Deck Ledger Bolt & Lag Screw Spacing Calculator (IRC R507.9)

How it works

Spacing from IRC Table R507.9.1.3(1) by joist span and fastener type (two staggered rows); count ≈ (length × 12 ÷ spacing + 1) × 2

Ledger failure is the leading cause of deck collapses, and the code answer is a prescriptive fastener schedule: half-inch lag screws or through-bolts in two staggered rows, at a spacing that tightens as the joists — and therefore the load per foot of ledger — get longer. Through-bolts carry roughly twice what lags do, so their spacing is wider, and stacked washers to bridge a thick sheathing gap cost some capacity. The edge distances keep the fasteners out of the splitting zone at the top and bottom of the members, and the rest of the section forbids the details that have actually killed people: ledgers screwed through brick or stucco, hung on cantilevered floors, or fastened with nails.

Worked example

A ledger carrying 12 ft joists needs ½″ lag screws at 15″ on centre in two staggered rows (through-bolts could be 29″); a 16 ft ledger takes about 28 lags, plus the pairs at each end and two hold-down tension ties for lateral load.

Frequently asked questions