Which plywood grade combination would NOT normally be used for sheathing or underlayment?

• A. A-A
• B. B-C
• C. A-C.
• D. C-D

Answer: (C)

Plywood grade on each face tells you how smooth and defect-free that face will be. For sheathing or underlayment, you want panels that sit flat, nail securely, and give you predictable thickness at a reasonable cost. A panel with one very high-quality face and the opposite face with more defects creates a mismatched sheet: the rough face can cause installation issues or show through when finished, while you’re paying extra for the premium face you won’t actually use on the visible side. That mismatch isn’t typically chosen for these applications, whereas two faces of more balanced, lower-to-mid quality (or a finish-grade panel only when a finished surface is needed) are standard options.