When I re did the deck on my 23 I used 1/2” plywood to replace the lamination inside the two skins of the deck it is very solid. If I were doing your deck removing the whole thing raising up and then setting in a new one. I am sure ½ to 5/8” would be plenty strong enough. The one thing I would do is laminating both sides of the deck with one or two layers of glass for stiffness and to prevent water from getting in. I did that both for my gas tank deck and for the deck part under the CC it is unbelievably strong.
FellowShip
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My motto: Just for the Grins
