Water intrusion at exterior decks is the movement of water into the wall cavity where the deck ledger board attaches to the wall.
Attaching a deck to an exterior wall disrupts the drainage plane. Any water at this disruption can't drain freely to the base of the wall. The deck ledger board is a piece of pressure-treated wood that's attached to an exterior wall to support the deck joists. As water moves down the wall, it may run behind or rest on top of the ledger board. Without proper flashing, water will pool there, potentially rotting the ledger board and deteriorating the ledger board fasteners. The water can then enter the wall cavity, leading to peeling paint, wood rot, mold growth, and even structural failure.
Using deck flashing
Flashing the deck ledger board helps ensure that water will drain away from the wall assembly and the ledger board. It's critically important to make sure the deck ledger board is flashed and that the deck flashing is properly integrated with the housewrap.
Deck flashing is installed to direct water in front of the deck ledger board instead of allowing it to run behind or rest on top. If the deck flashing above the ledger board is missing or improperly installed, water can be trapped behind the ledger board and corrode the fasteners, potentially leading to structural failure of the deck. Water intrusion can also occur if the housewrap is reverse-layered with the deck flashing; reverse layering will direct water behind the ledger board instead of away from it.
Reveal between exterior finish and decking
The reveal is the space left between the exterior finish and the decking. If there's no reveal between the two materials, water can wick up into the exterior finish, potentially damaging it.
Cause 1:
Lack of deck flashing above the deck ledger board
If the deck flashing above the ledger board is missing, water can be trapped behind the ledger board and corrode the fasteners, potentially leading to structural failure of the deck.
Solution
Install deck flashing above the ledger board to prevent water intrusion at this area. Fold the housewrap down over the vertical leg of the deck flashing to integrate the deck flashing with the drainage plane.
Cause 2:
Improper integration of housewrap with deck flashing
Water intrusion can occur if the housewrap is reverse-layered with the deck flashing. Reverse layering can actually direct water toward the home and behind the ledger board, eventually causing it to rot.
Solution
The top of the deck flashing should be layered under the housewrap, and the bottom should be layered over the ledger board in order to direct water away, rather than trapping it behind. The deck boards should layer over the deck flashing.
Cause 3:
Lack of reveal between the decking and exterior finish
Installing the decking too close to the exterior wall can trap water and prevent drying. If there's no reveal between the decking and the exterior finish, water can penetrate and damage both the wood deck and the home’s exterior finish.
Solution
Leave a reveal between the decking and the exterior finish to promote continuous drainage down the wall. The amount of reveal needed depends on the type of exterior finish installed.
