Type of plywood to use

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Dockside85

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Hey all!

On my lunch break today I went to Home Depot to do some cost reconnaissance for plywood for the new deck on my tin project (Yes, I shop at Home Depot and probably always will! My dad was a long time vice president for the company.). My first thought was to get a 4’x8’ sheet of the sanded pine, but then I saw the kind of gnarly looking subflooring boards made up of all the little shards of wood compressed together. I’ve never used this kind of wood before so I don’t know much about it, but it’s about a third to half the price of the nice and smooth sanded pine. Has anyone used this kind of wood in their build? I figured it would probably feel the same underneath carpet as the more expensive wood does. My only concern about it is that it seems to have some kind of thin film of varnish or glue on the surface, maybe to hold it all together? That could affect water resistant varnish being able to penetrate and cure. Has anyone used this type of wood with any success or is it recommended to use the pine?
 

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Dockside85 said:
My first thought was to get a 4’x8’ sheet of the sanded pine, but then I saw the kind of gnarly looking subflooring boards made up of all the little shards of wood compressed together.

Stick with your first choice and coat it with three solid layers of Spar Urethane. Not 100% certian, but the composite boards are not as structurally sound.
 
Don't use that OSB junk, it won't last. Don't use pressuer treated either.

Do use exterior grade pine or fir plywood. See if they have an ACX or BDX grade. The first 2 letters are the grades for the faces with A being the smoothest with very few or no knots, the X stands for exterior grade.
 
Thanks guys! Yeah I wasn't aware there was a letter grade to plywood. I'll definitely look for that next time I stop by. Thanks for the info!
 
OSB (oriented strand board) and water do not play well together at all. Get the exterior grade ply, fill any voids, and coat it with epoxy or some other water-proofing.
 
True "marine grade" plywood is made with a more water resistant glue but that fact is not the most important difference. The real cause of water related failure are the voids and joints and knot holes that are found in the lesser grades.

IMO, the most cost effective choice is "veneer core." It's skin veneers, on both sides are made of a continuous piece of a harder wood and its center layers, while softer/cheaper, are also continuous - no voids/seams/joints.

The main defense against water is the continuity of the veneers. You've seen those eye shaped patches on the "bad side" of construction plywood. The inside is worse - there are even more of those joints and seams and knot repairs hidden among the interior veneers. When you cut cheap plywood, you can see voids on the edges. Those voids and gaps and seams let the water in and then it stays in, then both glue failure and punky wood leads to delamination.

Veneer core is light too; the glue that fills/fixes the flaws in other grades, is heavy. It comes with a nice hardwood exterior veneer - show it off! Don't cover it with carpet and don't make it slippery with varnish. Just stain it. Let it breath.

We have tin boats, in part, because they are lightweight. Why load 'em up with heavy grades of plywood over studs all covered with urethane or epoxy resin coats followed with carpet?

Seal the edges to slow penetration up the end grains, stain it, and try to use a veneer core with a thick outer veneer skin.
 
I should add that the "X" of CDX for example, while it does mean "exterior," does not mean that it is made with a "marine" glue. You won't find true marine grade at any home improvement store.

Veneer grade may also be known as "cabinet grade."

In good light, look carefully at the various grades; the veneer grade is dead flat, no high spots, because it does not have gaps and patches in the midst of the interior veneers. It also tends to have less curve or warp. When you start looking carefullky, you will note how the cheaper grades are often curved or even potato chipped and how they have high and low spots and how there are gaps/holes in the edges.

OSB is too heavy, ugly, and since there are end grains on its surface and its edges, it presents too much end grain to atmospheric moisture.
 

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