How is plywood graded? Well, the answer is not how well the plywood did on the history exam. Yes, yes, we know, a bad joke. But the question actually has many, many layers (pun intended).
There are grades of plywood, types of plywood, plywood quality, and more. Each plays into your choice, including the intended purpose for which it will be used. Let’s peel these layers back and take a closer look.
What is Plywood?
Plywood is a manufactured wood product made from thin layers, also called “plies,” of wood veneer. These veneers are glued together, with each layer, or ply, having its grain rotated 90 degrees to its adjacent layer.
The grains are rotated for 3 reasons:
- Prevent splitting when nailed;
- Avoiding, or at least reducing, expansion and contraction from changes in its environment; and,
- Providing integrity to the strength of the plywood consistently in all directions across the board.
The types of plywood usually refer to the number of plies, usually in odd numbers, 3-ply, 5-ply, and multi-ply, with 3-ply being the most common. 3-ply plywood runs about 2 to 3 millimeters in thickness and is most often used in indoor projects because it appears more decorative than thicker plies.
Is Plywood A Wood?
Yes, it is layers of wood fiber glued together with a resin to form sheets or panels. The most common size of these sheets or panels is 4’ x 8’, although specialty sizes are also sold for particular purposes, like cabinet-making.
The project you are planning will determine the plywood you will want. Thickness numbers can be a little misleading, and you must be aware of this when choosing your stock.
Just like the cutting of 2x4s removes enough material to make the actual measurements a little less than those numbers, so does the sanding of plywoods during their manufacturing affect the actual thickness of the sheets. It doesn’t bear on the subject of this article, but it is something to keep in mind so your measurements will be accurate.
One other thing to mention about plywood being a wood – normally, the face veneers will be of a higher grade than the core layers.
What is Plywood Used For?
The answer is many, many things. When framing a house, plywood attaches to the exterior of the wall framing. When laying a floor in a house, plywood is used for the floorboard over the floor joists. Furniture and cabinets are also common uses for plywood.
Basically, when a project calls for a high stability factor, plywood is the go-to choice because of its strength.
Benefits of plywood include:
- Stability. As mentioned, the strength of plywood, which comes from multiple layers cross-grained in alignment, provides stability to any project or application.
- Strength to Weight Ratio. With good high impact resistance, stiffness to weight ratio, and stability against changes in temperature and moisture, plywood is the right material for flooring and shearwalls. The sheet shear is nearly twice as strong as solid lumber because of its cross-grain structure.
- Resistance to Chemicals. Plywood does not corrode, so it can be used in chemical works, concrete forms work, etc., as a cost-effective and reliable material.
Plywood comes in many grades, and your project will determine which is the right choice for you.
What Are The 5 Grades of Plywood?
Plywood grades refer to the quality and appearance of the sheet’s face and back plies. There are grades, and within grades, there are grades. It’s not necessarily as confusing as it sounds, though. Let us explain.
The grade chart, first grades, if you will, are:
- A grade. Highest quality, most expensive. Sanded smooth, surface will be blemish free, and paintable. This is the top of the line plywood.
- B grade. A solid surface, also sanded smooth but with some flaws in appearance. Repairs will have been made and will show as football-shaped patches or wood filler. While some knots may show, no wood is missing. Less expensive than A grade.
- C grade. Unsanded and with some defects apparent that you will have to repair yourself, including splits and knots up to 1 ½ inches, with wood missing. Discoloration is common. To be used when the sheets will not be seen, such as in floorboards.
- D grade. Also unsanded, with defects you might have to repair yourself, including splits, knots up to 2 ½ inches with wood missing, and discoloration. This is the least expensive plywood, and again, to be used where it will not be seen.
What Are Plywood Ratings?
Separate from, but equally important to your choice of plywood, are its ratings. These include:
- Exterior: Sheets have been waterproofed and may be used on outdoor structures that will be exposed to the elements.
- Exposure 1: One rating below exterior, these sheets have also been waterproofed and can be exposed to the elements during construction, but are not for prolonged exposure.
- Exposure 2: Waterproofed to a lesser degree (the glue used can withstand occasional exposure to water and moisture), but its use should be limited to interior projects. Remember, water is the enemy of wood, even plywood.
- Interior: Not waterproofed, and use should be limited to interior projects only.
- Structural 1: Earthquake resistant. Most of us in the US will not need to concern ourselves with this rating, except perhaps on the west coast.
Other Plywood Features
Some special features of plywoods may also affect your decision on which is the best for your particular project, including:
- Stainable: The surface has already been sanded and will accept a stain for a natural-looking finish. This would be an A-grade plywood, Type – Interior, suitable for a use that will be seen.
- Paintable: Again, an A-grade plywood, sanded smooth, ready for paint application, suitable for a use that will be seen.
- Pressure-Treated: Just as with PT lumber, these plywood sheets have been pressure-treated, and will resist damage from water, the elements, and insect infestation. Type-Exterior would be the choice, along with exterior use.
Within these grades, types, and features, plywoods can be broken down further into additional categories, such as:
Softwood, made with cedar, redwood, and pine. Good for shelving, for instance.
Hardwood, made with birch (one of our favorites), oak, walnut, and maple. Good for furniture, musical instruments, and such. Baltic birch plywood is the cream of the birch plywood, a higher quality than standard birch, and is made entirely of birch plies with no softwood plies or filler. Here’s a cool video about Baltic birch plywood, if you’re interested.
Aircraft, one of the most durable and highest grade plywoods, for obvious reasons.
Marine, although the name can be misleading. It’s not water-resistant, and this is more the grade of plywood than an implication it is used to make boats. It’s a very high-quality plywood, the most high-grade constructed plywood, often used on exterior projects like benches, decks, and other garden features.
There are more, but you get the idea that there are types within types when it comes to plywood.
Grades Within Grades
Now we’ll break down the grades within the main grades for you since this may have a bearing on the choice of plywood you will make for your project. It gets a little complicated, to be honest, and we’ll do our best. But don’t hesitate to ask the sales person at the lumberyard or the big DIY stores if you have any questions.
Within each sheet of plywood, there are two grades, such as AB, AC, BC, DD (refer to the grade chart above). The first letter refers to the face ply, and the second letter refers to the back ply. This would be, of course, in the event they are different.
Following this understanding, CD would refer to a face veneer of C-grade and a back veneer of D-grade.
Add an X to it, CDX, and the X refers to the glue, which is rated for temporary outdoor use. We say temporary because this plywood would be used on the outside of a house frame, then to be covered by brick or shingles or siding. The formal designation of this plywood, incorporating everything we have discussed above, would be:
CD Exposure 1 Plywood
A designation such as C2 would refer to C-grade face veneer, and the 2 would refer to the back veneer where 2 designates color variation or flaw.
A designation such as C+ refers to pine plywood that is used for such things as packing, decking, roofing, flooring, and other construction uses in buildings.
This is not intended to be an exhaustive discussion of all the different grades within grades when it comes to plywood. Rather, it suggests there are many designations of plywood, some of which might be pertinent to your project.
So, again, don’t hesitate to ask questions of the salesperson when purchasing plywood. It’s not as simple as asking to buy plywood – – what’s your project, what’s your budget, etc., all come into play, and you want to make the smart choice.
We consider this an essential Plywood 101 video that even seasoned woodworkers might want to watch.
Choose your plywood well, and your project will turn out as you want it to be.