Deck Ledger Board Fastener Calculator

On-center spacing and fastener count for 1/2" lag screws or 1/2" through-bolts attaching a deck ledger to the house band joist

Enter ledger length and the joist span from the ledger to the beam (no cantilever). Pick 1/2" lag screws or 1/2" through-bolts and 1 or 2 staggered rows. Spacing uses a published inspector rule of thumb, not the full IRC table. Attachment rules: guide. Field problems: troubleshooting. Hardware choice: lag vs bolt vs structural screw. Deck sequence: beam span, footing size, post spacing, deck stairs. Carpentry index: hub.

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How to Use This Deck Ledger Fastener Calculator

Measure the ledger along the house. Measure joist span from the face of the ledger to the beam those joists rest on — stop at the beam. Choose 1/2" lag screws if you cannot reach the inside of the band joist, or 1/2" through-bolts if you can. Leave rows at 2 (staggered) for a 2x8 or deeper ledger unless the adopted code says otherwise.

1/2" lag screw on-center (in) = 190 ÷ joist span (ft)
1/2" through-bolt on-center (in) = 330 ÷ joist span (ft)
Count = [ceil((ledger inches − 4) ÷ spacing) + 1] × rows

Worked example 16 ft ledger, 10 ft joist span, 1/2" lag screws, 2 staggered rows.
Spacing = 190 ÷ 10 = 19 in on center.
Usable length = (16 × 12) − 4 = 188 in.
ceil(188 ÷ 19) + 1 = 11 fasteners per row.
× 2 rows = 22 fasteners.

That 19 in figure is a rule of thumb, not a cell from the IRC table. Confirm before you drill. Full fastening rules: attachment guide. If an existing ledger looks wrong: troubleshooting.

What This Number Is (and Is Not)

The ledger is the one deck connection that hangs off an existing house instead of sitting on posts and footings. Fastener spacing is how that hang is shared along the board. This calculator returns a published inspector shortcut for 1/2" lags and 1/2" through-bolts, then a count from that spacing and your ledger length.

It does not pick live load, sheathing thickness, or band-joist species. Those are why IRC Table R507.9.1.3(1) has more than one column. It does not size flashing, lateral hold-downs, or the joists and beam themselves — use the joist span calculator and deck beam span calculator for those.

Hardware that does not count. Carriage / cap-head bolts and ordinary deck screws are not IRC-listed for this connection. Lag screws must fully penetrate through and beyond the inside face of the band joist. Comparison: lag vs bolt vs structural screw.

Framing Layout Gear for Ledger Work

Ledger fastening is a layout job as much as a hardware job. Square the board, keep it level, and cut clean before you commit the first lag or through-bolt into the band joist.

16×24 Framing Square

Mark the 2" end distance and the staggered two-row pattern on the ledger before you drill. Same square you will use on the rest of the deck frame.

View on Amazon →

48" Box Beam Level

A ledger that walks out of level before flashing is a water problem waiting to happen. Sight it, then fasten.

View on Amazon →

7-1/4" Circular Saw

Cut the ledger to the house, not to a guess. Crown and length are set before any fastener goes in.

View on Amazon →

Johnson Stair Gauges

When the same deck gets stairs, lock rise and run on the square. Ledger fastening and stringer layout are different jobs.

View on Amazon →

Run the Ledger Spacing

1/2" lag screw or 1/2" through-bolt on-center spacing from joist span, plus a fastener count for the ledger length.

Open Ledger Fastener Calculator →

How Ledger Attachment Works

Why this connection is not stacked bearing, plus staggered rows, flashing, and hold-downs.

Read the Attachment Guide →

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Deck Ledger Board Fastener Calculator

How to Use This Deck Ledger Fastener Calculator

What This Number Is (and Is Not)