Ryan Crooks – Plywood is engineered lumber composed of three or more layers. The layer faces are oriented 90 degrees to one another to increase the structural characteristics of the board in all directions. Grading the veneer by letter, such as “A,” “B,” “C” and “D,” describes the quality of the face of the plywood, with “A” being the highest quality. Veneer letter grades do not convey structural properties or applications, only visual characteristics.
Meaning of Letter Order for “AC” Grade Plywood
The first letter in “AC” is the exposed face grading, and the second letter is the reverse face grading. So, “AC” grade plywood will be installed with the “A” side visible. The middle layers of plywood are not graded, but they are usually lower quality than either of the face plies.
Veneer Grade “A”
Veneer grade “A” has a smooth surface that can be painted or stained. There should not be more than 18 repairs to the surface of a standard 4-by-8 sheet during fabrication, and noticeable knots and splits should not be present in the wood surface.
Veneer Grade “C”
Veneer grade “C” can have a rough surface, as well as visible repairs. Knots no larger than 1-1/2 inch are allowed. The wood can have splits and discoloration.
Use of “AC” Grade Plywood
“AC” grade plywood is one of the most common types. Although the “AC” only gives the visual quality of the plywood surface, the plywood can be used for interior and exterior applications, determined by a separate exposure durability classification and span rating. The exposure durability classification gives the plywood’s ability to resist water and moisture, where most plywood is “Exterior” or “Exposure 1” — glued with waterproof adhesive for exterior use. The span rating provides how far apart joists should be placed under the plywood for a floor or roof.
Structural Plywood
Structural plywood is usually a lower grade of plywood — “CD.” The veneer grading is the same as with “AC,” where “D” permits knot up to 2-1/2 inches wide, and splits and repairs are noticeable throughout the plywood surface. Because the plywood is used for structural purposes, there is no need for an attractive plywood-veneer.