Quick Plywood question

I have, what I hope to be, a basic plywood question; Are marine plywood and pressure treated plywood the same thickness? I am only having to replace a small section of floor and the boat was originally built with marine plywood. I do woodworking as a hobby and I know that different plywoods can be different thicknesses. .75" "Cabinet" plywood for example, is not exactly .75". Thanks!

Re: Quick Plywood question

I think they are the same thickness, dvan.

I don't think that PT, or "exterior" plywood is as void-free as marine and therefore not likely to be as strong.

A lot of guys here know a lot more than I about plywood. Let's see what they say.

Re: Quick Plywood question

i would not use pt ply as its not glass friendly best to use marine ply little more $ better results ... using pt will not let u glass it or epoxy it as it will not adhear to it for more then a week or two then delaminate you can opt for an (a-c)ply and its not that bad and alot less $ then marine and is almost as good hope this helps

Re: Quick Plywood question

That does help and I have thought of just going ahead and using marine plywood too so I can be sure. I may go ahead and do that since I have to go to the lumber yard for some other wood anyways.

Re: Quick Plywood question

Marine plywood for transoms and stringers. AC plywood is perfectly fine for decks and caps.

All wood should be encapsulated in glass, not just resin.

Re: Quick Plywood question

andy6374 said:

Marine plywood for transoms and stringers. AC plywood is perfectly fine for decks and caps.

All wood should be encapsulated in glass, not just resin.

Click to expand...

Ok, so let me add a follow on question. If I encapsulate the wood in resin and fiberglass, what thickness should this account for when I am cutting the shape? (Rough idea) 1/16" or 1/8", somewhere in there?

Re: Quick Plywood question

for decks id go with 3/4 im a large man 6 ft 220 id feel shakey in anything less then 3/4

Re: Quick Plywood question

dvan, 1/2 plywood is pretty typical for the original floor. I think 3/4 is overkill for a floor of a boat. Figure on 2 layers of light cloth, which is about 3/16 inch. Of course 1/4 inch is better. Also, I like to use PT plywood on a floor, because if you get some water into it (when you screw down the seats) it doesn;t rot as fast.

Re: Quick Plywood question

I agree that 3/4 is overkill. But I still like marine ply. When you screw things down, first drill with an oversize bit and then back fill with resin and cabosil. Then screw it down with a normal sized screw. That way if any water gets in, it won't be hitting any wood.

However, some argue (myself sort of included) that by doing this you are reducing the screw fastening strength because it is not biting into any solid wood...true! What you can do instead is put a nice bead of 3m 4200 or 101 around the flange of what you are securing and around the screw itself. This should prevent any water from getting to the wood. Don't ever use silcone.

Also, just because you bedded the screw in sealant, don't forget about it. The sealant will eventually shrink and then the water can get in. Every 3-5 years or so, rebed and you'll have no water intrusion worries. Rebedding thru-hulls on the same timescale is a great keep your transom drier longer (if not forever).