Re: custom patters on a deck
Sounds like this is a single level deck.
What I would do is frame it for the main14 x 51 run, then in the middle where you want the diamond, install blocking between the joists in the pattern of the diamond. This will support the ends of both the main decking and the diamond section. If you're going to separate the two with a border, you can still use the blocking, you'll just have to install two diamonds, one to carry the main deck ends and one to carry the diamond decking ends. The border pieces should land on the existing joists pretty well.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!