The Importance of flashing for 
Decks in Maryland

I  cannot stress the importance  of properly installed flashing for any  deck or screen porch project.   The severe weather of the Washington  Metropolitan area will destroy any  unprotected wood.  If your deck is  not flashed properly you are putting  yourself and your family in harm’s  way.  Last year I did over 200 deck  estimates.  The majority of these  estimates included either the  demolition and removal of an existing  deck or an addition to an existing  deck.  More than 50% of the decks I  visited were not flashed and open  to rain and snow damage.  It’s a  crime that  a contractor would not take  thirty minutes and install  flashing properly.  The material does not  cost very much and the  installation is quite simple.
However, left  undone you leave the deck  vulnerable to many problems such as mold  invasion and deck collapse.   There are  two materials we recommend using to properly flash a deck,  copper and  vinyl.  We tend to use vinyl because of the cost factors.   The flashing  material is going to come in contact with pressure treated  wood, so the  flashing needs to be resistant to the corrosion that ACQ  materials may  cause.  If you stick to vinyl and copper, you will be  OK.
There are  three steps to installing flashing correctly.  First,  the siding must be  cut away in the area where the ledger board is to be  installed.   

Second,  the flashing must go behind the siding.  You want the flashing  to be  approx. 6” above(behind the siding & ledger) your ledger  board.   Once the first piece of flashing is installed you can attached  the  pressure treated ledger board sandwiching the flashing to the  house.   The final step is to install a piece of ‘L’ flashing that sits  on the  ledger board and goes behind the siding.  This piece keeps water  from  entering the house and flowing in front of the ledger.  The first  piece  is for added protection.
Design Builders Recommendation:   Look at your deck.  If your ledger board is ‘sandwiched’ to the siding   and the siding has not been cut away, you are set up for disaster.    Water IS going inside your house.  You may have mold growing that you    do not know about and the structural members of your outside wall are  facing water damage.  Call Design Builders Inc. for a free consultation.
http://www.epa.gov/oppad001/reregistration/cca/acq.htm