How to Nail Deck Flashing

Sheet metal flashing prevents water from seeping into the seam between a deck ledger board and a building. The flashing is nailed in place after the ledger and deck joists are installed, and before installing the decking. Proper installation requires removing the siding or stucco along a section of the wall above the top of the ledger board. For this reason, many builders begin a deck by removing a section wide enough for the ledger and the upper fin of the flashing.


Measure the length of the ledger board and add 8 inches. This allows the flashing to overhang 4 inches at each end of the ledger. If the ledger is longer than 10 feet, it’s necessary to use an additional piece of flashing and overlap the joint where the pieces meet 4 inches.


Cut a piece of 3-by-4-inch galvanized Z flashing to length, or use a full piece if the ledger is longer than 10 feet. Position the piece of flashing at the intersection of the ledger and wall. The straight 4-inch fin is against the wall and one end of the piece extends 4 inches beyond the end of the ledger.


Mark the locations at each side of the joists on the lower fin of the flashing with a marking crayon. Make a 1/2-inch-long cut at the edge of the flashing at each mark with sheet metal cutters. This allows the lower fin of the flashing to fit tightly on the top of the ledger.


Install a tube of construction adhesive in a caulking gun. Apply a generous amount of adhesive along the back of both fins of the flashing. Reposition the flashing at the deck-to-wall intersection and align the cuts at the lower edge with the sides of the corresponding joists. Press the flashing firmly into place.


Attach the upper fin of the flashing to the sheer wall or exposed wood framing at the building with 1 1/2-inch galvanized roofing nails and a hammer. For sheer wall, drive pairs of nails 6 inches apart, from end to end of the flashing. For exposed wall framing, drive a pair of nails at each vertical framing member.