Here are some excerpts from recent Houseboat forum exchanges regarding plywood deck thickness:
dbric36: What is the plywood thickness that Gibson used on a 1982 houseboat deck from the factory? I need to repair a section of deck.
DitchRider: So far all of the marine plywood I've seen on our Gibson is 3/4 inches.
Dan: I’m assuming you have soft spots in your deck anyway, so I suggest you drill a hole and take a measurement. They may have done things differently in 1982. I would not be surprised if you find it to be less than 3/4 inches. If this is the case, you will just need to purchase the next size below what you have and pad your joists a bit. Also, it's not necessary to use marine plywood for the repair as they didn't use it originally. Just non-treated exterior glue plywood will do the job just fine.
dbric36: Yes, I will be repairing a section soon. I’ll be using exterior ply and epoxy, and I was searching for information but couldn’t find anything about the material thickness on the deck.
Frantically Relaxing: I repaired a chunk of my roof this summer, and you’re likely going to run into the same problem I did. It turns out that plywood these days is thinner than in the “old days.” So if you had 5/8-inch plywood before (which you probably do), today’s 5/8 is 19/32, and 3/4 is 23/32. So 3/4-inch is still too thick, and 5/8-inch isn’t thick enough. I ended up shimming my patch to bring it level. And I also found out that plywood swells up over the years. I could've ALMOST gotten away with using 3/4-inch, but not quite.
dbric36: Well, I think I will just go with 3/4-inch. I will probably just redo the entire deck over the next year or so anyway.
Ike: That’s because most plywood today is manufactured to metric sizes. It may be advertised as 1/2-inch but it's actually 12 millimeters.
Plywood today comes mostly in the following sizes: 4, 6, 8, 12 and 18 millimeters or larger.
A good wiki page on plywood is http://en.wikipedia.org/wiki/Plywood.
Another of the best sources with information on plywood is the Engineered Wood Products Association. Their website is www.apawood.org.
dbric36: Thanks. I will let you know how I make out. It should still be a couple of weeks before I get started.
Have you faced issues with plywood deck thickness before? Hop on the forum and share your experiences.