Deckhead repair

[Tim Stewart]

Over the years the plywood in the sandwich construction of my deck, which is supporting my deck stepped mast, has turned to mush. This has allowed the mast to form an indent in the deck at the base of the mast, due to years of tightening up the rigging screws a bit at a time.
I would be most grateful if anyone could suggest a repair that I may carry out. I have a idea how I am going to do the job, but I won't mention it just yet, because I don't want to influence any forth coming suggestion.
Thanks in advance.
Tim Stewart