Plywood comes in different types and grades to accommodate its many uses. Understanding the differences between plywood types ensures that projects are built with materials that meet or exceed the required design values or aesthetic desires.
Let’s look at what we mean by exterior plywood grades and their unique qualities.
What is Exterior Plywood?
Contrary to what its label suggests, “Exterior” rated plywood does not have any rot resistant properties. Rather the term “exterior” refers to the panel’s bond classification and type of adhesive used when manufacturing the plywood. Further, an Exterior rated panel can be used for both interior and exterior applications.
There are two bond classifications: Exterior and Exposure 1.
- Exterior panels use glue that will withstand repeated wetting and redrying or long-term exposure to moisture or high humidity. A rule of thumb for recognizing Exterior panels is that they have a “CC” grade or better. Examples of Exterior panels include BC Tongue & Groove (T&G) Underlayment; AB Marine Grade plywood, and CC Struct1 plywood.
- Exposure 1 panels use the same adhesives as Exterior plywood, however are not suitable for sustained exposure to weather, moisture and humidity. The panels can tolerate exposure to the elements as can occur on a jobsite. However, due to other compositional factors of the panel, when repeatedly subjected to high levels of moisture or humidity over time, the panel can delaminate or lose structural value.
An example of an Exposure 1 panel is the Rated Sheathing plywood (familiarly called “CDX”) used for wall sheathing and roof decking.
Exterior Plywood Grades and Ratings
Plywood is also classified by the quality and look of the face and back veneers of the plywood into four grades: A, B, C, and D. An A grade is the highest quality and higher cost compared to lower grades, whereas D is the lowest quality and has the lowest cost.
- A-grade plywood has a smooth, sanded surface that is free of knots. Any apparent flaws have been filled with wood or synthetic filler.
- B-grade plywood also has a smooth, sanded surface, although it allows for more repairs and tight knots up to 1 inch across.
- C-Plugged-grade plywood has an improved, touch-sanded C veneer face. Splits are limited to ⅛-inch wide and knotholes or other voids limited to ¼” x ½”.
- C-grade is unsanded and allows for tight knots up to 1.5 inches, discoloration and sanding defects that do impair strength. Limited splits are also allowed.
- D-grade is also unsanded and allows unrepaired knots and knot holes up to 2.5 inches across and discoloration and sanding problems. Limited splits are also allowed. D-grade veneers are only used in Exposure 1 panels.
APA – The Engineered Wood Association is one of the primary North American standards-setting bodies for the manufacture of plywood and other engineered wood products. APA identifies nine distinct rated panels types. The more common of these are:
- Rated Sheathing. Used for a broad range of construction and industrial applications, Rated Sheathing is unsanded and designed for subfloor, wall and roof sheathing. Rated Sheathing is span rated to indicate the maximum support spacings required for each panel thickness.
- Sturd-I-Floor. Designed as a single-layer subfloor-underlayment. This is a sanded panel manufactured with either a square edge or tongue-and-groove edge. Not rated for use as roof sheathing.
- Structural 1 (“Struct1”). Struct1 panels are most often Rated Sheathing panels manufactured to provide additional strength along a diagonal strength axis. Typically, the grain of panel veneers are alternated to be perpendicular to each other as each veneer layer is added. In Struct1 panels, veneers are also added at an angle to provide additional resistance to “racking” or stresses applied diagonally. Struct1 panels are frequently used in seismic zones or in any application where additional strength is desired.
- Underlayment. Sanded panels for use as the finished layer on top of structural subflooring. Provides a smooth surface for finished flooring products.
- Sanded Plywood. For use where appearance is of primary importance. No span ratings are given to Sanded Plywoods.
- Marine Grade. Marine Grade panels are made with AA, AB or BB veneers. As these grades are free of core voids, Marine Grade plywood is the highest grade plywood when smooth surfaces, rigidity and long term stability are critical. Marine Grade plywood is not inherently resistant to rot and decay.
About Curtis Lumber & Plywood
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. If you are a lumber retailer and have questions about plywood or treated wood products, or need to place an order by the trailer load, unit or piece, contact Curtis Lumber & Plywood at 703-972-1947.