pressure treated plywood

[kulas44]

Ok, someone else has admitted to useing this stuff. I have used it in places that it couldn't be seen but never told anyone. It's ugly, meant to be hidden, and the dust from sawing it is horrible, it will give you pneumonia if you don't wear a good mask. I like it, I'm cheap and so is it. My ? is, what do you guys think of useing it for the decking and topsides, glass covered with all the voids filled. What I have available is 3/4 cd. Good marine ply has to be shipped in. I can get underlayment grade treated ply but it's much more expensive and i would have to drive into okla. city, not fun ! Thanks

[Dale R. Hamilton]

I'd use PT plywood in a heartbeat- for a core material. That is, plywoods big value is its stiffness, and its ability to serve as a core to some composite. I use regular old CD plywood, and I cover it in epoxy and glass laminate. My only concern with PT ply would be if the PT part interferes with the epoxy bonding. I'd call the experts, System Three, or the Geugeons. I'm sure they have thought of this.
I recall some forumite saying that he had an old piece of 3/4 PT plywood in the bed of his old pick up. Had been there for many years, exposed to all the elements of Michigan- or whereever- and it was just fine- no rot, warpage - really a hell of a test.

[John Bell]

The seat bottoms on my pontoon boat are all 3/4" PT plywood. These things get wet and stay wet a lot longer than anyone would consider ideal. Even after 7 years, there is no problem with rot.

[kulas44]

that mirrors my experience of it, tuff stuff. Right out of the pile it is very wet, and heavey. I leave it in the sun a few days and it dries out. If i need to bend it around a curve I use it wet, I get a peice from deep in a new stack, it's wet all the way through. Glad to hear I can come out of the closet on this subject.

[Buddy]

Truth and consequences. Yes Virginia, once it is dry, it takes epoxy without any problem. There is some below the cabin sole in several boats I have helped repair, now in service pushing as long as 18 years, still no problems. But pretty it ain't.
One interesting deck job I saw, the builder let it dry, covered the bottom side with contact cement and off white formica, screwed it down over the varnished big box 2x fir deck beams and sheer clamps and proceeded to cover the deck side with fiberglass and epoxy, then nonskid rolled on paint. Looked very yachty, no home made bugaboo about it. This was on a fiberglass hull from a stalled out "kit" boat project purchased cheap and then remade into what he really wanted.

[Keith Wilson]

The only pressure treated plywood I've ever seen was utter crap - DDX, or something similar. Perhaps we could convince one of the manufacturers of good marine plywood to make pressure-treated stuff? Maybe there is a species of wood that absorbs preservative well, which they can't use now because it's not durable enough, and is enough cheaper that the cost of the preservative wouldn't be prohibitive. There are also less toxic preservative than CCA available now. It would be nice to get good plywood that really woudn't ever rot. Just a thought.

[Buddy]

I'm talking about 1/2" stuff, five ply of Southern Yellow Pine. Yes, some voids, lots of inserts on the "bad" side, but not utter crap. A utility sheet I would say, never meant to take just paint.
But I know one club racing boat, a 30 ft Pearson Flyer ,where the owner, tired of failing to eliminate deck leaks on a largely bilgeless boat, removed the rotted , ( needing yet again) varnished marine plywood bow berth tops and floor panels and made new ones of P.T 1/2" ply and did a neat, epoxy free job with rolled on light grey latex deck enamel and nonskid additive ( on the floorboards). Really looked nice and bright, and seems to be holding up fine. Goes to show you.
This man wanted a shipshape boat ( he paid real money for a air lift floating dock to keep the boat clean) but wants to spend his time sailing. His choice was not about spending the money, it was spending the maintenance time.

[kulas44]

agreed, very ugly, must hide. both sides are bad, they must use the worst of the bad wood for this stuff. lots of voids, but, if it won't rot the voids won't matter-right? How about pressure treated yellow pine for beams and stringers, laminated?

[Buddy]

P. T. SYP lumber is used all the time for below the cabin sole stuff like cleats, engine beds, mounts for tanks, cockpit dsole stiffeners, deck hatch surrounds. Sure us it for anywhere under the deck you'd use lumber but keep in mind the designs might be sized for something with the strength of spruce or ash. Badly knotted SYP might need to be upsized to insure the same stiffness and strength. How much overbuilt is you design already type of question. The voids won't cause rot, but they might end up being under the base of of gear mounted through the deck. They might show up under the morning dew where you don't get as much condensation on your paint. Any of these voids detected can be filled thru a bitty drilled hole with a syringe full of epoxy/ filler. But no way can you expect 3/4 five ply SYP to match 3/4" nine ply marine fir or mahogany for structural strength. But alot of times you design a deck to be thick enough to be stiff enough. Substantially thinner has already been calculated to be strong enough. You can reduce the spacing of your deck beams, or increase there depth to add more stiffness when using a more limber panel. 10 or 15% can make a lot of difference in how it feels under foot. Thats usually the limiting design factor on decks anyway. Its not that anything is going to pull apart or anybody step through, but a springy deck just feels "wrong". Years ago they added a aluminum deck sprung "free standing" over much of the pool area on the QE2 to provide shade below as well as sun above. Looks great being covered with a laid teak deck, and its used as a strolling, prominade area above the pool or for special events. Been there 15 years easy, so it's held up to the sea, but you can feel that baby stiffen up at a frame and then begin to deflect and bounce for 6 or 8 feet. No other deck on the ship flexes a bit, but I suppose they were concerned about adding additional weight up that high.Upsets the hell out of old ladies who just don't feel safe there and think something's wrong. Which in a very real way, it is.

[kulas44]

thank you all, and especially Buddy. Excellent replies and a wealth of information. Iv'e wasted a lot of time waiting for materials that ended up costing way more money when I could be useing whats at hand, and my little project would be much further along. Thanks again.