Plywood Grades Explained: A Contractor's Buying Guide

Plywood Grades Explained: A Contractor's Buying Guide

Ordering the wrong plywood grade is a common and costly mistake. Using finish-grade plywood for sheathing wastes money; using sheathing-grade on visible surfaces looks unprofessional. Understanding the grading system takes the guesswork out of ordering and helps you match the right product to every application on the job site.

How Plywood is Graded

Plywood panels are graded on two faces: the front (face) and the back. Each face receives a letter grade from A to D based on the number of defects, patches, and overall appearance:

 

• Grade A: Smooth, paintable surface. All knots and defects are neatly repaired. Best appearance.
  
• Grade B: Solid surface with tight knots (up to 1") allowed. Minor repairs permitted. Good appearance.
  
• Grade C: Knotholes up to 1.5" allowed. Some broken grain. Used on sheathing back sides and structural applications.
  
• Grade D: Knotholes up to 2.5" allowed. Not for exterior or finish use. Interior structural applications only.
  

A panel labeled AC has an A-grade face and a C-grade back. An AB panel has two good faces. Sheathing panels are labeled CDX — C face, D back, with X indicating exterior-grade glue.

Common Plywood Panel Types

CDX Sheathing (Structural)

The most widely used plywood on the job site. CDX plywood uses exterior-grade glue and is designed for wall sheathing, roof decking, and subfloors. It's not intended to be exposed to weather long-term but will withstand construction moisture. Available in 3/8", 1/2", 5/8", and 3/4".

OSB vs. CDX for Sheathing

OSB has largely replaced CDX for sheathing in production homes due to lower cost. However, plywood retains advantages in moisture resistance and edge performance — particularly relevant in wet climates and for roof decking.

AC/BC Plywood (Finish/Utility)

Used for cabinets, furniture, shelving, painted surfaces, and visible utility applications. AC is good for one-sided visible applications; BC works where both sides may be seen but finish isn't critical.

Marine Plywood

Highest quality softwood plywood with no voids in core plies and waterproof glue. Used in marine applications, concrete forms, and demanding wet environments. Significant premium over standard grades.

Span Ratings

Structural plywood panels display a span rating — for example, 32/16. The first number is the maximum spacing for roof framing (in inches); the second number is the maximum spacing for floor framing. Always match the panel rating to your framing layout.

Species and Strength Groups

Plywood is manufactured from various wood species grouped by stiffness and strength:

 

• Group 1 (strongest): Douglas Fir, Southern Yellow Pine
  
• Group 2: Coastal Sitka Spruce, Western Hemlock
  
• Group 3-5: Less common species used in some panels
  

For structural applications, stick with Group 1 or Group 2 panels to ensure adequate stiffness and load capacity.

What to Order for Common Applications

 

• Wall sheathing: 7/16" or 1/2" CDX or OSB
  
• Roof decking: 1/2" or 5/8" CDX (or OSB equivalent)
  
• Subfloor: 3/4" tongue-and-groove CDX or rated subfloor panels
  
• Cabinets/shelving: 3/4" AC or BC birch plywood
  
• Concrete forms: HDO (High-Density Overlay) or marine-grade panels
  

Ordering Tip

When buying plywood wholesale, confirm the mill certification (APA-rated) and ensure panels are stored flat and dry on the job site. Improper storage causes warping that wastes product and labor.