Deck ledger attachment
More decks separate from houses at the ledger than fail anywhere else. The connection carries half the deck's load into a band joist that was never designed for it, through a horizontal board that sits against a wall and collects water. Both problems have standard answers, and both are routinely skipped.
What the ledger actually does
On a ledger-built deck, roughly half the floor load transfers through the ledger into the house's band joist and rim board, and the other half goes down the posts. That makes the ledger a structural member with a load path behind it — and the load path continues into the floor diaphragm of the house, which is why the attachment point matters as much as the fastener.
Three conditions have to be true for the connection to work:
- The ledger is fastened into solid framing — a band joist or rim board — not into siding, sheathing or veneer.
- The fasteners are sized and spaced to a schedule keyed to the joist span, not chosen by eye.
- The top of the ledger is flashed so water cannot get behind it.
Fastening: the schedule, not a rule of thumb
The IRC permits a ledger to be attached with ½″ diameter lag screws or through-bolts, in two rows, staggered, with the fasteners set back from the ledger's top and bottom edges so they do not split it. Spacing is not a single number — it comes from a table where the spacing narrows as joist span increases, because a longer joist delivers more load to the same length of ledger.
That means "12″ on centre" is not an answer. It might be conservative, it might not, and the inspector will read the table. In the 2018 and 2021 editions this is R507.9; the clause numbering has moved between editions, so match the table to the code your jurisdiction adopted.
Through-bolts are stronger than lag screws in the same hole and are preferred where you can reach both sides — typically into an open basement or crawl space. Where the band joist is not accessible, lag screws are used, and the ledger must be at least 2×8 nominal so the fasteners have something to bite.
Flashing: the part that gets skipped
A ledger is a horizontal board pressed against a vertical wall. Water running down the wall reaches the top edge of the ledger and stops there. Without a flashing that laps over the top of the ledger and up the wall, that water goes into the joint between the ledger and the house — which is exactly where the band joist is.
The sequence that works, from the wall outward:
- Siding removed or cut back above the deck line, so the flashing can lap against the sheathing rather than sit on top of the siding.
- A self-adhesive membrane or metal flashing against the wall, extending above the ledger line.
- The ledger, fastened through the flashing into framing.
- A Z-flashing or cap flashing over the top of the ledger, tucked under the wall flashing above and lapping over the ledger face.
- Siding or trim reinstated over the flashing, holding it up — never nailed through it.
Caulk is not a substitute at any step. Caulk fails in a season or two, and the failure is invisible: the water enters behind the ledger and the rot appears two years later as a soft band joist.
Where a ledger is not allowed
- Brick or stone veneer. The veneer is not structural and is separated from the framing by an air space. A ledger cannot be bolted through it to the framing without a designed standoff detail.
- Cantilevered floor framing. The band joist at a cantilever carries the house's own load in the wrong direction; adding a deck to it needs an engineer.
- Manufactured or steel-framed housing. Different connection rules entirely — the manufacturer's detail governs.
- Where the band joist is not deep enough or not accessible. In that case the deck becomes freestanding, with its own posts and beams on both sides.
The freestanding alternative
A freestanding deck carries all its load on its own posts and beams, and connects to the house with a gap or a slip joint rather than structurally. It costs two more posts and a second beam, and it removes the ledger problem entirely. On a house with veneer, inaccessible framing, or a cantilever, it is usually cheaper than the engineered detail.
What this guide cannot decide for you
- It does not give a fastener schedule. That table depends on joist span, ledger species and fastener diameter. Read it, do not infer it.
- It does not size the band joist. Whether the house's framing can accept the added load is an engineering question, and on older homes it is a real one.
- It does not cover lateral load. A ledger attachment resists vertical load; the deck still needs bracing or a connector for racking, and that is a separate detail.
Questions people actually ask
How far apart do ledger bolts go?
It depends on joist span — the schedule tightens as span increases. There is no single spacing that is correct for every deck, and the table in R507.9 is what the inspector reads.
Can I attach a ledger to brick veneer?
Not directly. Brick veneer is a drainage layer, not structure, and it is tied to the framing with corrugated anchors that carry almost no pull-out load. A ledger needs a designed standoff that reaches the framing, or a freestanding deck.
Do I need flashing above the ledger?
Yes, always. It is the single most important detail on the whole deck, because the failure it prevents — a rotted band joist inside the house — is expensive and invisible until it is advanced.
Lag screws or through-bolts?
Through-bolts where you can reach the inside face, because they clamp the assembly and are stronger in the same hole. Lag screws where the band joist is only accessible from outside. Both must be galvanised or stainless.
What if my house has a cantilever?
Stop. A cantilevered floor band carries load in a direction that makes deck attachment a design problem, not a detail choice. That is an engineer's call, not a fastener schedule.