How does joist span affect ledger bolt spacing?

The joist span sets the tributary load carried by the ledger. A longer span means each foot of ledger supports more deck weight, so the code tightens the bolt spacing. The IRC table moves from roughly 30 in on center at short spans down to about 10 in on center near the 18 ft limit.

What is the difference between a lag screw and a through-bolt here?

A through-bolt passes completely through the rim joist and is secured with a nut and washer, giving higher capacity, so it can be spaced farther apart. A lag screw threads into the band joist only and must be placed closer together for the same load.

Why are the bolts shown in two staggered rows?

The IRC requires ledger fasteners in two rows, offset top and bottom, so the connection resists both the downward pull and the rotation of the deck. Staggering also avoids splitting the band joist along a single grain line.

Does this replace a structural plan?

No. It reproduces IRC Table R507.9.1.3(1) for common assemblies, but flashing, sheathing thickness, brick veneer, cantilevers, and local amendments all change the result. Have a building official or engineer confirm any deck ledger connection.

Can I attach a ledger to any wall?

No. Ledgers may not attach to brick veneer or stucco over foam, and never to a wall with a band joist hidden behind such finishes without an engineered detail. When in doubt, use a free-standing deck on its own posts and footings.

What is the Deck Ledger Bolt Calculator?

Compute the number and on-center spacing of ledger-attachment bolts from joist span, ledger length, and fastener diameter using IRC Table R507.9.1.3(1). Shows a staggered two-row bolt pattern for a code-compliant deck ledger connection. It runs free in your browser on Gera Tools, with nothing uploaded.

Deck Ledger Bolt Calculator

Name: Deck Ledger Bolt Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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A deck ledger is the horizontal board that fastens the deck structure directly to the house rim joist. It carries roughly half of the entire deck load straight into the building’s framing, which is why the ledger connection is one of the most critically engineered parts of any deck. Under-fastening a ledger is cited as one of the most common causes of catastrophic deck collapse, and the International Residential Code (IRC) prescribes exactly how many bolts are needed and how far apart they can be spaced for a given deck size.

How the required spacing is determined

The IRC ties ledger fastener spacing to joist span — the distance the deck joists travel from the ledger to the outer beam. A longer span means each linear foot of ledger supports a larger tributary area of deck, so more load transfers through each foot of the connection, and the fasteners must come closer together.

The tool looks up the maximum on-center spacing from IRC Table R507.9.1.3(1) for your span band and fastener type, then calculates how many fasteners fit along the ledger:

spacing  = IRC table lookup(joist span band, fastener type)
count    = floor((ledger_length_in − 2 × edge_offset_in) ÷ spacing) + 1

The two-inch edge offset keeps the outermost fasteners away from the ends of the board to prevent splitting. The total count is split between a top row and a staggered bottom row, as required by the code, so that the connection resists both vertical load and the rotational forces the deck exerts on the ledger.

Through-bolts versus lag screws

These two fastener types have different capacities and therefore different allowed spacings:

Through-bolts (½” diameter minimum) pass completely through the ledger, rim joist, and any sheathing and are secured with a nut and washer on the interior side. Because they engage the full cross-section of the rim joist and clamp both sides, they have higher load capacity per fastener and can be spaced farther apart.
Lag screws (½” diameter minimum) thread only into the band joist and rely on thread engagement for their holding strength. They must be spaced closer together than through-bolts for equivalent load transfer.

In most residential situations, lag screws are used because they can be installed from the exterior without access to the interior side — a significant practical advantage during construction.

Flashing and exclusions

The fastener pattern is only part of a code-compliant ledger installation. The ledger must also be:

Properly flashed to prevent water intrusion between the ledger and the house sheathing. Water trapped here causes the rim joist to rot, and the rotted joist can no longer hold the fasteners regardless of the pattern used.
Kept away from brick veneer, stucco over foam sheathing, or any wall assembly where the rim joist is not directly accessible. The IRC prohibits attaching a ledger to these finishes. In those situations a free-standing deck on its own footings is required.
Installed with fasteners at least 2 inches from the top and bottom edges of the ledger board to prevent splitting.

Important notice

This calculator reproduces IRC Table R507.9.1.3(1) for common assemblies and is a planning and reference tool only. Flashing details, sheathing thickness, cantilevers, local code amendments, and non-standard framing conditions all affect the final design. Have your local building official or a licensed structural engineer confirm any deck ledger connection before construction.