A ledger, sometimes called a header, is the piece of lumber that is attached to the wall of the home that allows your deck to be attached to and stabilized by the house.
Ledgers usually have joist hangers on them, so that each joist can be attached to the house securely. Joist hangers should be attached to the ledger with hardware that penetrates the ledger, but does not go through the house wall.
When a ledger is set to be parallel to the joists, the ledger is generally used to support the ends of the deck boards. This often happens when a deck is installed next to a porch, so a ledger board is attached to the porch to support the ends of the deck boards but not any of the joists.
Improperly installed ledgers are the number one cause of decks collapsing. Ledgers should not be attached with nails, always lag screws or lag bolts. Lag screws should be screwed into the house wall plate, the side of a truss, or the ends of trusses. Even a ledger installed with lag screws can fail if the lags missed the trusses, and only went through the house sheathing,
Improperly water-shedding around ledger boards can cause ledger failure, because of the ledger rotting and the joist hangers pulling away more easily. Ledger flashing or drip edge, whether galvanized metal or vinyl, is a key component in making sure you don't have water sitting on or getting behind a ledger. The flashing allows any water running down the house wall to run over the top of the ledger and drip off the front side of it. Allowing humidity to sit on top of the ledger or run behind the ledger tends to rot the ledger and even the house wall from the backside. Flashing tape is also used with flashing to make sure the seam between the sheathing, home wrap, and flashing is seamless. Flashing is sometimes run around the side of a ledger so that any water running down the side of the ledger doesn't creep along that side and get behind the siding. Caulking should also be used for nooks and crannies that tape cannot seal.
On homes where there is a house wall on both ends of the deck, (like in the photo directly above) ledgers can be configured a little differently.
Provided that the walls on both sides are fully supported and have footings below the walls, a ledger can be attached to each wall, and joists can be run between these ledgers. The joists are run parallel with the back house wall, rather than perpendicular to it, and therefore the decking direction can also be switched.
