To avoid butt joints from decking boards being applied lengthways on a deck longer or wider than available sizes, consider applying the decking at a 45 degree angle.  Synthetic decking mak require joists at 12 inches rather than 16″ OC, byt two by decking does not.  For example, a 16 foot board covers a 12 foot wide deck with almost no scrap at the ends.  Any boards cut short by a corner are used in the opposit corner.  Looks nice and no butt joints.