Deck Ledger Board Calculator
The ledger is the single most common point of failure on a deck — most collapses start where the deck meets the house. Spacing is not a matter of preference: the AWC DCA 6 prescriptive table sets an exact on-centre distance based on how far your joists span and what you are fastening with. Enter both and this returns the spacing, the fastener count, the row layout and the edge distances.
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Frequently asked questions
- How far apart should deck ledger bolts be?
- It depends entirely on how far your joists span. From the DCA 6 table, half-inch lag screws go at 30 inches on centre for a 6 ft joist span, 18 inches at 10 ft, 15 inches at 12 ft and 11 inches at 16 ft. Half-inch through-bolts allow much wider spacing — 34 inches at 10 ft and 29 at 12 ft — because they clamp the whole assembly rather than relying on thread withdrawal.
- Can I use deck screws on a ledger board?
- No. Ordinary deck screws and construction screws have no rated withdrawal or shear capacity for this connection, and ledger failure is the leading cause of deck collapse. Use half-inch lag screws, half-inch through-bolts, or a proprietary structural screw that carries an evaluation report listing it specifically for deck ledger attachment, installed at the spacing on that report.
- What size ledger board do I need?
- The ledger is sized to match the joists it carries — 2x8 joists get a 2x8 ledger, 2x10 joists a 2x10. It must be pressure-treated for ground contact or exterior use, and it must bear on the house's rim joist or solid blocking. A ledger fastened to brick veneer, stucco, foam sheathing or a cantilevered floor is not attached to anything structural.
- Do I need flashing behind the deck ledger?
- Yes, and the order matters more than the material. The house wrap or building paper laps over the top flange of the flashing, the flashing tucks behind the ledger and turns down over its face, and the decking sheds water away from the wall. Water trapped behind an unflashed ledger rots the band joist from the inside, which is exactly the member the fasteners depend on.
