Flashing is an essential deck building material and comes in a variety forms.  Flashing creates a barrier to prevent moisture from entering the house in the openings you create in the houses skin by attaching your ledger board connection. 

You can buy flashing either in roll form or in rigid Z or L shaped sheets usually 10’ long made of stainless steel, copper or vinyl. Never use galvanized stele or aluminum flashing in contact with an acq pressure treated wood ledger board because of corrosion.  If you are installing copper flashing from a roll make sure you use copper nails to secure it into place to prevent corrosion.  You can buy rolls of sheet copper from wholesalers that sell roofing materials.  Copper works well because it is compatible with the chemicals used in commonly sold acq treated lumber and is easy to bend. It can be soldered at corners and joints.  Because flashing comes in different widths make sure your flashing will be the right size to tuck up behind the house siding at least a couple of inches. 

New flashing products and methods are being introduced all the time.  Some self-adhering products that are applied like duck tape to the ledger board and house wall are also available.  Ask your local lumberyard for their suggestions and always follow the manufacturer’s installation instructions.