The typical grading system used most often for plywood, which is an engineered wood panel, uses the letters A through D (A,B,C,D) and N, to grade the quality of the plywood according to the number of blemishes.
Grade N plywood would generally contain the least number of blemishes, followed by Grade A and then through to grade D plywood containing the maximum roughness and number of blemishes, seams, knots and repairs allowed.
Most retailers of plywood have a two letter grading system. One letter is used to grade the front and another to grade the quality of the back of the sheet of plywood. For example, if a piece of plywood had a grade of A-C, the face of the wood would be finely finished with very few blemishes and they back of the piece would be a bit rough and unfinished compared to the face. No need to finish the side no one will see, after all.
Another example would be construction grade plywood. While the previous example, grade A-C, would be okay for indoor finished work, construction grade plywood is used mostly for structural work that will not be seen by most people. Construction grade plywood generally would be grade C-D and is commonly referred to as CDX plywood.
Four Basic Types of Plywood
Besides the grade, plywood is also categorized by its type or purpose. The four main types of plywood that are commonly known are:
1. Interior Plywood: Interior plywood is what would commonly be used for constuction on the inside of a home. It is not meant to be exposed to much moisture. It is bonded together with agents that don’t allow it to be exposed to the elements, so this type of plywood should never be used outdoors or in wet areas. Interior plywood is usually available in most grades and common ones are A-C, B-D, etc.
2. Exterior Plywood: Exterior plywood is glued together with glue that is meant to hold together better when exposed to moisture. Exterior plywood is designed for such applications as building a home’s outside structure or any other construction outdoors or in slightly moist areas. It is available in most grades and the most common are A-C, B-C and CDX. As mentioned earlier, CDX is the common construction grade plywood that is so easily found in stores and DIY centers.
3. Structural Plywood: Structural Plywood is designed to be extra strong. It is glued together with adhesive that holds the layers together firmly. It can be used inside or outside, but is not really meant to be seen. It is commonly only available in grades C-D or D-D, so is a rough piece of plywood. It is what construction crews generally use for cement forms. It can be used anywhere that you need a strong structure and don’t care what it looks like.
4. Marine Plywood: Marine plywood is the strongest, toughest, highest grade plywood you can get. It is fabricated with superior glue that is moisture resistant and bonds tightly. This type of plywood is generally only available in A-A grade and only in hardwoods that match marine like surroundings. Since it commonly comes in A-A grade, it is meant for both sides to be finished. So, you could use this perhaps in a finished basement where moisture is high and you want it to look good, but it may be a little over the top even for that. It would be good for constructing outdoor furniture or similar applications that call for weather resistance and finished surfaces.
Softwood plywood is available in thicknesses ranging from 1/8 inch to 1 inch, with the most commonly used thickness being ¼, 3/8, ½ and ¾ inch. The standard size of a panel is 4 feet wide by 8 feet long.
Most panels sold in the U.S. are stamped with grade marks complying with the National Plywood Standard PS 1-83, which contain all the information on the grade, thickness, and exposure durability rating (indoor vs. outdoor).
There is also a span rating on structural grade panels, in the form of two numbers, separated by a slash. The number on the left will indicate the max recommended span in inches when the panel is used as roof sheathing. The number on the right would indicate the max recommended span when the panel is used for subflooring. This rating only applies to panels placed the long dimension across 3 or more support members.
So, you know the four letter grades and the four basic types of plywood. Now all you have to do is understand how they work together. Most often plywood for sale will be called by both a type and grade so you know exactly what you are getting. For example you might see something like Interior/Grade A-C plywood.
Now, after reading this article, you should know exactly what that is. It is a sheet of plywood designed to be used indoors only that is finished good on one side and rough on the other. It could be used for furniture, cabinets or inside walls, but not outside structural work.
Photo by Hryck, Creative Commons Attribution License