Syndicated — May is National Deck Safety Month and the most common cause of deck collapses is improper attachment to a building.
Most decks are supported on one side by the building, and on the opposite side by the earth. The photo below shows a deck collapse that happened in Minnesota. This is exactly how most decks collapse. The cause of collapse is quite obvious. It wasn't attached properly.
It's not always possible to know for sure, but let's go over a few different ways of attaching a deck to a building. It all centers on something called "the ledger." The piece of wood that connects a deck to a building is called the ledger, or ledger board. So get ready to hear this word used a lot!
Traditionally, lag screws have been the most common method of attaching decks to buildings. To properly attach a deck ledger using 1/2" lag screws, 5/16" holes need to be pre-drilled through the ledger and rim joist. After that, a 1/2" hole should be drilled through the ledger only.
For specific spacing and installation instructions, you can turn to page 12 of the Prescriptive Residential Wood Deck Construction Guide. There's no way of knowing if all of these steps were followed just by looking at a deck, but if lag screws are visible, you can feel a little better about the deck attachment to the building.
One problem that I occasionally find with lag screws is that they're not attached to anything substantial behind the ledger. When a home is constructed with floor trusses and there is no rim joist for the deck to attach to, it's important to figure out what the screws are going in to. In the photos below, the lag screws at this townhouse were only attached to the fiberboard wall sheathing, which is basically worthless. You wouldn't want to put too many people on that deck.