The Deck Connection That Kills People

Every deck failure statistic in North America lands on the same joint. It is not the railing, the stringers, the posts, or the footings. It is the band of lumber bolted to the side of the house. Here is how to get the ledger right — and how to know when to walk away from it.

Why ledger failure is the #1 residential deck collapse cause

Data from the North American Deck and Railing Association and major insurance investigations points at the same joint. Roughly 90 percent of catastrophic deck collapses in single-family homes start at the ledger — the band of lumber that carries one whole edge of the deck and every pound of live load on the near half. When the ledger lets go, the deck drops as a unit, usually full of people, usually during a barbecue or party where the live load is double what the designer assumed. The failure modes are monotonous: rotten rim joists behind un-flashed ledgers, nails (not bolts) driven through brick veneer into nothing, lag screws pulled through an I-joist rim, ledgers attached to a 1920s solid-brick wall with no structural backing. None of these are edge cases. Every one is a code violation a careful inspection would catch. The schedule on this page covers the prescriptive case; the notes cover the traps.

Lag screws versus through-bolts — the real-world difference

A lag screw grips the rim joist by the threads biting into wood. When that wood dries, shrinks, or starts to rot, the threads have less to hold. A through-bolt passes all the way through the ledger AND the rim, capped with a washer and a nut on the inside face of the joist bay. The clamping force is absolute — the nut and washer spread load across a large washer footprint on the back side of the rim and the bolt cannot withdraw as wood dries. Test data from the American Wood Council shows through-bolts at the same diameter allow about 50 percent wider spacing than lag screws in DCA-6. So why do most decks get lag screws? Because most rim joists are not accessible from the inside — they are behind drywall, insulation, or finished space. Where you can get at the inside (walkout basement, open crawlspace, garage) always choose the through-bolt. Where you cannot, lag screws are acceptable, but you pre-drill every hole (5/16 inch pilot in the ledger, 3/8 inch in the rim for a 1/2 inch lag) and you accept the tighter spacing the schedule demands.

Ledger to brick veneer — why it is never allowed

Brick veneer is not structure. It is a single wythe of brick tied to the real wall behind it with small corrugated metal ties spaced every 600 mm or so, and those ties are rated for wind and weather, not for deck load. A deck ledger bolted through brick veneer does one of two things. It bolts into mortar joints, which have zero withdrawal capacity — the ledger is held only by the friction of the masonry drill hole, which fails gradually as water cycles through. Or it bolts into the bricks themselves, concentrating a deck load that wants to pull the brick face off the building. Ontario inspectors reject this detail on sight, as they should. The right answers are two. First, remove the brick veneer in the ledger footprint, exposing the sheathing and rim joist behind, and bolt directly to the structure. Flashing the patch afterwards is its own skill. Second, and better on heritage or tight-budget jobs, build a freestanding deck on its own beam line 300 to 400 mm inboard of the house wall, with no connection to the wall at all. You sleep better. So does the homeowner.

Flashing the top of the ledger — the detail that decides the next 30 years

A deck ledger sits horizontally against a vertical wall. Every drop of rain that runs down the siding above has to go somewhere when it hits the ledger. Without flashing, it runs between the ledger and the sheathing, into the rim joist, behind the sill plate, and onto the top of the foundation. Six or seven years later the rim joist is a sponge of rotten pulp and the lag screws are held in by memory. The fix is a self-adhered flashing membrane — Grace Vycor, Henry Blueskin, Resisto, any rubberized asphalt or butyl-based peel-and-stick — applied directly over the top edge of the ledger and up the house wall at least 150 mm. The membrane laps UNDER the WRB (housewrap) and UNDER the siding above, so water runs down the siding, onto the membrane, past the ledger, and out onto the deck surface. A piece of bent metal Z-flashing works as a kick-out at the bottom edge where the membrane transitions to open air. This detail takes twenty minutes. Skipping it costs the homeowner a deck in a decade.

The ledger-to-engineered-I-joist trap

Any Ontario house built after roughly 1995 probably has engineered I-joists — TJI, BCI, LPI, Nordic Lam I. You can see them from the basement: a dimensioned lumber flange top and bottom, a thin OSB web in between, a blue or red mill stamp on the web. The I-joist rim looks from outside like a 2× solid rim but it is not. It is the same OSB web, about 10 mm thick, with narrow flanges. Drive a 1/2 inch lag screw into it and the web tears through as the screw seats. Load that joint and the lag pulls clean out, ledger and all. The DCA-6 prescriptive schedule DOES NOT apply to engineered I-joist rims. Every manufacturer publishes a specific ledger-attachment bulletin — Weyerhaeuser has TJI-TL-6, Nordic has a similar document — and the detail is almost always a through-bolt with a solid lumber backer block sandwiched inside the rim. Some manufacturers sell a proprietary ledger bracket. The key rule is this: if the house has I-joists and you cannot produce the manufacturer detail, the deck goes freestanding. No exceptions, no site improvisation.

Bolt pattern — staggered rows, edge distances, end distances

Bolts sit in two rows — a top row about 50 mm (2 inches) down from the top edge and a bottom row 50 mm up from the bottom. The rows are staggered, so no two bolts line up vertically. Staggering prevents the ledger or rim from splitting along a line of fasteners — wood splits in a straight line much easier than in a zig-zag. Each bolt must be at least 2 inches (51 mm) from the end of the ledger, and the first bolt from each end must be no more than 6 inches (152 mm) inboard so the ends of the ledger cannot cantilever off unsupported lumber. The spacing this tool returns is the centre-to-centre distance along the ledger; alternate bolts between the top and bottom rows at that pitch. Pre-drill every lag screw — dry SPF splits under a driven lag, and a split ledger holds nothing. Finish by running a wrench over every bolt to full seat, then a second time after the first rainy week. Then flash the top and sleep well.