Lickel Architecture - Blog

...WOOD...

Do you know your wood?

Do you know the difference between Marine Grade Plywood and Exterior Grade Plywood and Pressure Treated Plywood.  My guess is that you’ve seen all of these called out in your drawings, but do you know the difference…or where one should or should not be called-out.  Below is a description which should answer those questions?

FIRST:  What is Plywood?

Plywood is made from thin sheets of wood veneer glued together with adjacent plies having their grain at right angles to each other for greater strength. There is usually an odd number of plies so that the sheet is balanced—this reduces warping. Because of the way plywood is bonded (with grains running against one another and with an odd number of composite parts) it is very hard to bend it perpendicular to the grain direction.

SECOND:  What does AB or CDX Plywood mean…is that a Grade of some kind?

There are Five Main Grades of plywood that run from A, which is the highest, to D. The reason there are five grades in total is because the C grade has two grades: C grade and C plugged grade.  The grades specify how many knots and patches and other imperfections the plywood can have. Most plywood is described by more than one grade.

Basic Rule: First Grade - First Side... Second Grade= Opposite Side

The first grade describes the face panel while the second describes the back panel (i.e. B-C grade means the front is B grade and the back is C grade).

Plywood Grades:

·        Grade A is smooth. It can be painted and has a limited number of patches.

·        Grade B grade is solid, and has a few more knots and may even have very minor splits.

·        Grade C allows for limited splits and discoloration or sanding effects, provided the plywood strength is not affected. C plugged grade plywood is an improved version of C grade. The difference is that the splits and cracks are not as big as those found on C grade.

·        Grade D plywood features knotholes of up to three inches across the grain and even some cracks. This grade of plywood should never be used outdoors, as when it gets wet it will swell and break down.

Some typical grades look like this:

·        AA is great stuff. It is smooth on both sides with no knot holes. It's used to build nice furniture type objects. Book shelves that are open, thus allowing both sides of the plywood to be seen.

·        AC is more common. One side is the good stuff and the back is ok. We used AC for flooring that is going to been seen by audience members. That is, seen up close and that is not going to be covered with carpet, etc.

·        CDX is ugly stuff. But it's cheap and works. The surface has big unfilled knot holes on the back (D side). Keep in mind that since this is just the surface ply, it doesn't show through as a knot hole in a piece of wood would. This is perfect plywood to use on platform tops when the platform is also getting covered with something like Homosote.

·        Then there’s OSB. It's ugly and heavy. But it's cheap and strong. As strong as AC ply and as cheap as CDX. It will dull your saw blades faster because of the resin used to glue the Oriented Strands to form the Board (hence OSB) together.

QUESTION:  Where does the letter “X” come from in grades?

The letter "X" in CDX indicates for what purpose this plywood should be used. Some mistakenly think the "X" stands for "exterior" - which is not true, exactly. The letter stands for "exposure" - which means the plywood is tough enough to withstand a little moisture - but for only a short time. The letter "X" in CDX indicates for what purpose this plywood should be used. Some mistakenly think the "X" stands for "exterior" - which is not true, exactly. The letter stands for "exposure" - which means the plywood is tough enough to withstand a little moisture - but for only a short time.

 

THIRD:  Now that I understand the basics of plywood…what’s the difference between Marine grade – Pressure Treated – Exterior Grade.

Pressure-treated plywood, often called "Wolmanized" or P.T. plywood, is NOT " Marine grade" plywood, and those designations do not make the two products arbitrarily interchangeable.  Pressure treated plywood is common plywood that has been subjected to pressure treatment with chemicals to prevent the wood from decaying, or rotting. To some degree, it also discourages insect damage because of the chemicals involved . Pressure treated plywood, however, is not suitable for marine use. The treatment of plywood with copper and arsenic compounds under pressure simply does not make the plywood waterproof, and worse, continuous exposure to water will leach the preservative chemicals from the pressure-treated wood.  Again, pressure treated plywood is ordinary, interior-grade plywood that has been chemically-treated, and it is often made with softer woods to enable the penetration of the wood treating chemicals, with no special care effected to eliminate all gaps or voids.

SIDE NOTE:  For you older folks, you might remember seeing some wood called out as CCA plywood or blocking.  CCA stand for chromate copper arsenate and it’s the chemical they used to create pressure treated wood.  In 2002 the EPA banned this process.  The common practice now is ACQ (alkaline copper quat) and CA (copper azole).

Exterior grade plywood is made with water-resistant glue, but the exterior shell is the only layer that is made void-free. There may be gaps, voids and the resulting points of weakness in the interior layers. When you cut a sheet of exterior grade plywood, you may expose a gap on the cut surface.

Marine grade plywood is a different creature. Marine grade plywood is assembled gap and void-free in all layers, and laminated together with special, water-proof glue that holds the various layers together. When immersed, water has absolutely no effect on the glue or the strength of the lamination of marine grade plywood. Marine grade plywood will not commonly delaminate, bubble, buckle, or warp. Upon cutting marine grade plywood, no voids will be discovered on the cut edges. It is also usually constructed of harder woods such as Douglas Fir, or Western Larch.

Marine grade is a superior grade of plywood, and a substantially better product.

So my big question…when do I use the different plywood:

Well, I’m not going to answer this too directly since each situation calls out for a unique solution, but in general:

Marine Grade Plywood:

This is typically used in super high water/moisture areas.  Think of boat docks or marinas!!  This wood is super expensive, so I would pick and choose when you want to stick this into your projects.  Consider a 4x8 sheet can run from $75 to $100 per sheet!!

Exterior Grade Plywood:

This is far more common.  I would consider this when you’re in areas that you know are going to get wet, but not constantly.  So, think of canopy framing, exterior side of a cavity wall, or any other surface that will be exposed to the elements (by that, I mean when you use plywood as a backing material to a product that will be completely exposed to the exterior, like stone veneer).

 Pressure Treated Plywood:

This is pretty common as well, but I would use it in areas where the exposure to moisture is going to be limited.  I’ve always considered this type of wood to have a much shorter life span that the others.  Think of your neighbors deck…the one that is made of wood, but it’s turned gray over time and is starting to bow…ya, that’s pressure treated wood.  Instead, think of it as a bottom track to a stud wall attached to basement floor.  There may be some moisture time-to-time that creeps up from the concrete, but it’s exposure to somewhat limited.

I hope you found this helpful!