Roger, something that I like to do is not bolt the boards in! I just use some flat stock at both ends and capture the boards, let'em float. They'll stay cleaner and the bolt holes won't promote rot! Plus they can expand / contract as they need to! And if a board does get damaged, simply cut the weld on one end and remove the bad board! I've been doing it this way for 20 - 25 years and never an issue!