How To Look At A House
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What is the most common cause of a wood deck collapse?
Tuesday, May 30, 2023
Failure of the ledger connection, where the deck meets the house, is the most common cause of failure of a wood deck. A ledger is the board secured to the wall of the house that the deck joists sit on top of or are connected to the side of. It’s absolutely necessary to use thru-bolts or structural lag screws to secure the ledger board to the wall of the house—and never nails. Nails can slowly work loose over time, causing the ledger to pull away from the wall and collapse the deck. Look for the zig-zag pattern of bolts or lag screws, both of which have large heads, across the length of the ledger board, like in the drawing below. A good builder uses spacers to create a small gap between the ledger and the wall to allow rain flowing down the wall to pass behind the board, thereby avoiding what carpenters call a “water trap”—where water seeps into the crack created when a ledger is pressed against the wall and then cannot drain out. If there is no spacer and water is trapped between the wall and ledger board, then wood rot and accelerated corrosion of the fasteners will eventually cause the ledger to fail.
The other big safety priority in deck building is providing a continuous and securely connected load path. This means constructing a series of sturdy connections from the top of the deck structure to the bottom that transfers the load of the people and furniture on the deck, along with the weight of the deck itself, through each part of its frame safely to the ground or an adjacent structure (typically a wall of the the home). Because we have hurricanes to consider in Florida when designing a deck, the connections must also be able to resist uplift forces from strong winds roaring under the deck.
So, when you look at a deck from a safety standpoint, focus on the structural connections first. They should be clearly visible. The only exception is a contractor-built deck that used concealed fasteners at some locations–which should be verified with the contractor. Nails alone are not acceptable structural connectors; especially nails set at an angle to tie together two structural components, such as a joist to a beam. This is called “toe nailing” in the construction industry and is a sign of inferior construction. Metal connector plates, straps, and bolts, like the ones shown in the drawing below, are what you should find. Every hole in the connectors should have a nail in it. Also, the connections should be snug. Any lumber that is pulling away from its connection point needs immediate repair.
Diagrams and photo - Simpson Strong-Tie
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