I'm assuming that you are currently in the process of designing a new home and are working through the checklist to determine if adding this deck using the FSC certified raw materials will provide points toward a LEED for Homes certification.
The LEED-H reference guide states that as long as 90% of the materials used to build the deck are FSC certified you are eligible to receive the 1/2 point under MR 2.2 Environmentally Preferred Products. As a point of note, if the process of treating the wood removes the FSC label you should ask for the paperwork stating that the materials are FSC certified for documentation purposes.
Also, if the wood was locally sourced and the product supplier has the chain of custody paperwork you can receive another 1/2 point. To receive this credit the product would have to be extracted, processed or manufactured less than 500 miles from your construction site.
Lastly, I thought it would be prudent to mention based on the format of the question that LEED certification for an existing home is a detailed process. I recommend downloading the rating system document here.
In addition, I highly recommend purchasing the reference guide. This book was my point of reference to answer this question.
Best of luck on your project.