 f'c, at most 7,500 psi (AWC TR12 Table A1; the basis of NDS Table 12E, which assumes f'c ≥ 2,500 psi and 6 in embedment); the embedment is the bearing length lm. The anchor's own strength in the concrete (ACI 318-19 Chapter 17, for example from the Anchors in concrete calculation, or the anchor's ICC-ES report) is the engineer's entered design value per anchor (φVn for LRFD, the allowable load for ASD), for this spacing and edge distance.
• Grouted masonry: f'm, the masonry dowel bearing strength (no published value exists for masonry: the concrete relationship 3 f'm ≤ 7,500 psi is prefilled as the engineer's judgment, or the engineer enters Fe), and the anchor: a cast-in headed or bent-bar anchor bolt designed by TMS 402 (embedment, fy and fu, the edge distance lbe in the direction of the load and the wall thickness), or a post-installed anchor with its entered design value.
• Ledger: 2x6 to 2x12 sawn lumber; species G from NDS Table 12.3.3A, or G entered; the ledger length (for the elevation and the fastener count); E for the group action of a vertical pair. IRC R507.9.1.1 asks for a 2x8 or larger No. 2 preservative-treated Southern Pine, incised hem-fir or naturally durable ledger.
• Rim (band) joist: sawn lumber (species G) or an engineered rim board with the manufacturer's equivalent specific gravity for dowel bearing; its thickness and depth; the distance from the top of the rim to the top of the ledger (the rows' position in the rim); E for a vertical pair.
• Between the ledger and the support: the sheathing and the stacked washers (or a spacer). Their total is the gap g of the TR12 equations; the sheathing's own bearing is neglected (conservative). Without a gap the equations are NDS Eqs. 12.3-1 to 12.3-6.
• Fastener pattern: staggered (one fastener per station, alternately in the top and the bottom row, IRC Figure R507.9.1.3) or pairs (two rows aligned); the on-centre spacing s along the ledger; the distances of the two rows from the top and the bottom of the ledger; the end distance.
• Loads: from the deck joists (D, L, Lr, S in psf on half the joist span: 10 psf dead and 40 psf live are the IRC deck table basis) or line loads along the ledger by load type; optional point loads spread over a length of ledger (the fasteners there carry them with the line load). Every ASCE 7-22 combination is evaluated with its own CD (ASD; at most 1.6 for connections, NDS 11.3.2) or KF φz λ (LRFD; λ at most 1.0 for connections, Table N3 footnote 1); the load duration factors of the code options section apply.
• Service conditions: moisture (CM, Table 11.3.3: an exterior deck ledger is usually in service wet, CM = 0.7; the engineer decides) and temperature (Ct, Table 11.3.4); the IRC comparison on or off.

Auto-design (editor toolbar) varies the fastener type (bolt or lag screw, on a wood rim only) and its diameter, the largest on-centre spacing that passes (36 in down to 2 in), and, when unlocked, the ledger size (locked by default: the ledger usually matches the joist depth for the hangers). A lag screw's length follows the candidate (through the ledger, the gap and the rim with the tip past its inside face). Objectives: least fastener steel per foot; fewest fasteners per foot; lowest cost index. The support, the gap, the loads and the pattern's edge distances are kept; see Auto-design.

Checks performed #

The IRC table is not an NDS design: it rests on load tests of ledger connections (Virginia Tech and Washington State University, Carradine, Bender, Loferski and Woeste) with test-based allowable loads per fastener of roughly 385 lb (1/2 in lag screw), 725 lb (1/2 in bolt) and 615 lb (bolt with stacked washers), back-calculated from the table at 50 psf on half the joist span. The NDS yield model is two to three times more conservative (JLC, "Load-tested deck ledger connections", 2004), and the gap and wet service lower it further, so the NDS spacing is much closer than the table's. The calculation designs by NDS and shows the table beside it.

Limitations #

The drawing sheet shows the section at the ledger (the support, the sheathing and washers, both fastener rows with their washers, nut, head or embedment, the thicknesses, the rim's edge distances and the line load) and the ledger elevation near its end (the pattern as designed: staggered or paired, the spacing as equal spaces, the end distance and the edge distances). Flashing, the joists and their hangers, the lateral load connection, the rim's support and the anchor's strength in the wall are not part of the calculation; the sheet notes say so.

• Vertical load only (perpendicular to grain in the ledger and in a horizontal rim, θ = 90°). Withdrawal and the deck's lateral load connection (IRC R507.9.2) are notes, not checks.
• Not checked: the rim joist's bending, bearing and its connection to the house; the ledger's bending between fasteners (negligible); the joist hangers; local stresses of NDS Appendix E; hollow (ungrouted) masonry; brick veneer (not permitted); a ledger carrying girders or beams (IRC R507.9.1.1).
• The anchor's strength in concrete and the strength of post-installed anchors in masonry are the engineer's entered values. No published dowel bearing strength exists for grouted masonry: the 3 f'm relationship is the engineer's judgment.
• Structural screws with an evaluation report (proprietary ledger screws) are not covered: use their report.
• One ledger ply; a built-up ledger and its ply fastening are the engineer's design.
• Point loads are spread uniformly over the entered length; overlapping point loads are not combined.
• φ of the anchor bolt steel in shear is 0.65, which is not confirmed. TMS 402-22 moved the steel strength from fy to fu, and the code text of 9.1.4.3 is not on hand. The 402-16 value of 0.90 goes with fy, and it does not fit fu: the Designers' Guide 2022 prints a steel design tension of 6,390 lb for a 1/2 in. A307 bolt, which is 0.75 × 0.142 in² × 60 ksi. No shear value is printed. The allowable forms against the nominal forms suggest about 0.65 in shear and 0.75 in tension, and 0.65 is used. Masonry crushing governs at usual sizes, so the value rarely decides a design.
• The TMS 402 projected areas use pairwise overlaps of the nearest anchors (conservative where three overlap) and assume the masonry extends at least lb beyond the anchors on the wall face.

Verification examples #

Verification status: 76 values compared in 46 examples (41 published, 5 hand calculations from the cited equations): 76 within tolerance, 0 recorded deviations. Code basis: partial. Only some provisions were read in the target edition; the rest rests on older editions, examples or summaries.

Published tables and examples (backend/tests/test_verify_wood_ledger.py; tables rounded to 10 lb, compared within 5 lb or 2 %; the module computes Fe from G, the tables round Fe to 50 psi):

Hand calculations (closed form, written out in the test file):

No free, complete NDS worked example of a deck ledger connection was found (research/DOCUMENTS_NEEDED.md): the AWC Structural Wood Design Examples and the DCA 6 Commentary would add one.