How to Flash a Deck

Trace the outline of the ledger board onto the siding of the house, with the help of an assistant. The ledger board is a board installed on the side of the house to which the deck is attached.

Cut away the siding along the tracing of the ledger board. Use tin snips for aluminum siding, and a zip tool for vinyl siding.

Place J-channel or undersill trim along the bottom edge of the hole you cut in the siding, and nail it into place with 11-gauge aluminum nails. The J-channel and trim prevent water from entering the joint along the bottom of the ledger board.

Insert the top edge of a piece of Z-flashing underneath the shingles along the top edge of the hole in the siding, and nail it to the side of the house with 16d nails. The top edge of the Z-flashing should extend at least two to three inches underneath the siding. If you need to install multiple pieces of Z-flashing to cover the length of the ledger board, overlap the edges of the adjoining pieces by at least six inches, and seal the joints with silicone caulk, to prevent leaks from occurring.

Nail strips of J-channel along the side edges of the hole in the siding.

Nail Z-flashing along the bottom edge of the hole, using the same method you used when installing it along the top edge. When installing the wide, flat area of the Z-flashing along the bottom edge, it does not get tucked underneath the siding; instead, it is placed flush against the side of the house.

Install the ledger board, following your design plans.