We replaced redwood with Trex Transcend over 5 years ago. Not the slightest hint of sagging on 16" spacing. Our decks aren't on a diagonal though but I doubt that will be a problem. If it's an attached deck, you may want to beef up the connection to the house as well as the supports. This stuff is probably 5 times heavier than wood. Never having to stain again will make it worth the effort. Bonus that it's fire resistant too (Class B rating).