Marine Plywood for Deck

[kazenza]

Am replacing a rotten aft deck on my trawler. While the rest of the boat deck is teak over glass, the rear is teak over plywood with some kind of water barrier sandwiched between.

Looking for a recommendation on best marine plywood to use and what to use as a moisture barrier including whether to glass it over.

I should mention it includes a lazarrette hatch I have to rebuild as well

Randy

[Cheechako]

Maybe Minaret will chime in, the "best" used to be Bruynzeel marine plywood. Not sure if there are others now equally as good. I would opt to glass it over... being sure to saturate the plywood before laminating over it. If you do that, then spending the big bucks for Bruynzeel (sp?) isnt necessary. If you are considering just painting over ply.. then get the best ply you can get and epoxy it prior to painting I guess.

[zeehag]

do use penetrating epoxy on anything even marine ply that you use for this adventure so it stays waterpruf..... goood luck.

[Pauls]

There is no need to use marine ply for the deck. Use ABX, available at local lumberyards for a small fraction of the price of the marine grade plys. The glue used for both is identical. As re wood types, you will find ABX available in fir. It is also commonly available in what looks like a pine from Chile. The chilean ply is exceptionally high quality, even thickness lams, most sheets have no voids. It's nice stuff. There is no need to use marine grade ply for decks, IMHO.

The plywood will have a long life if it is fully sealed on the exterior from water ingress. Epoxy/fiberglass is the standard for this. If you lay teak on top of this by screw fastening into the ply you will have created hundreds of water ingress points and the ply will rot pretty quickly. If you have to lay down teak I would recommend you follow the Gougeon Bros approach, which is to adhesive bond a fairly thin layer of teak. You can use epoxy or resourcinol, particularly the gap filling Aeordux version.

Regards, Paul

[GordMay]

See ➥ Boat building lumber and plywood 5

North American Marine-Grade plywood is made entirely of Douglas-fir or Western Larch. The grade of all plies of veneer is B or better. B-grade veneer may have knots but no knotholes. A-grade veneer has no knots or knotholes. Both A and B grade may contain wood or synthetic patches. Panels are sanded on both faces or Medium Density Overlay (MDO) or High Density Overlay (HDO). The maximum core-gap size permitted is 1/8 inch. Its exposure durability rating is EXTERIOR and the glue used is a fully waterproof structural adhesive. It is considered a "premium" panel grade for use in situations where these characteristics are required. It is available in 4x8-foot sheets of 1/4, 3/8, 1/2, 5/8 and 3/4-inch thickness. Sheets up to 5x12-feet are also available. Available grades are A-A, A-B, B-B (face-back), MDO and HDO.
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.
The detailed description of veneer grades and Marine-grade plywood is contained in Voluntary Product Standard PS 1-95 PS1-07 Construction And Industrial Plywood.

PS1-07 ➥ http://gsi.nist.gov/global/docs/vps/PS1-07.pdf

[minaret]

Originally Posted by kazenza

Am replacing a rotten aft deck on my trawler. While the rest of the boat deck is teak over glass, the rear is teak over plywood with some kind of water barrier sandwiched between.

Looking for a recommendation on best marine plywood to use and what to use as a moisture barrier including whether to glass it over.

I should mention it includes a lazarrette hatch I have to rebuild as well

Randy

Why use ply? I would look at a synthetic replacement such as Coosa Board. It is about the same price as Marine Grade, but will never rot and is actually stronger, depending on what type you get. It's very easy to work with. The only drawback is that it will not hold a fastener, so if you are doing a teak overlay you will definitely need to use an adhesive method instead of fasteners. But no one uses fasteners anymore anyway. Look at Coosa overlayed with TDS or similar. A much smarter solution.

Coosa Composites, LLC - Manufacture of high-density, fiberglass-reinforced polyurethane foam panels

[minaret]

Originally Posted by Cheechako

Maybe Minaret will chime in, the "best" used to be Bruynzeel marine plywood. Not sure if there are others now equally as good. I would opt to glass it over... being sure to saturate the plywood before laminating over it. If you do that, then spending the big bucks for Bruynzeel (sp?) isnt necessary. If you are considering just painting over ply.. then get the best ply you can get and epoxy it prior to painting I guess.

Bruynzeel is to my knowledge still the best marine ply. The big difference between really high quality marine grade like this and the regular stuff is number of plies. More plies means more stiffness for the same weight. Regular American ACX is generally five-ply. Good marine grade Bruynzeel can be seven-ply or even nine-ply in thicker panels. It's really good stuff, but you sure do pay for it.

[Wotname]

Perhaps I am biassed but I have never found a better marine plywood than the Austral Hoop Pine marine ply built to AS/NZS 2272 - 2006. And yes I have used Bruynzeel but I believe the above standard is tighter.

However I realize it won't be available for the OP!

As to coating it, I suggest 100% solids epoxy (West etc) with fibreglass cloth.

Some interesting reading Marine Plywood - Highest Quality Marine Ply

[mrm]

I have a question related to the subject I think.

Saturating plywood with epoxy is an accepted and recommended practice with a known and good track record and, with that many years of use, could be already described as a 'traditional' approach.

The question is: were there any attempts to saturate plywood with moisture cured urethanes instead of epoxy? Looking at their properties, urethanes seem to be at least as attractive as epoxy, plus they are more resistant to UV degradation and remain elastic.

Are there any real world results from urethane use in this application?

[Cheechako]

Good question. The ones I've used are pretty thick and sticky... would be difficult to spread around... but maybe there is a thinner viscosity version? I would be reluctant to use Epoxy only over ply with no glass buildup etc....as you mentioned unpigmented epoxy can fail significantly from UV and then it's a real mess...

[Nemo55]

This would be the choice for me,,why use wood when synthetic is superior in some cases?

Originally Posted by minaret

Why use ply? I would look at a synthetic replacement such as Coosa Board. It is about the same price as Marine Grade, but will never rot and is actually stronger, depending on what type you get. It's very easy to work with. The only drawback is that it will not hold a fastener, so if you are doing a teak overlay you will definitely need to use an adhesive method instead of fasteners. But no one uses fasteners anymore anyway. Look at Coosa overlayed with TDS or similar. A much smarter solution.

Coosa Composites, LLC - Manufacture of high-density, fiberglass-reinforced polyurethane foam panels

[Cheechako]

Does fiberglass layup stick to Coosa well?

[minaret]

Originally Posted by Cheechako

Does fiberglass layup stick to Coosa well?

Yes, that is one of it's strong suits. It sticks amazingly well with zero prep. It's a high density foam with fiberglass laminate in it, so the surface is kinda porous. Perfect for laminating to and takes coatings very well. I have used it for over a decade now in all sorts of marine applications, it's great stuff. Lighter than ply but stronger and guaranteed to never rot. Comes in several densities and amounts of laminate for different applications. Much cheaper than regular cored composite panels, but obviously not as light as a high end composite panel. For a deck ply replacement I would use Coosa and laminate a couple of plies over the top to provide durability and a proper seal all around, making sure to tie in to existing laminate everywhere. About a thousand times better than a ply deck for the same money. I would probably use the Bluewater 26 for a deck, but the 20 would be fine.

[mrm]

Originally Posted by Cheechako

... but maybe there is a thinner viscosity version?...

Yes, there are moisture cured polyurethane wood varnishes which are very low viscosity and penetrate deep. Mostly formulated with Toluene diisocyanate (TDI) and Methylene diphenyl diisocyanate (MDI) (afaik).

This stuff has been around for quite some time, however, is there any track record in plywood saturating for marine uses?

[kazenza]

Thanks everyone for the great feedback. One more question is I've seen several references to the Gougeon Brothers method for adhering a teak deck w/o fasteners, but can't find a summary of it anywhere. Book seems to be out of print, and don't know that I want to buy another boat book (have quite the library now). Anyone with information on this, I would love to know more of the details.