Choosing the right plywood for a project comes down to numerous factors: a client’s budget, the proposed goals for the structure, its exposure to weather and moisture, and more. Among plywood options, CDX and ACX panels are two common choices. From variations in pricing to differences in utility, these two options are not necessarily interchangeable.
Understanding how these plywood options differ and what they are suited for is important to ensure that the appropriate materials are used to meet the project’s goals.
Here is an overview of how plywood panels are graded and the difference between CDX and ACX plywood.
Understanding Plywood Grading
Historically, plywoods were graded.
The industry-standard plywood grading system established by APA – The Engineered Wood Association defines plywood panel performance criteria based on the intended end-use of the material. Panels are rated for three end uses: single-layer flooring (Rated Sturd-I-Floor), exterior siding (Rated Siding), and sheathing for roofs, floors and walls (Rated Sheathing).
To qualify for a particular use, panels must meet performance criteria in three areas: physical properties, bond performance and structural performance.
Physical properties are described using a visual grading scale ranging from D to A, with A being the highest and D being the lowest. The grade reflects the number and size of visual defects in the veneers used to make the plywood panel.
A veneer graded as D can contain many large knots and some voids (open holes).
C grade wood will have knots and other minor imperfections. Voids and some knots may be filled with wood or resin.
B grade veneers may have small knots, some repairs and may be sanded. B grade plywood provides a smooth finish for underlayment as well as exposed, finished applications like shelving or painted cabinetry.
An A grade veneer is nearly free of visible imperfections, though it may have some pin-knots, a few repairs, and is also sanded.
Plywood is classified in one of two bond classifications: Exterior or Exposure 1.
Exterior panels have bonds capable of withstanding repeated wetting and redrying or exposure to weather. Exterior panels do not have rot resistant qualities. They must be coated, sealed or treated to protect against mold and decay.
Exposure 1 panels are for applications that are not exposed to the weather or repeated wetting and redrying. However, Exposure 1 panels may be used when exposure to weather is on the underside only, such as in roof overhangs, without compromising the integrity of the panel bonds. With that said, like Exterior classified plywood, Exposure 1 panels must be coated, sealed or treated to protect against mold and decay.
Spoiler alert! Both Exterior and Exposure 1 panels are manufactured using the same exterior adhesives. The difference in panel performance when exposed to rewetting and redrying comes from other compositional elements of the plywood like the grades of veneers used to assemble the panel.
Based on its intended end use, plywood is subjected to a battery of tests to assess structural capacity, dimensional stability and bond performance. The tests are not the same for all panel types, but can include measuring how the panel reacts to changes in moisture, uniform loads, concentrated static and impact loads, and the force required to withdraw a nail.
What is the Difference Between CDX and ACX Plywood?
With this understanding of how plywood is graded, we should be able to more easily determine the difference between “CDX” and “ACX”.
“CD” and “AC” refer to the physical properties of veneers used for the face and back of the panel. A visual comparison between the two panels shows that the AC panel has a smooth face with few pin-knots or repairs. By contrast, the CD panel is a much rougher panel. Both its C-face and D-back are unsanded and will have large knots. Plus, the D-back can also have some voids where knots have come out.
There is a common misperception that the X indicates a panel graded for exterior use. This is not the case. The X does not mean that the panel is an exterior grade.
As we now know, the “X” denotes the use of an exterior grade adhesive to join the panel’s veneers together. As all plywood panels utilize exterior grade glues, the X does not offer any distinguishing information about the panel. Yet, it remains in building industry jargon as a legacy of pre-performance-based industry standards.
Instead, remember that there are two bond classifications: Exposure 1 and Exterior. A quick way to determine if a panel is an Exterior panel is by the veneer grades. To achieve an Exterior rating, the minimum veneer grades used for both the face and the back must be a “C” grade or better. Thus, a CD plywood is an Exposure 1 panel, not an Exterior panel.
To quickly summarize the differences between AC and CD panels:
- AC panels are Exterior rated panels, suitable for non-structural uses where only one side will be exposed and a smooth surface is desired.
- CD panels are Exposure 1 panels. They are a construction workhorse commonly used for wall and roof sheathing.
- Exterior panels will use at least a C-grade veneer for both the face and the back.
- Exposure 1 panels will have a face and/or back veneer that is less than a C-grade.
Remember that both Exterior and Exposure 1 panels must be painted, sealed or treated to protect against rot and decay. Neither panel has any intrinsic properties making it resistant to rot and decay.
Learn More About Plywood Options from the Pros
Curtis Lumber & Plywood, founded in 1957, is an independently owned and operated wholesale lumber dealer exclusively serving premier lumber retailers throughout the mid-Atlantic and Southeast regions of the United States. Based in Northern Virginia, we specialize in fire-retardant and preservative treated lumber and plywood for both indoor and outdoor applications.