The IRC R507.9.1.3 ledger fastener table lookup that every deck builder + inspector needs but nobody memorizes. Enter ledger length + joist span + snow load + house framing — and the calculator returns the required on-center bolt spacing, total fastener count, row configuration (1 or 2 staggered rows for deeper ledgers), edge + end distance compliance, and Z-flashing requirement. Four fastener options compete: ½″ × 5½″ lag screws (IRC standard, 250 lb per fastener), ½″ through-bolts with nut + washer (380 lb — preferred when backside access exists), Simpson SDS25500 ¼″ × 5″ structural screws (IRC alternate path, no predrill, 220 lb), and Simpson SDWS22600 ¼″ × 6″ (280 lb for heavier loads). Plus 5 house framing types. Ledger attachment is the most failure-prone connection in residential deck construction — this is the math that prevents collapse.
Below 30 psf — IRC no-snow column · looser spacing allowed
IRC R507.9.1.3 Table lookup for 12' joist span at 25 psf snow. ½″ × 5½″ lag screw + washer gets 1.0× the baseline.
14 per row × 2 rows. End distance 4″ from each end + on-center spacing 15″. Total fastener cost $62.
IRC R507.9.1.3(2) edge distance compliance. Top edge ≥ 0.75″ (3/4″) keeps fastener away from ledger top; bottom edge ≥ 2″ prevents splitting under load.
Alternate fasteners between rows — no two bolts in a vertical line. Row spacing 1.625″ vertical (1-5/8″). Prevents the wood from splitting along the fastener line under cyclic load.
Metal Z-flashing tucked under wall siding above ledger + lapped over ledger top edge. Directs water around the ledger instead of behind it. Not optional — this is the #1 cause of house framing rot when omitted. ~$4.20/lf for 24-gauge stainless.
Open the Deck Load Calculator to check joist + beam + post + footing capacity at the same time. Or jump to Joist Span Calculator for joist sizing only.
IRC R507.9.1.3 + Table + R703.4 + Simpson Strong-Tie ESR-2236 (alternate fastener path). Not a substitute for stamped structural engineer letter on engineered I-joist house framing, brick veneer, log home, or heavy point loads (hot tub, outdoor kitchen).
How to use
From end to end where the ledger will attach to the house. Standard 5/4 PT deck board or 2× lumber. Typical ledger length matches the deck length (parallel to house wall).
Distance from ledger out to the beam — this is the joist length, which equals deck width (or width minus cantilever). Bigger joist span = more tributary load on the ledger = tighter bolt spacing required.
12″, 16″, or 24″ on-center. IRC R507.6 standard is 16″ for parallel decking, 12″ for diagonal composite. The calculator uses this for tributary math.
ASCE 7 ground snow PSF for your region. Below 30 psf uses the no-snow column of IRC Table R507.9.1.3(1) — looser spacing. 30+ psf uses the snow column — tighter spacing. North-snow zones (MN, WI, ME, etc.) commonly have 35-50 psf which dramatically reduces the allowable spacing.
½″ lag screw is the IRC standard (250 lb allowable per fastener). ½″ through-bolt gets 50% more capacity (380 lb) but needs backside access for the nut + washer — viable only when the rim joist is open from inside (basement, crawl space). Simpson SDS25500 ¼″ × 5″ structural screw is the modern alternate (no predrill, smaller hole, IRC R507.9.1.3.1 alternate path, 220 lb). Simpson SDWS22600 ¼″ × 6″ gives 280 lb with a longer reach for 2x12 ledgers.
2×10 dimensional rim joist (most homes), 2×12 (basements + newer construction), engineered I-joist / LVL (requires solid blocking behind I-joist web — get engineer involved), log home (special — needs longer lag + engineer letter), or unknown (don't attach until you've verified). The framing type affects edge distance compliance + whether standard fasteners are even permitted.
Top of results shows required on-center spacing in inches and total bolt count. Bottom shows the compliance check — edge distance (¾″ from top + 2″ from bottom of ledger), end distance (4″ from each end), row spacing (1-5/8″ between rows when 2-row), staggering requirement (mandatory for 2-row), Z-flashing requirement (always — required per IRC R703.4).
Material guide
Three honest paths. Composite wins the 25-year math for most homeowners, wood wins on upfront cost, and PVC is unbeatable around water. Each card below answers in one glance — recalculate the bill of materials by clicking a brand in the picker above.
How we calculate
The Ledger Bolt Calculator does the IRC R507.9.1.3 Table lookup that every deck builder + inspector needs but nobody memorizes. Enter your ledger length, joist span (deck width), joist spacing, snow load region, and house framing type — and the calculator returns the required on-center bolt spacing, total fastener count, row configuration (1 or 2 rows for deeper ledgers), edge + end distance compliance, staggering requirement, and total fastener cost. Four fastener options compete: ½″ × 5½″ lag screws (IRC R507.9.1.3 standard, $2.20 each), ½″ through-bolts with nut + washer (50% higher capacity but requires backside access, $3.80 each), Simpson SDS25500 ¼″ × 5″ structural screws (IRC R507.9.1.3.1 alternate path, no predrill, $2.65 each), and Simpson SDWS22600 ¼″ × 6″ for heavier loads ($3.20 each). Plus 5 house framing types (2×10 rim, 2×12 rim, engineered I-joist / LVL, log home, unknown). Ledger attachment is the most failure-prone connection in residential deck construction — get this math right.
IRC R507.9.1.3 + Table R507.9.1.3(1) for on-center bolt spacing by joist span + snow load. IRC R507.9.1.3(2) for edge + end distance + row spacing. IRC R507.9.1.3.1 alternate fastener path with Simpson Strong-Tie SDS25500 + SDWS22600 ICC-ESR-2236 allowable values. IRC R703.4 for Z-flashing requirement. Per-fastener allowable load for ½″ lag (250 lb) + ½″ through-bolt (380 lb) derived from AWC NDS yield-mode capacity in 1.5″ SPF rim joist.
Ledger carries half the joist load (the other half goes to the beam). 12 ft joist + no cantilever = 6 ft tributary width per LF of ledger. 75 psf design load × 6 ft = 450 lb per LF of ledger. This determines required fastener spacing.
Baseline is for ½″ lag (1.0× multiplier). Through-bolt gets 1.5× (50% looser spacing). SDS25500 0.95× (slightly tighter). SDWS22600 1.15× (slightly looser). Snow region (≥30 psf) drops to the tighter column.
End distance subtracts 4″ from each end of the ledger. Plus 1 because both ends need a fastener. Multiplied by row count (1 row for 2×6/2×8 ledgers, 2 rows for 2×10/2×12 ledgers with deeper joists).
Top edge from top of ledger ¾″ min, bottom edge from bottom 2″ min. End distance from each end of ledger 4″ min. Row spacing in 2-row config = 1-5/8″. These prevent the wood from splitting under load.
Metal Z-flashing tucked under the wall siding + over the ledger top edge prevents water from running down behind the ledger and rotting the house framing. Required per IRC R703.4 — not optional. ~$4.20 per LF for 24-gauge stainless or galvanized.
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