A ledger board is a structural attachment member running horizontally which receives rafters or joists similar to the way a rim joist or ridge beam. Rafters or joists ataching to a ledger are commonly done so with a joist or rim hanger if they have no other ledge to bear on. The more common use for a ledger board is for a new suspended joist floor or deck joist attachment. The next common application is for an attachment point for adding on a roof rafter system to an existing vertical wall.  The size of the adjoining joists or rafters should be the same as the ridge board height.  The most commonly used ledger board sizes are 2×6, 2×8, 2×10, and 2×12. Treated lumber will need to be used if being exposed to weather for a period of time. If being attached to existing wood, lag bolts are commonly used for attachment. If being attached to concrete or masonry, power driven fasteners will be utilized.