A plywood grade tells you the quality of the face and back veneers, the glue bond strength, and the appropriate exposure level. The APA (Engineered Wood Association) grades plywood from A (smooth, paintable) to D (structural use only). Selecting the right grade saves money and ensures your project meets code.
Click a grade to see details. Face/back combinations determine the plywood designation (e.g., C-D = CDX sheathing).
Select a grade above to see details
Common Face/Back Combinations
| Designation | Face | Back |
|---|
Span ratings indicate maximum on-center spacing of supports (inches). The panel must run with its long dimension perpendicular to supports.
| Span Rating | Roof Span (in OC) | Floor Span (in OC) |
|---|
| Nominal | Actual | Weight / sheet |
|---|
| Property | Plywood | OSB |
|---|
How to Use the Plywood Grades Reference
Choosing the right plywood grade can save significant money without compromising structural integrity. This APA plywood grade guide covers everything from face veneer quality to exposure ratings and span ratings used in real construction.
Understanding APA Face Grades (A through D)
The APA (Engineered Wood Association) grades plywood veneers on a scale from A to D. Grade A is smooth and paintable, with no open defects — ideal for cabinets, furniture, and any surface that will be seen. Grade B has minor repairs and a solid surface, commonly used as an underlayment or cabinet back. Grade C Plugged limits knotholes to 1/4 inch and is used where moderate appearance is needed. Grade C allows larger knots and is the minimum grade for structural sheathing. Grade D is the roughest grade, used only in interior structural applications where it will never be exposed.
Reading the Plywood Grade Stamp
A plywood panel is labeled with both its face grade and back grade, separated by a hyphen (e.g., C-D). CDX — the most common sheathing — means C face, D back, and Exposure 1 glue. The exposure rating letter follows the grade combination. When you see "APA RATED SHEATHING 32/16 15/32 INCH" on a stamp, you know the span rating (32/16), nominal thickness (15/32"), and APA certification. Always check for this stamp before purchasing structural panels.
Choosing by Exposure Rating
Exposure ratings are critical for any outdoor or moisture-prone application. Exterior plywood has waterproof glue AND C-grade or better veneers throughout — use it for siding, fences, and soffits. Exposure 1 has waterproof glue but may have D-grade inner plies — fine for roof sheathing that may get wet during construction but will be covered. Interior and Exposure 2 panels are for permanently dry indoor conditions only.
Using Span Ratings for Structural Work
Span ratings appear as two numbers (e.g., 32/16). The first number is the maximum roof rafter spacing in inches (on-center); the second is the maximum floor joist spacing. A 32/16 panel works on roofs with rafters up to 32 inches OC and floors with joists up to 16 inches OC. Standard residential construction uses 24-inch OC rafters and 16-inch OC floor joists, so a 24/16 rating is often the minimum code requirement for subflooring.
Plywood vs OSB: Which Should You Choose?
OSB (Oriented Strand Board) is manufactured from wood strands glued under heat and pressure. Both OSB and plywood carry identical APA span ratings and meet the same building codes. OSB is typically 10–15% less expensive. However, plywood holds fasteners better at cut edges, resists edge swelling from moisture, and performs better in high-humidity environments. For most wall sheathing and roof decking, either works well — but in wet climates, many contractors prefer plywood for roofing.
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What does CDX plywood mean?
CDX means the plywood has a C-grade face veneer, a D-grade back veneer, and Exposure 1 (X) glue bond. It is the most common structural sheathing grade, used for wall sheathing, roof decking, and subfloors. The 'X' does not mean exterior — it means the glue can withstand temporary moisture during construction.
What is the difference between Exterior and Exposure 1 plywood?
Exterior plywood uses fully waterproof glue AND has C-grade or better veneers throughout, making it suitable for permanent outdoor exposure. Exposure 1 (such as CDX) uses waterproof glue but may have D-grade inner plies, making it fine for construction delays but not permanent outdoor use.
What do the two numbers in a span rating mean (e.g., 32/16)?
The first number is the maximum recommended roof rafter spacing in inches (on-center), and the second number is the maximum floor joist spacing. So 32/16 means the panel can span up to 32 inches on a roof and 16 inches on a floor. The rating assumes the panel is placed with the long dimension across supports.
What is the actual thickness of 3/4 inch plywood?
Plywood is sold in nominal thicknesses but the actual thickness is typically 1/32 inch less. A nominal 3/4" panel measures approximately 23/32" (18.3 mm). A nominal 1/2" panel is about 15/32" (11.9 mm). Always confirm actual thickness with your supplier for precision cabinetry or flooring applications.
When should I use Marine-grade plywood?
Marine plywood uses B-grade or better veneers throughout with no voids and fully waterproof glue. Use it for boat building, docks, or any application with sustained water contact. It costs significantly more than ACX exterior plywood, so for most outdoor furniture or decks, ACX or even MDO is sufficient.
Is OSB as strong as plywood?
OSB (Oriented Strand Board) has similar structural properties to plywood for sheathing and subfloor applications, and carries the same APA span ratings. OSB is typically less expensive but is more susceptible to edge swelling from moisture. Plywood holds fasteners slightly better at edges and is preferred for high-moisture applications.