Well, you could use 3/16" plywood, or 1/4" plywood. Or, if you're really stingy on weight, you could use 1/16" plywood... Does that answer your question, or are you asking about alternative materials to plywood? I've seen several other woods used, along with plastic (polycarbonate, I think), and one person even fiberglassed metal into his boat to use as a subdeck. Personally, I prefer aircraft-grade baltic birch plywood. I find it easy to work with, reliable, and it looks good, unlike scratched-up plastic or holy rusted metal.
I make make my subdecks out of spruce usually 48X3/8X1/4". I then lay a deck rim on top of the subdeck made out of 1/8X1/4" aircraft plywood. This method has worked for me for years and looks very clean when completed. Thanks,
Check out the cleveland build, I am using 1/8th for the sub-deck and top deck. It's inexpensive when you consider you can cut it with a jig saw or sissors (heavy-duty), doesn't need waterproofing (and no extra weight), and sands smooth.
