How the fasteners are counted
The IRC’s ledger table (R507.9.1.3(1) in the 2021 and 2024 editions) gives the most on-center spacing of ½ in lag screws or bolts for joist spans from 6 to 18 ft, in 2 ft steps. The calculator looks up your span, using the next longer column when the span falls between two, then spreads the fasteners evenly along each ledger piece with the first and last at the distance you choose from the ends.
The table is for 40 psf of live load and 10 psf of dead load, and the ledger carries the half of the joists nearest the house. Worked backwards, its lag screw spacings give each lag screw about 385 lb of deck to hold and its bolt spacings about 725 lb per bolt, with nothing farther apart than 36 in. Longer joists put more deck on each inch of ledger, so the fasteners move closer.
Worked example: a 16 ft ledger, joists spanning 12 ft
- ½ in lag screws through ½ in sheathing into a 2× band joist: at most 15 in apart.
- Run: 192 − 2 × 2 = 188 in. 188 ÷ 15 = 12.5, rounded up to 13 gaps: 14 lag screws.
- Spacing: 188 ÷ 13 = 14.46 in, 14 7/16 in on the tape, alternating between the two rows: 7 in each.
- With ½ in bolts instead, the table allows 29 in: 188 ÷ 29 = 6.5, so 7 gaps and 8 bolts, 26⅞ in apart.
A ledger in two pieces is worked piece by piece: each piece gets its own fasteners near both ends. Enter the pieces separated by plus signs, like 12 + 8.
Where the fasteners go
The fasteners are staggered in two rows along the ledger. The IRC’s placement table (R507.9.1.3(2)) sets the edge distances; JLC’s guide to the same rules adds the outer limits:
| Distance | On the ledger | On the band joist |
|---|---|---|
| From the top edge | at least 2 in | at least ¾ in |
| From the bottom edge | at least ¾ in | at least 2 in |
| From the ends | 2 to 5 in | — |
| Between the rows | 1⅝ to 5 in | — |
JLC also gives a least depth for the lower row: 5½ in below the top of a 2×8 ledger, 6½ in on a 2×10 and 7½ in on a 2×12. The calculator shows the lower row’s range for your ledger, from that depth to ¾ in above its bottom edge, and puts the upper row 2 in down, or lower if the rows would otherwise be more than 5 in apart.
Holes, lengths and washers
- Lag screws: predrill with two bits, a ½ in hole through the ledger and any sheathing and a 5/16 to 3/8 in hole into the band joist (R507.9.1.3). Fairfax County’s details ask for the tip to go at least ½ in past the band joist, so through ½ in sheathing into a 2× band a lag screw is at least 4 in long. Each takes a washer under its head (DCA 6).
- Bolts: holes 17/32 to 9/16 in (Fairfax County), with a washer under the head and another under the nut.
- Finish: bolts and lag screws for decks are hot-dip galvanized to ASTM A153 Class C, mechanically galvanized to ASTM B695 Class 55, or 410 stainless steel (IRC Table R507.2.3).
What the ledger may fasten to
The ledger is at least a 2×8 of treated Southern pine, incised treated hem-fir or naturally durable wood, and it may not carry the concentrated load of a beam (R507.9.1.1). It may not hang on stone or masonry veneer. The band joist behind it is at least 2× solid-sawn lumber or 1 in engineered rim board, bearing fully on the house’s structure (R507.9.1.2). The table covers up to ½ in of sheathing between the boards for lag screws, and up to 1 in of sheathing or stacked washers for bolts.
Flashing and hold-downs
New in the 2024 IRC, R507.9.1.5 sets the ledger flashing: it rises at least 2 in above the ledger, and either reaches at least 4 in out beyond the ledger’s face or comes to the face and at least ¼ in down it. The deck also needs a lateral connection to the house: two hold-downs rated 1,500 lb within 24 in of each end of the deck, or four rated 750 lb (R507.9.2).
The IRC also allows other ledger arrangements built to carry the code’s loads (R507.9.1.4): the route for anything the table doesn’t cover, such as joists spanning more than 18 ft. For the joists, beam and posts that go with the ledger, see the deck framing calculator; for the footings, the deck footing calculator.
Questions people ask
How many lag screws does a deck ledger need?
It depends on the joist span. For a 16 ft ledger carrying joists that span 12 ft, the IRC table allows ½ in lag screws at most 15 in apart (through up to ½ in of sheathing into a 2× band joist). With the end ones 2 in from each end, that takes 14 lag screws, about 14 7/16 in apart, staggered seven up and seven down.
Should I use lag screws or bolts?
Bolts can go farther apart: 29 in instead of 15 in for 12 ft joists, so the same 16 ft ledger takes 8 bolts instead of 14 lag screws. Bolts are also allowed through up to 1 in of sheathing or stacked washers, where lag screws stop at ½ in. But a bolt needs a washer and nut on the inside of the band joist, so you need to reach behind it.
How far from the edges do ledger bolts go?
The IRC’s placement table puts them at least 2 in from the top of the ledger, ¾ in from its bottom and 2 in from its ends, with the two rows at least 1⅝ in apart. JLC adds that the end ones should be no more than 5 in from the ends and the rows no more than 5 in apart, which for a 2×10 ledger puts the lower row at least 6½ in down.
Can a deck ledger go on brick or stone veneer?
No: the IRC says deck ledgers shall not be supported on stone or masonry veneer (R507.9.1.1). The ledger has to fasten to the house’s band joist, which must bear fully on the house’s structure. Where that isn’t possible, the deck can be freestanding, on its own beam and posts next to the house.
What about engineered rim boards?
The IRC accepts a band joist of 1 in nominal engineered rim board but leaves the fastener spacing to the rim board’s maker. AWC’s DCA 6 gives spacings for 1 in and 1⅛ in rim boards, closer than for lumber: 12 in and 14 in for joists spanning 12 ft. The calculator uses DCA 6’s figures; check them against the maker’s.
Does a deck need hold-down brackets?
The IRC asks for a lateral connection between the deck and the house (R507.9.2): hold-down devices in at least two places, within 24 in of each end of the deck, each rated at least 1,500 lb; or in at least four places, each rated 750 lb, using the second detail the code shows.
Sources for the numbers on this page
- California Building Standards Commission, via Codebook — 2025 California Residential Code, section R507.9: deck ledger attachment and lateral connection (2024 IRC text) · checked October 4, 2026
- California Building Standards Commission, via Codebook — 2025 California Residential Code, section R507: exterior decks (2024 IRC text; footings, posts, beams) · checked October 4, 2026
- Colorado Chapter of the International Code Council, via Adams County — Single family residential uncovered decks and porches, 2024 IRC (ledger fastener spacing and loads, PDF, 03/20/26) · checked October 4, 2026
- Journal of Light Construction — Ledger bolting requirements in the 2012 IRC (Glenn Mathewson, May 2012) · checked October 4, 2026
- Fairfax County, Virginia — Typical deck details, version 2021.0 (1/5/2024, PDF) · checked October 4, 2026
- American Wood Council — DCA 6: Prescriptive residential wood deck construction guide (2015 IRC edition) · checked October 3, 2026
Corrections and updates are logged on the sources page.