Pressure Treated or Not??

Get-it-Wet · Nov 23, 2010

I just ran across an article that told me not to use PT Plywood. Another article told me that it was the best to use. What is the better plywood to use: pressure treated or non-pressure treated?

5150abf · Nov 23, 2010

Re: Pressure Treated or Not??

Welcome to iboats.

It depends what you are using the wood for, if you want to glass it I don't think you want the pressure treated, I'm not a glass guy but I remember reading that.

If it is for an aluminum boat it will react with the aluminum but only needs to be seperated with a layer of foam.

So, what are you wanting to do with this wood?

Bob_VT · Nov 23, 2010

Re: Pressure Treated or Not??

Welcome to iboats.

PT is really not good due to the moisture content and chemicals. If it comes in contact with 90% of fastener's available they will deteriorate .................

Exterior plywood is the same glue as marine plywood however the exterior may have a few voids.

Most of our people doing restorations use exterior and seal it up and never use PT wood at all.

metriccrescentwrench · Nov 23, 2010

Re: Pressure Treated or Not??

I can't think of many applications where PT would be the best choice. For a deck or stringers on most boats I'd chose exterior grade plywood (with fiberglass over it of course), for a transom only marine grade plywood because it doesn't have any of the inevetable voids in regular exterior grade which would weaken it.

Get-it-Wet · Nov 23, 2010

Re: Pressure Treated or Not??

Thank you for the welcome. I am starting a rebuild project for an 18' shadow bass boat and everything is rotten, the only thing i am keeping is the hull! thanks for the advice, coming from the construction side made me think it was a no brainer till i came across a thread that suggested using exterior grade ply. i need it for the transom, stringers, and the deck. all of which i am planning on glassing in so i will not have this to do again while i own the boat!

while i am here, what about the foam that they use for the boat. it looks like they injected foam in pockets behind the transom, below the deck, and in two boxes in the back of the boat on either side. what is the purpose for the foam, other than support, and what is the best way to remove and replace it?

DaNinja · Nov 23, 2010

Re: Pressure Treated or Not??

You'll probably get your info here, but you should check out the "Resto Forum" for some great projects similiar to yours.

Welcome!

Get-it-Wet · Nov 23, 2010

Re: Pressure Treated or Not??

Thanks for the heads up. i will go to the correct forum to ask.

oops! · Nov 24, 2010

Re: Pressure Treated or Not??

hi....welcome to i boats.

lots of plywood threads in the restoration section i wont go into it here because it will start a riot !

.

but the tread in my sig can and will help you

cheers
oops

Chris1956 · Nov 24, 2010

Re: Pressure Treated or Not??

Unlike some, I recommend dry PT plywood for decks and dry dimentional PT lumber for stringers. You will likely need to dry the lumber yourself, since it is sold very moist.

I believe I have a sound reason for my recommendations. Solid PT lumber is much more rot resistant than exterior plywood, and plenty strong enough for stringers. Also if the new plywood stringers ever get wet, they will quickly rot and fall apart, and the reason we are replacing them is that the original stringers did get wet. What makes you think you can waterproof the replacements better than the original builder, who was working with new dry fiberglass and did the original monolythic lay up?

I also would rather have a PT plywood transom for the same reason. Although new PT plywood is a bit weaker than new Marine plywood, non-rotten PT plywood is much stronger than any rotten Marine plywood. If any water penetrates the marine plywood, it will start to rot. This always happens when the transom bolts are installed, despite your use of silicone, some water will always get in. PT plywood will not rot for a long time, even if it gets wet.....

Get-it-Wet · Nov 24, 2010

Re: Pressure Treated or Not??

This does bring up a very good point. I bought the PT ply ahead of time because i knew it would need drying time, I just do not know about the adhesion to the resin. I certainly do not want to take a chance, but it does make a lot of sense that there would be more protection with PT ply than with exterior or marine grade. Has anyone used PT and have you ever had problems with it and the resin?

dan t. · Nov 24, 2010

Re: Pressure Treated or Not??

I had a 81 21.5 ft Campion that I completely gutted and replaced stringers and deck. Did it all with PT ply and 2x12 for the stringers. Used fibertec resin ,seamed to work and stick just fine,ran the boat for 5 years, sold it 6years ago, as far as I know it is still solid .BTW I used all stainless screws and I dried the wood for 3 weeks in a heated shop before it was installed

Isaacm1986 · Nov 24, 2010

Re: Pressure Treated or Not??

Chris1956 said:
Unlike some, I recommend dry PT plywood for decks and dry dimentional PT lumber for stringers. You will likely need to dry the lumber yourself, since it is sold very moist.

I believe I have a sound reason for my recommendations. Solid PT lumber is much more rot resistant than exterior plywood, and plenty strong enough for stringers. Also if the new plywood stringers ever get wet, they will quickly rot and fall apart, and the reason we are replacing them is that the original stringers did get wet. What makes you think you can waterproof the replacements better than the original builder, who was working with new dry fiberglass and did the original monolythic lay up?

I also would rather have a PT plywood transom for the same reason. Although new PT plywood is a bit weaker than new Marine plywood, non-rotten PT plywood is much stronger than any rotten Marine plywood. If any water penetrates the marine plywood, it will start to rot. This always happens when the transom bolts are installed, despite your use of silicone, some water will always get in. PT plywood will not rot for a long time, even if it gets wet.....

I agree...

Get-it-Wet · Nov 27, 2010

Re: Pressure Treated or Not??

Thanks for the input. I think that I am going to go with the PT ply, bought it already, started this post, now it has been drying for about 2 weeks. I do not plan on the stringers or the transom getting wet while I own the boat.......

I have enjoy demo so far, just don't think I want to do it again to the same boat. Can anyone think of any special way that the wood needs to be prepared? or can I just dry it and then sand it before resin?

Bob_VT · Nov 27, 2010

Re: Pressure Treated or Not??

Since we now know you are working on a fiberglass boat....... PT will be somewhat safe..... needs to be really dried out a bit before resins will properly (and permanently) adhere to it.

Chris1956 · Nov 27, 2010

Re: Pressure Treated or Not??

Just dry it out and cut to shape. No sanding is necessary. If the surface is a bit rough, the resin will fill in the roughness.

BTW - the P/T sawdust is nasty stuff. Protect your lungs.

Lou C · Nov 27, 2010

Re: Pressure Treated or Not??

I used ext ply coated in CPES and then glassed for much of my deck replacement but for some things like the wood in the big sunpad and backing pieces for the gas lifts I did use PT wood. I have two dinghys on the beach, both have a piece of plywood to mount wheels to. As an experiment, one is regular unprotected ext ply and the other PT. The PT one looks like new, the regular one will need to be replaced soon...it does hold up better to the constant moisture (these dinghys are left on the beach all year)....

NSBCraig · Nov 28, 2010

Re: Pressure Treated or Not??

Don't forget to factor in that pressure treating ply de-laminates the ply's, a solid piece of cdx is still batter than a piece of pt.

Mark42 · Nov 28, 2010

Re: Pressure Treated or Not??

There is a lot of mis-information repeatedly posted on these boards. The best thing to do is to get the facts. The best place is from the people who rate and test plywood: The Engineered Wood Association (aka APA)

There are a number of good documents produced by the APA that should be mandatory reading for anyone working with plywood on their boat:

For BEST performance of plywood in boat manufacturing, APA recommends the use of treated plywood according to the following specification.
Treated in accordance with AWPA Standard C9 with < CCA, ACQ, ACZA or ACA> to 0.40 pcf retention. Kiln dry after treating (KDAT) to 18% or less.

That is the common pressure treated ply found at the local lumber yard. http://www.glen-l.com/wood-plywood/boatbuilding-plywood.html

The influence of the treatment and moisture content on fiberglass bondability was assessed by two measured results:

Whether failure occurred in the plywood or at the fiberglass/plywood interface

The load at which failure occurred

All tests on the untreated and treated specimens at the 65% relative humidity failed predominately in the plywood, indicating a good fiberglass bond. This indicates that the treatment did not have an adverse effect on bonding at this moisture level. Furthermore, the strength of the treated and untreated plywood were similar.
The effect of the 90% relative humidity led to some failures at the plywood/fiberglass interface indicating poor bond. This occurred for some untreated and treated plywood types.

http://www.glen-l.com/wood-plywood/fiberglass-plywood.html

Marine-grade plywood is not treated with any chemicals to enhance its resistance to decay. If decay is a concern, it should be pressure-preservative treated to an appropriate standard.

http://www.glen-l.com/wood-plywood/marine-plywood.html

http://www.gp.com/BUILD/DocumentViewer.aspx?repository=BP&elementid=3839

Regardless of all the nay-sayers, you will not find any posts on the restoration forum from people who used pressure treated plywood and had problems as a result of it. If you check the home supply stores, there majority of outdoor fasteners now say they are safe for use with pressure treated ply. I have to admit that over the last 20 years or so, I have never had any fastener fail that was used with pressure treated ply, either in a boat or on a home deck.

Isaacm1986 · Nov 28, 2010

Re: Pressure Treated or Not??

Bob_VT said:
Since we now know you are working on a fiberglass boat....... PT will be somewhat safe..... needs to be really dried out a bit before resins will properly (and permanently) adhere to it.

Just my oppinion, and i know i have said this before.

Why use resin? Pt plywood Will outlast your interest in the boat even if it got wet. If you are using pt plywood, then the resin is a waist of time and money.

I know there are a lot of people who dissagree with me on this point. But to me it just does not make sense to use resin on top of pt plywood. A pt transom will last a good 10+ years, if not a LOT longer. And at that point if i still had the boat, i would probably want to redo it anyway.

Just my oppinion.

NSBCraig · Nov 29, 2010

Re: Pressure Treated or Not??

Here we go again!

No it's not the same as lumber yard pt that they are recommending, Mark your making that part up. They are recommending you use PT marine lumber not the crap they sell at Home Depot.

Love the way you don't list the link I sent you explaining that you should never use lumber store PT on your boat (jamestown help files).

Wow I never seen someone so hell bent on spreading misinformation to try and make himself feel better about the things he's done, but yet you keep at it.

To the OP use what you want it's your boat, but no there are zero facts supporting that you should use lumber store PT in your boat and there are real one's saying don't.

(my last post on this thread cause we all know how Mark's fixing to get.)

Pressure Treated or Not??

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