Fastener Guide

What Size Lag Bolts for a Deck Ledger?

The ledger-to-house connection carries half your deck — and it is the joint inspectors fail most. Code demands 1/2 in lag screws or through-bolts (never nails, never deck screws), spaced per IRC Table R507.9.1.3 by joist span, staggered in two rows, with flashing and a lateral-tension tie. Here is the sizing that passes inspection.

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What to Look For

  • Minimum 1/2 in diameter — lag screws, through-bolts, or code-listed structural screws only.
  • Spacing follows joist span per IRC R507.9.1.3 — roughly 16 in staggered for 12 ft spans, tighter as spans grow.
  • Two staggered rows — 2 in from ledger top and bottom edges, 2–5 in from the ends.
  • Ledger must be 2x8 minimum, bolted to solid rim joist or concrete — never brick veneer or stucco.
  • Hot-dip galvanized or stainless with treated lumber — plus continuous flashing and a lateral-load tie.

Top Picks

1. GRK RSS structural screws, 1/2 x 5 in Best overall

Code-listed ledger fasteners that drive with an impact driver — no pre-drilling, no socket wrench, published spacing tables the inspector already knows.

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2. Hot-dip galvanized lag screws, 1/2 x 4 in, with washers Best traditional

The classic ledger bolt — 1/2 in HDG lags with flat washers, pre-drilled per code, torqued to spec. What the span tables were written around.

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3. Hot-dip galvanized through-bolts, 1/2 in, with nuts and washers Best strongest connection

Where you can reach the back side, through-bolts beat every lag — clamping force both sides, inspectable, and the gold standard for heavy decks.

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What to Avoid

Avoid

  • Nails or deck screws in a ledger — the failure mode behind nearly every deck collapse.
  • Attaching to brick veneer, stucco, or cantilevered floors without an engineer — veneer carries nothing.
  • Skipping flashing — rot behind the ledger destroys the connection no fastener can save.
  • Forgetting the lateral-tension tie — code requires hold-downs resisting 750 lb at two locations minimum.
Bottom line: Half-inch structural screws for speed, HDG lags for tradition, through-bolts for maximum strength — all staggered per the span table, all flashed, all with a lateral tie. That is a ledger that passes.

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FAQ

What size lag bolts for a deck ledger board?

1/2 in diameter minimum — lag screws, through-bolts, or code-listed structural screws. Length must reach through the ledger with full thread engagement in the rim joist, typically 4–6 in overall.

How far apart are ledger bolts spaced?

Per IRC Table R507.9.1.3 by joist span — about 30 in for 6 ft spans tightening to 16 in or less past 12 ft. Always staggered in two rows, never a single line of bolts.

Can you attach a ledger with deck screws or nails?

Never — nails and deck screws have no structural rating for ledger loads and are the documented cause of ledger failures. Only 1/2 in lags, through-bolts, or listed structural screws.

Do you need flashing behind a deck ledger?

Yes — continuous flashing behind and over the ledger is code-required. Water trapped behind the ledger rots the rim joist, and no fastener holds in rotted wood.

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